■ ■ .- ' ■ ' '. - m& £-<&?> 15-18 (?) „ cow 40—41 >> 21 horse 42 )> 21 man 46—46 2/3 » 23 v3 » Here, once more, we are face to face with a problem requiring solution. Why should the simple relation between the two units, obtaining in rabbit, mouse, or cavy, have been altered? What advantage, or advantages, are to be gained by the new procedure? It is, probably, sufficiently clear, that one of the main objects in prolonging the gestation of the higher mammals has been the bringing of the young into the world in a more advanced degree of development. This explanation is a very obvious one, and has been urged by others1); but, as we have seen, mere length of gestation will not effect this, for it is conceivable, that one might have a very- prolonged gestation of two, or even of one, critical unit, and in such a case the new-born young would be brought forth in a more or less immature condition. If it be correct, that the gestation-period of the kangaroo has a length of some 38 days, as Owen stated, it follows, that the com- paratively prolonged gestation in this instance fails to accomplish more, than is reached in 8 days by the opos- sum, or in 10 days by the mouse, while it falls far short of the 1) The name of Robert Chambers, author of the celebrated "Vestiges of the Natural History of Creation", is sometimes cited in this connection, but he seems to have regarded the effect of increase of gestation -length as causing the passage from a lower to a higher genus. - 58 - degree of development arrived at by the rabbit in several days fewer than 38, or by the mouse in little more than half the time. It must, therefore, be accepted, that mere length of gestation, apart from the influence of other factors, does not determine the degree of development at birth. And this can be rendered more certain by other considerations. We have seen, that in certain cases, rabbit, cavy, and mouse, the critical unit is a very short one, and, moreover, that it must be considered as originally falling just short of an ovulation-unit. Now, the critical unit is, as we have stated, the average time requisite in a given form for development to the critical period, and this unit, owing to its relations [to the ovul- ation-unit, governs the gestation-length in such a fashion that the latter must always be some multiple of the former, the degree of development attained by the time of birth being determined by the number of critical units in the gestation. There appears no reason to doubt, that a simple crit- ical unit, like that of the mouse, i. e. one approximately equal to an ovulation-unit, could, if necessary, suffice as the unit of such forms as man, horse, sheep, or pig. What the cavy, or mouse, reaches in a period of probably rather less than an ovulation-unit ought, so it would seem, to be possible of attainment by a human, sheep-, pig-, or horse- embryo in a like interval. There is no doubt whatever, to my mind, that in by- gone days this held true, but at the present time double this interval, i. e. almost two ovulation-units, is the period required; or in other words, as already stated, the critical unit in these forms is somewhat less than two ovulation-units. — 59 — Before inquiring further into the matter, it may be well to see what would happen, if in man, pig, sheep, and horse the critical unit corresponded approximately to one ovulation - unit, whilst the number of the former in the whole gestation-period were to remain as at present. The obvious result would be a shortening of the gestation, and in each instance it would be reduced to half of its present length. Thus, in man it would be 138 — 140 days, in the horse 168 — 175 days, in the sheep 721/2 — 76 days and in the pig 54 V2 — 61 V2 days. It would follow from this, that, if the milk-nutrition of a preceding litter did not interfere, the number of offspring produced would in the long run be double what it is at present. The reverse of the picture is this; that, when by the addition of new critical units the gestation-period most advantageous for the species had been reached, i. e. that with which the young came into the world in the con- dition 1) of development most suited to the needs of the species, when no further advance in uterine development was desirable, or needful, if it became necessary, or useful, to further prolong the gestation without altering the degree of development at birth, this could only be attained by increasing the critical unit. In a cavy an increase of 10 days in this would yield a further prolongation of gestation for 30 days. In a form with a critical unit of 21 days and a gestation of 7 such units, an increase of 20 days in the unit would lengthen the gestation by 140 days. But, in enlarging the critical 1) Not the highest degree of development possible, for, theo- retically, that would coincide with sexual maturity! — 6o - unit Nature could act in no such haphazard a fashion. She had also to take the ovulation - unit into account, and either prolong this in a corresponding degree, or else adapt the new critical unit in such a manner, that it would correspond approximately to two ovul- ation - units. If she had adopted the former course in any instances, we should not be able to point to evidences proving it, and could only suspect it from the conditions in other forms, in which it had not happened 1). That she chose the latter in the higher mammals is demonstrated by the fact, that in man, pig, sheep, and horse, the critical unit is rather less than two ovulation-units. In the present discussion we are less concerned with the objects of certain phenomena, than with the things themselves. It is, indeed, sufficient for the end in view, to determine the facts, without troubling much about purposes. But the arrangement just noted and its results are of so curious a nature, that one is tempted to inquire into their possible meaning. If, as here maintained, the critical unit governs the length of gestation in such a fashion, that the latter is always some multiple of it — and the truth of this does not appear to be in the least doubtful — the one obvious result of a duplication of the critical unit must be a doubling of the whole gestation-period. As we have seen, the number of critical units in the gestation determines the degree of development at birth, the length of the unit having absolutely no influence in this matter. i) As already indicated, the comparison of the short critical unit of Didelphys with the nearly two-fold one of Lepus leads one to suspect, that once upon a time this eventuality also was realised. — 61 — It follows from all this, that the doubling of the whole gestation - period, brought about by a duplication of the critical unit, can have no effect whatever on the degree of development, attained by the time the birth-period arrives; and, therefore, some other reason must be at the bottom of the matter. A very obvious consequence is, that the number of gestations possible in a given time will be reduced by half. There would appear to be no conceivable advantage to the species in a Malthusian arrangement leading to this : indeed, some compensation for this loss of possible progeny might seem called for under it. Apparently, the solution must be sought for elsewhere. When the forms are studied, in which the critical unit is "simple", i. e. in which it approximates to one ovulation- unit, it is noticed that they are all of small size, for example, mouse, rabbit, cavy, mole, and hedgehog1). Those, on the other hand, which have a compound critical unit of approximately two ovulation-units, are, in the adult condition, usually much larger in size, as for example, man, horse, cow, sheep, and pig 2). In all these cases it may be assumed, that at corres- ponding periods of uterine or post-uterine life the young of the latter set must be much larger, than those of forms with simple critical units. i) That mole and hedgehog must be placed in this category appears to me to be certain from the circumstance, that, with a short gestation-period of either one, or two (according to some), months, the young are born in a very helpless condition. 2) Something of the same kind possibly prevails in the marsu- pials, and it may account for the longer gestation of the kangaroo (recorded by Owen), as contrasted with the much shorter one of the opossum, or kangaroo-rat. — 62 — So far as the actual sizes of the various embryos at the critical period itself are concerned, there is, perhaps, not so much difference, but even then it is marked, and, whereas a rabbit-embryo of the critical period measures about 12 — 13 mm (greatest length) 1), one of a pig is then between 19 and 20 mm in length2). Thus, the only reason 3), so it would appear, which can be assigned for this duplication of the critical unit, must depend on an increase in size having taken place in the course of the evolution of the higher mammals. This has been proved by Marsh, Cope, and others in the case of the horse, Phenacodus not being larger than a sheep4) and the other known ancestors in proportion. 9 1) See appendix t. 2) In marsupials at the critical period Trichosurus measures 14 mm, Macropus thetidis probably under 20 mm, and Macropus major, according to Owen, 25 mm. 3) But it is by no means suggested, that other possible and reasonable explanations should be excluded. Geddes and Thom- son note regarding the length of gestation that "it also depends on size, being about 280 days in the cow and 150 in the sheep". (Evolution of Sex., p. 247.) Compare also the following: ,,So sind nehmlich eben die Thiere, bei denen sich eine geringere Fruchtbarkeit zeigt, auch solche, die eine grossere Zusammen- sezung ihres Korpers haben, und bei denen die Jungen nach der Geburt weit ausgebildeter erscheinen, und so sind's ferner sie, zu deren Hervorbringung die Natur bei weitem am meisten Zeit braucht; um einen Elephanten zu bilden, dingt sich die Natur bei dem unfruchtbaren Elephanten zwei Jahre aus, wahrend wenige Wochen zur Bildung einer Ratte hinreichen", u. s. w. (C. F. Kiel- meyer, Ueber die Verhaltnisse der organischen Krafte etc. Eine Rede, den n. Februar 1793 — — gehalten. Stuttgart 1794, p. 29.) 4) One species being as big as a bull-dog. - 63 - VIII. The Influence of Lactation. Discussion of this important matter has been post- poned until now because of its complexity. The subject is a very big one, and, undoubtedly, one of the very many awaiting the attentions of coming comparative physiologists. It is rather strange, that it should hitherto have attracted so little notice, for in the human subject the problem is one, that is constantly intruding itself upon the gyneco- logist. Leaving the conditions in man on one side, it is certainly the fact that there are many mammals, 'in which the milk-nutrition, and it alone, suffices for the sup- pression of ovulation for certain periods, varying in length in different species, and, even, in some cases in the indivi- duals of a species, though this is of rarer occurrence. At this juncture, and with the slight material available from my own incidental observations, and from what can be gathered from books, very little can be done towards elucidating the question. In some animals the milk-nutrition appears to have absolutely no influence on the ovulation, and, unquestion- ably, this must be regarded as the primitive mammalian condition. In such a case, as we have seen, the female is practically always pregnant, for a new ovulation and gestation ensue almost immediately after the birth. Under favourable conditions of nourishment this appears to be the state of affairs in cavy and mouse. My own experiences with the breeding of these animals are too limited, to enable me to exclude the possibility, - 64 - that suckling may have a disastrous effect on a new- gestation ; but, certainly, from my reading no evidences of its having had such an influence have been encountered * ). Moreover, they both appear to ovulate with strict regu- larity throughout the greater part of the year. As a rule, the same may be said to hold good for the rabbit also, and a breeder of rabbits informed me, that it was not at all uncommon for a rabbit to have eleven litters in a year. Some years ago, for the space of a year, the breeding of rabbits on a large scale was attempted in Freiburg i/B., with a view to the acquisition of a complete series of embryos. The experiment came to sudden and unantici- pated close owing to the acceptance of an appointment in Edinburgh. Interesting observations were, however, made : in some cases too interesting, because of their negative properties in helping the collection of material. In order to keep up the stock, the young of many rabbits were allowed to live, when it was intended, that the mother should be sacrificed at an early interval after the birth, to obtain the embryos of a gestation following on the birth. This meant, that while the mother was supposed to be pregnant, the young were allowed to suckle. Many such suckling females were sacrificed at dates from the eighth to the fourteenth day of gestation, and in some of them it was found that, while the mother had certainly I) Since the above was written, experiments with mice have been made to test the matter, and it has turned out, that lactation may in this animal lead to the suppression of a new gestation. - 65 - ovulated within the usual period of twelve hours after birth as determined by Bischoff, the resulting embryos had been unable to hold their own against those suckling, and had died, before advancing very far in their development. This was proved by their remains. Since then many such cases have come under notice. In such instances it was clear, that the mother had been unable to satisfy the demands of two sets of young, those outside the body at the mammae, and those in utero, and, that lactation was responsible for the starvation of the latter. The nutrition of the mother could not have been the cause, or, rather, only indirectly, for it was possible, that the abundant nutriment furnished to her had enabled her, to bring forth a greater number of young, than that, which she could nourish, as well as a new set in utero. This suppression of a gestation, already commenced during a milk-nutrition, is probably fairly common in the rabbit, but it is very far from being the rule in this animal. In the pig, on the other hand, ovulation, and with it gestation,, entirely ceases during the lactation-period, which here, remarkably enough, is about equal to the length of gestation, i. e. four months. About the end of this time the young are weaned, ovulation and heat ensue, and the sow may again enter upon a new gestation. Much the same sort of thing happens in the cat and the dog, the difference here being due to a shorter gestation, and, thus, the lactation-period is almost equal to that of two gestations. It is a curious fact, that the cat especially seems to have divided up the whole year in such a way, that two Beard, Span of Gestation. c — 66 — gestations can occur, with prolonged lactation - periods, mainly filling the intervals. In pig, dog, and cat, the effect of the lactation in suppressing, or rendering ovulation abortive, and with it gestation, is very apparent. In man, though naturally the exceptions are now, perhaps, far more numerously represented, than cases con- forming to the rule , one can again note an average lactation-period, about equal to that of pregnancy. In the sheep *), on the other hand, with its single gestation during winter, we encounter the influence of climate, rather than that of lactation, and the same thing probably holds in a great many cases. But so little is known with reference to this, that it would be futile to attempt its discussion2). Summing up the matter, it must be concluded, that originally lactation had no influence on gestation, and that this is still so in many cases, but that in others it has acted as a check on a new gestation, and still does so. Where this obtains, the lactation-period will normally cor- respond to one or more gestation-lengths, i. e. it will keep up the rhythm of the gestation and ovulation. i) Bonnet (Arch. f. Anat. u. Entwickl., 1884) makes the follow- ing interesting citation: — "Franck giebt an, dass das sich durch grosse Fruchtbarkeit auszeichnende chinesische Ong-te (a variety of sheep) kurze Zeit nach der Geburt wieder trachtig werde und zweimal im Jahre Junge bringe" (p. 172). 2) According to Ryder, the diversion of nourishment from the ovary during the period of gestation tends to starve the re- maining ovarian ova, and this check to fertility is further prolonged during lactation. This citation is taken from the writings of my friend, J. Arthur Thomson, and it was noted too late to find the exact reference. - 67 IX. The Special Case of Man. On account of the difficulties and complications, which beset the elucidation of the gestation-period of the human subject, including under these ovulation- and fertilis- ation-epochs, as well as menstruation, it is no doubt convenient to consider this instance by itself in all its bearings. Of the factors involved, except the birth-period itself and the menstruation-interval, no exact information is obtainable. The data, upon which the age of any parti- cular embryo can be fixed, are always deficient ; and, thus, it happens that, given an embryo of the critical period, its age can only indirectly be calculated. The date of copulation may be left out of account, even though it be accurately known ; for, while in certain animals the time of fertilisation can be approximately reckoned from that of copulation, it would be idle to suppose, that in man similar information could be made at all a certain basis for the calculation of the moment of fertilisation. The most important factor is only approxi- mately known, and this is the time of ovulation. There is no doubt, that in any instance, where this is known, and where the relationships between ovulation and copul- ation can be stated in days or hours, the problems re- maining will turn out to be very easy of solution. But the most vital points in the case of man are the determination of the ovulation-unit and of the time of ovul- ation. The ovulation-unit is, however, not far to seek, and almost by common consent — whether rightly or 5* — 68 — wrongly remains to be discussed — it has been set down as 27 — 28 days. It may, indeed, be taken as the general opinion, that, whatever be the relationship between menstruation and ovulation, there is a correspondence between the two, of such a kind, that the interval of the one is equal to that of the other. But, before an attempt can be made to estimate the probable time of ovulation, it is necessary, that the pheno- mena of menstruation should be studied. This study is all the more forced upon us by the circumstance, that not only is the gestation-period, as a rule, calculated from the occurrence or non-occurrence of a menstruation, but the time of fertilisation also. X. The Meaning of Menstruation. It is hardly necessary to say, that this is a subject, about which much has been written. For one, who is not at all versed in gynecology, and who has no observations of his own to put forward, it may appear presumptuous to venture an opinion. But there are numerous facts avail- able, and to the writer it has, rightly or wrongly, appeared, that the things, he has fallen in with regarding the critical period, and the critical unit, etc. do have bearings on the problem of menstruation. It is not proposed in this place, either to recite the various theories of menstruation, or to attempt to disprove them. Most of them can hardly be termed explanations at - 69 - all, for they are either far-fetched and largely hypothetical, or they leave in mystery as much as they profess to solve. The points, which mainly concern us, are the phenomena of menstruation, its average-interval, and its connection, or non-connection, with ovulation. Regarding its correspondence to the heat-period or rut of mammals, as supposed by T y 1 e r Smith and others, very little is required in disproof. The mere fact, that heat in mammals is the time of sexual intercourse par excellence, while menstruation in those few forms, in which it occurs, is just the exact opposite of this, sufficiently illustrates how untenable this position is1). Of the various theories of menstruation a critical account has recently been given by J. C. Webster2), and to this, as also to the memoirs of Walter He ape 3), the reader may be referred. The facts, as laid down by others, and as Heape's researches on the menstruation of Semnopithecus and Macacus have proved them to be, will be made the basis of the following discussion. Menstruation is a periodic process, occurring in the majority of cases with great regularity in cycles of 271/3 to 28 days, though subject to variations. Some individuals have a period of greater or less 1) If a heat-period ever existed in human ancestry, the length- ening of the menstrual flow has sufficed to extinguish it. 2) Webster, J. C. The Biological Basis of Menstruation. (Reprinted from the Montreal Medical Journal, April 1897, pp. 19.) 3) See appendix w. Heape concludes that "the primary cause of menstruation remains unexplained". — 7° - amount than the above, and, even in healthy women, the flow may recur irregularly, so that, as quoted by Hen sen from Loewenhardt, of 46 and 50 consecutive menstrual periods of two women, these occurred at intervals of 16, 21, 22, 23, 24, and 32 days (once), 25 days (nine times), 26 days (eight times), 27 days (twenty-one times), 28 days (seventeen times), 29 days (fifteen times), 30 days (eight times), 31 days (five times), 33 days (thrice), 34 days (four times). The average is 27 i[3 days, and it is worthy of notice, that more than half the cases closely approach the mean. In a menstrual cycle four stages are recognised1): 1) The constructive stage, resulting in the formation of a menstrual decidua, and lasting approximately 7 days. 2) The destructive stage, including the whole period of the catamenial flow, and usually embracing 5 days. 3) The stage of repair, during which the uterus is recovering from the destructive changes. To this period 3 to 4 days are assigned. 4) The period of quiescence, of 12—14 days duration. The events of the constructive phase are compared with those, occurring in the placental lobes of the rabbit's uterus from the fourth to the eighth day of gestation, and for a fuller account the reader may be referred to other sources 2). The phases of repair and quiescence require no elucidation, but it is obvious, that the events of the period of repair may be regarded as similar in kind to those, which succeed birth or an abortion. 1) Compare H e a p e , appendix w. 2) Compare appendices u and w. — 7i — There remain, then, the actual phenomena of the cata- menial period itself for explanation. Prior to the appearance of the menses the uterus has formed a decidua, which is regarded as equivalent to that, which would arise, when a fertilised egg became affixed to the uterus. If an egg were affixed in this position, and if then menstruation took place as usual, we should regard this as an abortion1). Indeed, the whole of the phenomena of the constructive, destructive, and repair-periods point, to the conclusion, that menstruation is an abortion of a decidua at a time, when the "egg" ought to have become fixed to the uterine wall, but where this was impossible, because of the failure of fertilisation. It is an abortion of something prepared for an egg given, off at or after the close of the preceding menstruation, and it takes place, because this egg has escaped fertilisation. If there be, as I hold there is, any truth in the doubling of the former simple critical unit in man, it is an abortion which in its period corresponds to this latter. It represents an abortion at the old critical period, and is one, in which the decidua belongs to a missing 22l/2 to 23x/2 days embryo, and, thus, in all its phenomena it resembles a parturition, as first noted by Burdach, and is, in fact, an abortive one. This view of the matter is one, which is in agreement with all the facts2), and, so far as my reading goes, it is the only explanation covering all the i) As a matter of fact medical experts have long been of opinion, that early abortions of the first month of pregnancy are very often unnoticed, because they are taken to be normal menstruations. 2) For Minot's views compare appendix u. — 72 — phenomena. If this conclusion be right, it follows, that men- struation and ovulation cannot, as a rule, coincide ; or, to put the matter more exactly, that ovulation cannot be connected in such a way with menstruation, as to happen just before or during this, any more than that birth and ovulation could exactly coincide in rabbit, mouse, or cavy. This is con- firmed in the strongest possible manner by He ape's researches1), as well as by the observations ofBischoff, Kolliker, Coste, and others on the bodies of women, who had died during menstruation. The explanation of the nature of menstruation just given has thus far been based solely on the interpretation of the phenomena before, at, and after the catamenial period, but in the sequel it will be manifest, that other facts can be drawn upon in support of this conclusion. Menstruation is, thus, a phenomenon, which, as others have noted, lies on the borderland of pathology. It probably dates from a time, when the reproductive rhythm was such, that the mature female was always pregnant, and, possibly, when pregnancy was far shorter, than it now is. It persists, probably, because its existence is entangled in the reflex mechanism of ovulation. Ovulation cannot be supposed to occur during the constructive stage, for that would assume the preparation of a decidua for the reception of an egg, before it was fertilised, and the analogy of other cases is entirely against this. Again, in other animals, like mouser cavy, rabbit, or horse, the period of quiescence of the uterus is not that of ovulation, but it ensues in these very soon after a birth l) Compare appendix w. — 73 — has taken place, and, of course, at other fixed intervals, when the animal has not just borne young. This leaves us with the periods of destruction and repair, embracing some 8 or 9 days, in which ovulation may happen. If menstruation be comparable to an ab- ortion, it cannot be supposed, as we have seen, that, as a rule, it and ovulation will coincide; for, again, the analogy of other forms favours the idea, that ovulation cannot hap- pen, until the (premature) birth or abortion is complete. If the abortion of the decidua, prepared for an un- fertilised egg, only occupied a very brief interval of two or three hours, in other words, an interval comparable in length to the birth-time of the rabbit or cavy, ovulation could, as in these animals, ensue within a few hours of the end of this. But at the present time menstruation is a process extending over several days, and an egg, released from the ovary during the earlier part of menstruation, would require to wait a long time for fertilisation. From these considerations it appears, that the most likely time of ovulation must be, as a rule, towards the very end of the menstrual flow. This result is also in agreement with, among others, the opinion of H e n s e n. He writes : — „die bisher vorliegen- den Thatsachen sprechen zu Gunsten der alteren Ansicht, dass namlich die Follikel in der Regel ' ) gegen Ende der Menstruation platzen"2). The experiences of Hyrtl, Bischoff, Kolliker, and C o s t e , as cited by Hensen, support this view. 1) Italics in the original. 2) Hensen, V., Zeugung. Handb. d. Physiol., 1881, p. 75. — 74 — Apparently opposed to it are the majority of the examples of embryos, whose histories have been recorded by His1), and in these it must be assumed, either that the menstrual flow was slight, and had been overlooked — a likely and an unlikely thing — or that an unusual ovul- ation had happened 2). This appears to be the more likely explanation. There seems to be no reason for doubting, that some- times an egg may be given off at unusual times in the middle of a menstrual cycle; and upon the views of the nature of mammalian development advanced in the present writing, it is far more reasonable, than the supposition, that ovulation might just precede menstruation. The ex- periences of He ape and others are also opposed to this latter. The fertilisation of such an egg would suppress the menstruation, and with it the ensuing ovulation would be rendered obortive, and its egg or eggs would degener- ate in the ovary. This, if correct, is a very important conclusion, for the fertilised egg would develop, as if it belonged to the succeeding suppressed ovulation, and, thus, the phases of its subsequent uterine life would not chime in dissonance with the rhythm, which must of necessity underlie mammalian reproduction. It will be taken, then, that, as a rule, ovulation slightly succeeds menstruation, and the question now arises "what is the ovulation-period" ? The answer to this is not so simple, as it would appear to be. At the first glance it looks, as though its interval were equal to that of menstruation. i) His, W., Anatomie menschlicher Embryonen, II, p. 74 etc. 2) See the latter part of appendix m (on "Length of pregnancy in man") for a fuller discussion. — 75 — The acceptance of 27—28 days as the ovulation-unit fails to establish anything about the pregnancy beyond its supposed correspondence to ten menstrual periods, which I hold to be erroneous. Quite apart from the circumstance, that on this as- sumption no correspondence between it and the critical unit can be set up, as I know full well at the cost of much wasted labour, it must be noticed, that the men- strual interval is often different in different women, and even in the same individual at different times. Hensen cites two interesting examples drawn up by Loewenhardt: "So betrug bei zwei Frauen die Zeitdauer von 46 und 50 sich folgenden Perioden 1 mal 16, 21, 22, 23, 24 und 32 Tage, 9 mal 25, 8 mal 26, 21- mal 27, 1 7 mal 28, 15 mal 29, 8 mal 30. 5 mal 31, 3 mal 33, 4 mal 34 Tage. Diese Schwankungen traten sehr un- regelmassig ein" 1). From what follows, I am convinced, that a far greater regularity would have appeared, if the intervals, from the end of 'one menstruation to the be- ginning of the next, had been noted and recorded. Such an interval on the average must be equal to about 23 V2 days- A few of the lower numbers seem to be against this, but it by no means follows, that, because there is a "show" externally, the menstruation has not really ceased inter- nally 2). Without allowance for something of this sort, a post-menstruation, it is nevertheless curious to note, that about half of the cases approach the mean, i. e. 27 l/3 days. If menstruation, as already indicated, correspond to 1) loc. cit, p. 64—65. Previously quoted in English on p. 70. 2) As long ago urged by the gynecologist, Dr. Tyler Smith. - 76 - an abortion, its effect, if prolonged, is to delay the ovul- ation ensuing upon it. It may be concluded, that originally menstruation was very much shorter than it now is, so short at first as to occupy a very brief interval of time, possibly only an hour or less. There seem to be no grounds for assuming, that its present average duration was fixed at its initiation, or that it was then longer than it now is. If it were very brief, the interval between any two periods would be equal to the present average time, between the end of a menstruation and the beginning of the next. Moreover, if we carry the menstrual cycle over into the period of gestation, calculating the latter as equivalent to ten of the former, we fail to remark, that during preg- nancy, where the menstrual flow is suppressed, the be- ginning and the end of a menstruation are the same thing. And — so it appears to me — there can be no reason for including a certain number of days in each cycle for a process, which varies in length in the non-pregnant in- dividual, and which has no actual dimensions at all in pregnancy. I conclude, therefore, that, as nearly as one can guage it at present, the human ovulation-unit has a length of about 23 1/3 days, or perhaps rather more, though hardly 24 days. It will be seen, that on this estimate a satis- factory measure of the length of pregnancy may be made, and, what is quite as important from my point of view, a perfect relationship, between it and the critical unit, can be demonstrated. — 77 — XL The Human Critical Unit1). From the researches of H i s , as recorded in the three parts of the "Anatomie menschlicher Embryonen", from the conditions presented by Keibel's human embryo H. s. Brn., as demonstrated in a former publication 2), and from the data concerning a human embryo now in my own possession, it is quite clear, that the critical period is reached in human development in 45 — 47 days. From the facts recorded by H i s and K e i b e 1 con- cerning embryos of about 17 mm (Nackenlange) this is as near an estimate of the critical unit, as can be made, but of the embryo in my possession, which I owe to the great kindness of Dr. Anderson of Stonehaven, a more exact determination can be arrived at. As this embryo and its history are of vital importance for the line of inquiry in hand, as the specimen is almost the exact counterpart of another, given me without its history by a former pupil, Dr. Martin Corry, and, finally, as the membranes of these two are typical of a number of such, which, minus embryos 3), have passed through my hands, it appears desirable to note this par- ticular case in detail. The Stonehaven embryo is of the critical period. The grounds, on which this conclusion is based will, be given elsewhere, here the fact and the history of the embryo alone concern us. The patient last ceased a menstruation 1) Compare also appendices n and o. 2) Certain Problems of Vertebrate Embryology, p. 27 — 32. 3) To be more exact, either without, or with crushed and distorted embryos, or only parts of such. - 78 - before becoming pregnant on Dec. ist 1896, and the abor- tion took place on Jan. 17 th 1897. Assuming the ovulation to have occurred at the end of the catamenial flow, and to have been followed by almost immediate fertilisation, the embryo has an age of at most 46 — 47 days. As already indicated, its position in the normal table agrees with this. But of special interest, in their bearings on the results of the preceding section, are the following considerations. If, following Harvey, Tyler Smith, and others, we est- imate the gestation as equivalent to ten menstrual periods, and if in this special instance we seek for the occurrence of a menstrual period at this abortion, as Tyler Smith was wont to do, we find, that the latter falls some 4 — 5 days short of the second catamenial period. But if, on the other hand, the mode of reckoning the menstrual period during pregnancy, advocated in the pre- sent writing, be adopted, a very different and rather sur- prising result is arrived at. Calculating from the end of the last menstrual period on Dec. ist to the beginning of the next (suppressed) men- struation with an interval of 23^3 days, the date of Dec. 24th is obtained, as that of the latter. As in preg- nancy it is impossible to assign any length of time for the duration of the flow, the next cycle must be taken as beginning on the same day. A further interval of 23 ^ days brings exactly the date of the abortion, Jan. 17th. It is impossible to over- estimate this result. Important for us, though, possibly, insignificant compared with its other bearings, is the proof of the equivalence of the human critical unit to rather less than two ovulation-units. — 79 — XII. Abortions. In a former publication the importance of the critical period as one, at which abortions might be expected to be of frequent occurrence, was commented upon. It was insisted, that, owing to the mode of development, and to the absolute necessity of completion of all the parts of the embryo by the time the critical period was reached, failure to comply with the unwritten, but, none the less, inexorable, rules of the development prior to the critical period, was bound to revenge itself and result in abortion. Whilst any intention of assigning these factors as the causes of all early abortions must be disclaimed, everything since met with in the research has tended to strengthen the conviction, that a large percentage of abortions in man, and mammals generally, can be traced to causes, connected with the critical period, the critical unit, or the ovulation- unit. The instance described in the preceding section is a very significant one, and it is undoubtedly typical of a great many others. Although abortions *) are of daily occurrence, it is very difficult to get at the facts concerning them. Whitehead of Manchester, as quoted by Tyler Smith, found, that of 2000 women, patients at the hospital, 37 °/0 had aborted before reaching the age of 30, and Tyler Smith himself estimated, that of the women, who had lived in wedlock, and passed through the child-bearing period, go°l0 had aborted once or oftener. I) In the present section no distinction will be made between abortions and miscarriages. — 8o — It must be stated, that there is absolutely no novelty, in the idea, that abortions might be connected with the periodicity of ovulation and menstruation. Klein and Burdach early in the 19th century noted, that abortions, where not due to external causes, appeared to be of most frequent occurrence, at what ought to have been menstrual periods. Tyjer Smith remarks: — "there is, in fact, in all women a tendency to abort at the times represented by catamenial periods" x). And further on he adds : — "I have observed in cases of abortion, occurring at what would have been a cata- menial period, that menstruation has appeared at the end of a lunar month from the abortion, as if it were dating itself from a simple menstruation"2). In many standard works , such , for instance , as Tanner's3), tables exist of the frequency of abortions at various periods of pregnancy, but these are drawn up in such a way, as to be absolutely useless for detailed examination. Whitehead's table, dealing with 602 cases, classifies the data by the number of months of pregnancy, but "each figure in the first column (number of cases of each particular month) embraces a period of four weeks, extending from a fortnight before to the same length of time after the month indicated". It will be apparent, that of a table of this sort no exact analysis is possible. But from this and other tables certain things can be deciphered. 1) Smith, W. Tyler, Parturition etc., London 1849, P- Ir5- 2) loc. cit, p. 119. — A very significant statement in the light of the conception of menstruation here advocated. 3) Tanner, Diseases of Pregnancy, 3rd Ed. 1867. — 8i — For various reasons ') it might be anticipated, that abortions would be most frequent at the critical period and at some multiple of it in the human subject 2), thus, i) at the 46th— 47 th day, when a menstrual period should recur; 2) at the end of the third month; 3) at about the end of the 20th week; 4) towards the end of the 27th week; 5) at 33 1/2 weeks; and, lastly, 6) towards the end of the 40th week, i. e. the end of pregnancy. Of these the first and second are unquestionably periods, when abortions are very apt to occur. My own material proves this for the first (the critical period), and the common testimony of gynecologists, as well as Whitehead 's table 3), speaks emphatically re- garding the second, whilst the sixth is the actual normal birth-period. Thus, even with the meagre data available, three of the six hypothetical periods can be established. But, probably, there are other intervals of importance. In mammals generally actual ovulation appears to be suspended during gestation. If it were to come into operation during that epoch, it would certainly lead to abortion. It is highly probable, that during pregnancy the ovaries, with their nervous mechanism, mark the time with the regularity of clockwork, and that no period, 1) Briefly these are: — a) owing to a certain degree of ab- normality in the embryo, or in the "membranes", b) failure in the proper formation of a placenta, c) reversion to an ancestral birth- epoch, d) and owing to ovulation. The two latter are closely connected, and they might come into play not only at the critical period, but at some multiple of the critical unit. 2) And for similar reasons in other higher Eutheria also. 3) In this out of 602 cases, extending from the second to the eighth month, no fewer than 275 are assigned to "three months". Beard, Span of Gestation. 5 — 82 — which ought to be one of ovulation, is allowed by them to elapse without notice, without some inner change. The ovary, with its nervous apparatus, from the moment of puberty to the time, when its functions naturally cease, may, indeed, be compared to a clock 1), wound up at the start, and striking the hours with extreme regularity by periodic emissions of one or more eggs. During pregnancy the clock does not cease to run; it still strikes the hours, but in muffled fashion, so that, if all goes well, the sound is not heard outside of the ovary. And, just as some clocks are so arranged, as to give an indication of the approach of the hour shortly before the arrival of the striking- period, so the ovary announces the normal advent of birth some little time before the ovulation, due soon after the birth - period. What form the muffled strikings take during gestation is a problem for exact investigation, but it is not unlikely, indeed, highly probable, that they are represented by abortive ovulations of eggs, which de- generate in the ovary. Only in this way can regularity in the functionings of the ovary be conceived as being maintained. In this connection the corpus luteum and its history during Mammalian gestation afford interesting material for reflection. This structure appears to be entirely absent below the mammalia, and its increase in size during gestation would appear to be a contrivance for diminishing the i) von Baer remarks somewhere, that natural processes have any number of times been likened to the movements of a clock, but it is the only convenient machine, appearing to "go" of itself with rhythmic movements, to which one can refer. - 83 - nutrition of the ovary and ova, and, thus, of preventing a normal ovulation 1). After the initiation of gestation the striking of the clock first becomes audible again soon, often immediately, after birth2). This is the case in mouse, rabbit, cavy, horse, cow, and other ruminants, and probably in some form, i. e. as a wasted or an abortive ovulation, in man, cat, dog, pig, and sheep, in which a new gestation does not shortly ensue on the birth. The muffled strikings during gestation, the normal periods of ovulation, may then, as Tyler Smith and others have insisted, be frequent times of abortion ; if, for no other reason, then because the development is going on wrongly, and this is the best way of putting an end to it, and of obtaining a new start. In this connection cases of death of all the embryos in utero at early stages of the gestation in rabbit and cavy must be mentioned. During the last nine years many such have been encountered in my work. When in such in- stances all the young have died at an early stage, owing to the influence of lactation, or to some other unknown cause, they must either be absorbed or aborted prior to the next ovulation due. Usually, if not too large, they are disinte- grated and absorbed in situ, but, if this does not happen, there can be no doubt, that an abortion must shortly pre- cede the next ovulation. There would appear to be no advantage in drawing up a list, of what ought to be abortion-periods, corresponding to suppressed ovulations included in human pregnancy, as i) Compare appendix q. 2) Compare appendix v. - 84 - there are no data available at the present time, upon which to decide the matter. But it may once again be insisted, that during pregnancy they must be counted, not as intervals of 27 — 28 days, but of 23 — 23' /3 days, or thereabouts. What holds good for man will, mutatis mutandis, be found true of other placental mammals. I am firmly of the conviction, that to many with prolonged gestation- periods, such as horse, cow, sheep, etc., the considerations, outlined above, with due regard to the ovulation- and crit- ical units, will be found to apply. Thus, as an instance, may be mentioned certain state- ments regarding sheep, which were made to me by Prof. Wallace of Edinburgh University. It is said, that shepherds find, that sheep, which have been served, often refuse the ram at the next ovulation- (more exactly rutting) period, and then receive him at the next following one. Shepherds believe these sheep to have aborted, and de- scribe them as having "held", and then failed to "hold". These facts agree well with abortion at the critical period followed by a new fertilisation at the ovulation almost immediately succeeding. XIII. The Cause of Birth. Although this question figures largely in the title of the present writing, although the whole of the latter has worked up to it as to a climax, and although it has hitherto been wrapped up in a mystery, which resisted all attempts at unveiling, its resolution does not appear to be anything like so important, or so interesting, as the - 85 - solving of the riddle of the duration of uterine life. Its explanation comes, it is true, as a culmination, but it is so simple, that little acumen is now necessary for its perception. Wheresoever one seeks information regarding this physiological problem, little else is found, than an ad- mission, that absolutely nothing is known concerning it. In a standard work on physiology we read : — "we are utterly in the dark as to why the uterus, after remaining apparently perfectly quiescent (or with contractions so slight as to be with difficulty appreciated) for months, is suddenly thrown into action, and within, it may be, a few hours gets rid of the burden it has borne with such toler- ance for so long a time; none of the various hypotheses, which have been put forward, can be considered as satis- factory. And until we know what starts the active phase, we shall remain in ignorance of the exact manner in which the activity is brought about" 1). Werth, in Hensen's JPhysiologie der Zeugung", remarks: — "der Eintritt der als Wehen bezeichneten Uteruscontractionen gehort zu denjenigen Processen, welche in Bezug auf die veranlassenden Momente den Erklarungs- versuchen einen hartnackigen Widerstand entgegengesetzt haben. Im Ganzen treten die Wehen ziemlich pracise 40 Wochen nach der letzten Menstruation auf etc."2). According to Tanner, "we are really unacquainted with the exciting cause of parturition" and he proceeds to comment on "the devout remark of the Arabian physi- 1) Foster, M., Text-book of Physiology, 2n<* Edition 1878, p. 554. In later editions the existence of the problem is still admitted. 2) loc. cit, p. 279. cian, Avicenna — that at the appointed season labour comes on by the command of God" * ). Spiegelberg is no more explicit. He writes : " — die Schwangerschaft ist eine periodische Function, und es ist wie bei anderen periodisch ablaufenden Vorgangen ganz miissig, nach der immanenten Ursache der Periodicitat, ihrer Dauer, hier also nach der Ursache des Eintritts der Geburt zu fragen; die Art der Periodicitat ist eben durch die Organisation bestimmt, nur die Bedingungen, unter denen sie in die Erscheinung tritt, sind uns zu- gangig" 2). Explanations have been offered, but, as yet, they have not carried conviction. Thus, Harvey, as cited by Tanner, after making some observations on the way, in which "prudent matrons calculate", proceeds to say, that the}'-, "after ten lunar months have elapsed, fall in labour, and reap the fruit of their womb the very day, on which the catamenia would have appeared, had impregnation not taken place" 3). It is interesting to note, how here a kernel of probable truth is wrapped in a thick husk of error. The period of pregnancy does not correspond to ten menstrual cycles: for, as we have seen, these are variable in dif- ferent individuals, and in the same person at different times. Moreover, H e n s e n , with other authorities, estim- ates 281.86 days as "wohl zu hoch" for the duration of pregnancy. And, although at the time of birth a men- 1) loc. cit, p. 178. 2) Spiegelberg, O., Lehrbuch d. Geburtshulfe, 1878, p. 125. 3) The works of William Harvey, M. D. Translated for the Sydenham Society by Dr. Willis, London 1847, p. 529. - 87 - strual period is undoubtedly due1), it is not the tenth period, but the twelfth from the initiation of pregnancy. Finally, since menstruation must be regarded as in a certain sense an abortion, the birth itself is the menstrual period due 2). Another author, Dr. Tyler Smith3), like P o u c h e t , regarded heat or rut and menstruation as the same thing, and he looked upon the birth as a heat-period, in which, instead of the passage of an unfertilised ovum, that of a fully developed one took place. Although he noted, that in many animals an ovulation ensued soon after birth4), one searches in vain for a statement, that the approach of this is the cause of birth. It would be of no service to continue these citations, as they would but needlessly extend this paper. Harvey would have been quite right, apart from the wrong number of menstrual cycles and the failure to notice the correspondence of pregnancy to a certain number of ovulations, had he compared menstruation and parturition more closely together, showing, that in a sense they were identical. Tyler Smith went to the other extreme, in making menstruation, abortion, and parturition the equivalents of i) Harvey, Naegele, Rigby, and Berthold have all in- sisted on the supposed correspondence of the duration of pregnancy to some multiple of a menstrual period. 2) Compare appendix u. 3) Considerable pains have been taken to grasp the full significance of Tyler Smith's views, more particularly, because of some superficial resemblances certain statements of his, when studied apart from their context, bear to conclusions of mine. In appendix 1 quotations at length are given from his two chief works, and it remains only to say, that in their essence his arguments and conclusions, because based on what appear to be false premisses, are diametrically the opposites of those contained'in this paper. 4) Compare p. 112. rutting periods. In doing this he explained neither the one nor the other. It is no explanation to say, that men- struation, abortion, and parturition are all comparable to heat-periods, or that in this sense "parturition is essentially a menstrual period". Tyler Smith thought, that he had interpreted menstruation, in concluding its equivalence to heat in animals, and, thus, felt entitled to reduce parturition to the same thing. In another direction Tyler Smith was obviously on the right track, for he was apparently the first to assign the exciting source of parturition to the ovary, whilst failing to perceive, that a coming ovulation must be the real cause. In concluding this discussion it is possibly a work of supererogation to treat of the cause of birth in detail. From the results of preceding sections the reason of the climax of pregnancy, parturition, must be apparent. It is part of the same necessity, whose conditions we have studied, which dominates the developmental history of Metatheria and Eutheria from the moment of fertilis- ation of the Qgg, through the time of the building-up of the future animal, to the end of its uterine life, and it is the inevitable and rhythmical sequel of all, that has gone before. The length of gestation in any given species of placental mammal is fixed by a certain charac- teristic number of critical units, and this determines the degree of development, probably, in general terms, that most favourable for the species, at which the young shall be born. A critical unit corresponds to either one or two ovulation - units in different species. And, as in the Marsupial ancestry the ovulation -period was fixed by birth at the critical phase, so now necessity forces any form, in which the gestation has been lengthened to embrace two or more critical units, even when these latter have been doubled, still to carry out the extension in such a way, that it shall keep strictly rhythmic step with the periodicity of the ovulation. Thus, the gestation is prolonged so as to fall just short of a given number of ovulation-units. During pregnancy the ovary is still marking time, and in muffled tones striking the hours. When the na- tural term of the gestation approaches, the warning, that the time-limit is nearly run out, comes from the ovary, and, in obedience to this intimation, which is of the nature of a command, birth takes place "at the season appointed", in order that a new ovulation, which may, or may not, be expended in vain beyond inducing the birth, shall be car- ried into effect, and Nature's law observed. XIV. The Rhythm of Reproduction in Mammalia. A Summary. The problems dealt with in the preceding pages have of necessity been treated of singly; and, now that the main facts have been cleared up, there may be, at any rate for the reader, a certain convenience in bringing the whole together in a connected and brief form. In this way it will be easier to comprehend the order and rhythm of repro- duction in the Mammalia. The following lines may also serve the purpose of a — go — summary, of what has gone before in the preceding- pages. Of certain theoretical items, i. e. the origin of an egg- case and large yolked-egg, no outline will be given, but the reader may be referred to the section, in which they are treated of1). The inquiry really begins with an examination of the conditions, under which reproduction can be, and is, effected, when uterine development and nutrition of offspring by means of mammary glands are initiated. In the memoir, "on certain Problems of Vertebrate Embryology", published last year, among other things it was demonstrated, that the evolution of mammary glands and the initiation of lactation had been the primary factors in reducing the yolk in the egg of ancestral mammals. This reduction, due to the mammae, could, however, be carried no further back than the critical period, i. e. than the epoch, at which the embryo, being first complete in all its parts, could, and must, provide for its own nutrition. The mammary organs could not, and cannot, begin to function prior to that time. If, as has happened, a yolk- sac placenta, or trophoblast, be developed, it, as a structure forming a part of the phorozoon, can only function until the critical period, when it must begin its degeneration, for its normal life-span is reached. The combined workings of these two factors, mammary and trophoblastic nutritions, from opposite points, the one beginning at the soonest at the critical period, the other ending at the latest at this important epoch, have brought it about, that, where no allantoic placenta is developed, there uterine life is only possible until the critical period, when i) p.*i3— 18. — gi — the young must, perforce, be born. This is probably the case in all aplacental marsupials, and it certainly holds in Hypsiprymnus, Didelphys, Trichosurus, and Macropus, as either demonstrable, or already established. Once the birth was fixed to take place at the critical period in ancestral Metatheria and Eutheria, an alteration of this arrangement could only be brought to pass by the evolution of an allantoic placenta. If this happened, the birth could be postponed to a later time, for the nutrition of the foetus beyond the critical period1) was then provided for. Before an allantoic placenta had been, so to speak, in- vented, the coincidence of birth- and critical periods had led to deeply-reaching effects on the ovulation. Prior to all these changes, evolution of mammae, trophoblast, etc., ovulation may have occurred at regular periodic intervals, but of this there is no evidence available as yet in the developmental history of mammals2). However this may be, and it is a matter of no apparent import, the initiation of birth at the critical period put restrictions upon ovulation, if none previously existed. Ovulation was rendered impossible during gestation, and under the most favourable circumstances it could only recur shortly after every birth. Under the conditions imposed by uterine gestation the number of offspring is at the best a restricted one, no case being known, in which i) The critical period is one of starvation rather than, as suggested by Houssay, of asphyxiation (compare Houssay, F., Le rappel ontogenetique d'une metamorphose chez les Vertebres, Anat. Anz., Bd. XIII p. 33—39). 2) The conditions in Monotremes might be interesting in this connection. — 92 — the uterus can normally harbour more than 27 developing embryos, the average number being much smaller, even in the most prolific forms. It, therefore, became neces- sary, to repeat the ovulation as often as possible under the limitations of frequency and number of eggs imposed. Thus, it arose, that the ovulation-period became almost equal to, but actually a little longer than, the interval of time necessary for development to the critical period, or, in other words, to the birth. This latter interval may be termed "the critical unit" in any given case, and in the same way the period of time between any two ovulations may be described as "the ovulation-unit". It is probable, that the critical unit first fixed the ovulation-unit; or, at any rate, that the inter- action of the two led to a correspondence, such that the latter always very slightly exceeded the former. Owing to this it resulted, that a coming ovulation, i. e. a reflex mes- sage from the ovary, was the direct cause of birth. Tyler Smith in 1 849 almost foresaw this ; but, though he assigned the cause of birth to the ovary, he failed to recognise, that a coming ovulation was at its basis. When an allantoic placenta was initiated, with conse- quent prolongation of the gestation, the correspondence and relationships of the critical- and ovulation-units determined the conditions, under which this prolongation might take place. It could only be extended in the first instance by being made to embrace two critical units instead of one so as to maintain the correspondence of the critical- and ovulation-unit, i. e. so that birth might still be induced and succeeded by a new ovulation. The carrying-out of this extension of gestation to two — 93 — critical units afforded the passage from the Metatherian to the Eutherian stand, and it led to the loss of the marsupium, as a structure no longer required. This type of gestation of two units is still in existence in many of the lower Eutheria, as mouse, rabbit, etc., and, probably, in mole and hedgehog. A gestation of two units, although it gave an advance on the condition at birth, characteristic of forms, whose gestations were made up of one unit only, did not yield the advantages to be gained by a still further prolongation of the gestation to include three or more critical units. These advantages were seized upon in course of time by various mammals, and utilised to varying extents, their gestations gradually coming to include a greater number of critical units. But, wherever further prolongation was acquired, it was always still effected under the rules originally laid down as to a correspondence between the length of gest- ation and a certain number of critical- and of ovulation- units. By a curious coincidence, for it is nothing more, in the appended table (p. 98) forms with from one to eight critical units in the gestation are included. An examination of the list will make it clear, that, as a rule, forms with a greater number of such units in the gestation are born in a condition more advanced than those with a less. Mere length of gestation does not effect this, for a mouse attains in 20 days a degree of development as advanced as a rabbit in 30, or as a dog in 62 ; or, again, an opossum reaches in 8 days, the stage of development arrived at by a rabbit in 15 days, a cavy in 22, a sheep in 30, a horse in 42, or a human embryo in 46 days. — 94 — The reason of a greater degree of development at birth with an increased number of critical units lies deep, and is to be sought in the circumstance, that forms with gest- ation-periods of several critical units must, in their an- cestral history, have passed through conditions, in which the number was smaller. They must have started with a gestation of one critical unit, and have gradually increased this number. While many of the lower Eutheria still maintain very conservative traditions, among the higher ones in certain orders, thus, in Ungulata and in man, a rather different state of affairs obtains, probably as a secondary condition. Even in the lower forms, such as rabbit and cavy, it is rendered likely, from a comparison with the short crit- ical unit of the opossum or Hypsiprymnus , that the crit- ical unit, and with it the ovulation-unit, has been doubled, or trebled. However this may be, something of the same sort has taken place without affecting the ovulation-unit in Ungu- lata and man. In horse, sheep, pig, and man the critical unit is no longer the equivalent of one ovulation-unit, but almost of two such. This is interpreted as meaning, that the gestation - length has been doubled without altering the grade of development at birth, and it is put into association with a probable increase of size in the course of the ancestral history, without ex- cluding the possibility, that other factors may have come into play. In all mammals ovulation during gestation is either abortive or suppressed: and this is necessary, for normal ovulation during gestation would lead to abortion. — 95 — The corpus luteum is probably a contrivance for the suppression or rendering abortive of ovulation during gestation. The commencing degeneration of this structure some little time before the end of the gestation (like its rapid atrophy, where fertilisation has not taken place), allows of preparation being made for a new ovulation. As already indicated, the approach of this ovulation is, in a reflex manner, the direct cause of birth 1). In preceding chapters consideration is given to the causes of many abortions in various mammals and to the natures of menstruation. It has been insisted, as was already done in the memoir "on certain Problems of Vertebrate Embryology", that the critical period (with the consequences following in its train), multiples of this, and the ovulation - periods, normally - suppressed during gestation, must be very fre- quently times of abortion in various mammals, particularly in those, like horse and man, where the gestation is a prolonged one of several units. Menstruation is comparable to an abortion prior to a new ovulation, and it is an abortion of a decidua pre- pared for an egg, which was given off" subsequent to the preceding menstrual period, and which has escaped fertilisation. It is an abortion corresponding in its periodicity with a former critical unit, which obtained before the present one arose by a duplication. Thus, it is comparable to an abortive birth at a former critical period. i) See appendix q. - 96 - So far it 'has appeared, that, under the conditions governing it, gestation in Metatherian and Eutherian mammals is, like their ovulation, dominated by a rhythm, and a similar and connected one really underlies lactation. This latter originally, and even now in many cases, had no effect on gestation, and a female, suckling young, could, and in some species can, become pregnant again immedi- ately after the birth of a litter. This, as is well known, holds true of the mouse, rabbit, and cavy. But in others the effect of lactation has been either to destroy the fruit of the new succeeding gestation, or to render the ovul- ation ensuing on the birth useless, beyond inducing the latter. When a new gestation now no longer follows on the birth of a litter, one can in many cases make out, that lactation still keeps up the rhythm of the gestation in corresponding to one or more gestation-lengths. It has to do this in the mouse and rabbit; and, regarding the former, Sobotta has put it on record, that lactation finishes only a day or two before a new litter is born 1). But in the pig lactation suppresses a new gestation 2), lasting as long as the latter would have done, i. e. about four months, while in cat and dog the lactation-period is equal to about two gestation-lengths. Thus, ultimately, lactation, gestation, the ovulation- and critical units are all connected together in a remarkable way, and their relations are such, that they all typically conform with the rhythm of reproduction in the Mammalia. i) In the work cited in appendix v. 2) And occasionally in mouse and rabbit. — 97 - This latter is of such a nature, that without it there would be no regulated course for the gestation, no peri- odicity in the ovulation could be maintained, and the birth of offspring would be a matter of chance and accident, instead of one of law and necessity. y In mammals the uterine development from beginning to end, gestation, birth, ovulation, and, in many cases at least, lactation, all obey one rhythm, whose basis is in the ovary. By ovulation this rhythm is proclaimed throughout the reproductive life of the female, in gestation the same rhythm is maintained, but in a modified fashion; and, as the span of uterine life draws to a close, it again asserts itself, and induces birth. Harmony and law reign in the reproductive life of Mammalia, and, where previously chance and chaos ap- peared to rule in this, there we have learnt to recognise "a certain seeming order", the dominion of necessity and the action of regulated change. What the visible universe would be without the law of gravitation, holding the planets in their orbits and dictating to each its "vorgeschrieb'ne Reise", that on a far smaller scale the microcosmos in the uterus of a pregnant mammal would present without the domination and sway of an iron necessity. The reign of law prevails in the infinitely little no less, than in the immeasurably vast, and, that this should be, is probably not less momentous and vital for human existence, than that the law, which moulds a tear, should guide a planet in its course. Beard, Span of Gestation- n 98 - -fc>. U> kO w O Q o o o o 3 3 *d -d P pa R i-t o> O) pa Sfl -a ►d ^ •o O) cd 3 3 & CL 3 * T3 w a> 3, 3 R" K' k' cr -r- M. — ° P a4 p >d CD 5 1 Co Co •§■ § § a ^8" to CM o m ^ ° ^ (TO* ^ CD •«■*■ S> O Co Co o 5 & S Co a p ;-« a, o § a. 01 Cc 8 Co s a ^ Co — *. £• O § I Co «S. ^- a o W> e — i Co < a, o o 0 w pa O 3 rr crq crq ■ — s »j" •Si. a H Co s ?" <*5 5*« ^-* Co £ a w CT Cn Cn w r-H HH (M .Co S -v & s ^ fl o 3 3 CD e^ a. si* I Co 8 a Co § .Co ^ 1-1 a\-j* Qrq o -J O t/i -— v_^ v_- 4^ -1^ KJ N> N> K) C/i MO o o o 1 o t 1 o 1 H-t 1 1^ 1 1 c«|to 1 3 3 3 1 4^ N) 1 o> 1 Ck) 1 00 1 Ck) I 1 O *o nd t3 p pa as >-t i-< J-t ON o o N) N) at m(st en o> a> ^5- -vj pa P M "d 'd T3 'd 'd >o 2 2 o B 0 g Dj CL £b x' R- x" o oo - OJ K> N> to to g op n c^in o CLpj P !-' 3 3.5; o'H.o S ST1-* CD o" s. cn CD N3 Co 00 0 00 O 00 Cn Cn O ON ON CO M Cn 0 ^Q OO tOM C«|-* Cn j 1 I Ck> ^J ON I I I 00 o^u< ON OJ 1-1 4^ m vO i-t p P ■zf.crq O CD crq CD 99 — Appendices. a) Tessier, Recherches sur la duree de la gestation et de l'incubation dans les femelles de plusieurs quadrupedes et oiseaux domestiques. Memoires de l'Acad. des Sciences, Paris, Annee 1817, p. 1-40. The memoir contains observations on the apparent gestation- lengths in 575 cows, 442 mares, 912 sheep, 25 sows, and 161 rabbits. Of the cows1) 544 (94,60/0) had an average gestation of 282u/17 days Of the mares served once 272 (81,6 o/0) „ „ „ „ „ 346% „ Of the mares served oftener 128 (75%) „ „ „ „ „ 341 V4 » Of the sheep 912 (100 0/0) „ „ „ „ „ 151 1/8 „ Of the sows 25 (100 0/0) „ „ „ „ „ ii5V2 „ Of the rabbits 161 (100 %) „ „ „ „ „ 31 */„ „ b) Remarks on the table and on Tessier 's results. As stated in the text, no attempt can be made to do more than illustrate general averages. In many, nay, almost all, of the animals included in the list exceptions to the rule will readily be found. Each such exception would have to be considered on its own merits, and to do this it would first of all have to be demonstrated, that it did not really conform with the rule. Obviously, to prove this certain data are requisite. Since in all such cases the time of birth is known, the actual duration of the gestation can only be stated, if the approximate time of fertilisation be obtainable. Now, although, in dealing with aver- ages, it can be determined with sufficient nicety from that of copul- ation, this is by no means to be taken for granted, when an irregular and anomalous instance is under discussion. In such cases various factors may come into play. Thus, the ovul- ation may have been delayed, resulting in a postponement of the moment of fertilisation; the individual may have an abnormal, as compared with the usual, ovulation-unit, and with this an unusual critical unit. Mares are known, in which heat and probably ovulation return every 14 days, and analogous cases have been met with in the human female. Tyler Smith remarks, that an unusual type 1) Earl Spencer found in 764 cows an average gestation of 285 days. 7* — ioo — of menstrual cycle appears in his experience to be associated with a departure from the normal duration of pregnancy. This is quite what one might expect from general morpho- logical considerations and from the intimate relationship, which, as we have seen, must always subsist between the critical- and ovul- ation-units J). What applies to the species must with even greater force hold good in the individual. Unless in any given individual the two units be in agreement in the mode described in the text, it is difficult to see how the rhythm of the development can be maintained. Moreover, although this point was not mooted in the general memoir, it has not been overlooked, that, even at the present day, there may be a tendency in certain species to a still further lengthening of the gestation. This may, or may not, influence the critical- and ovulation-units, it may be met with in few, or in nearly all, of the individuals of a species. In the same way a shorter gestation-period in some individuals of a species may be due either to conservatism, or to a reversion to a nearer or more remote former such period. One cannot be expected to solve an equation, if two or more of its factors be unknown quantities, unless it be so stated that these can be guaged. In all such unusual instances of longer or shorter gestations all the factors, except the moment of birth, are unknown. Given these, there would in all probability in most of them be no problem left to be solved, for all the difficulties would then disappear. From Tessier's work it appears clear, that in more than one of the domesticated animals (cow, and horse) a prolongation of the gestation, not accounted for by a delayed ovulation, does take place. Without all the details concerning these it is out of question to explain them. Some of them may be such, that the underlying anomaly influenced all the factors, ovulation- and critical- unit etc. in the same way, while others may be determined by some other reasons. It may be — and this agrees with what must have happened in the past, when at one time gestations were shorter, and only gradually got longer by the annexation of a new critical unit — it may be, that some, at any rate, are explicable as the result of the adoption of a further such unit. The gestation-periods of children born with teeth might be interesting in this respect. But the matter has in the case of man important medico-legal aspects, and these are dealt with elsewhere2). i) The ovulation-periods of pig and sheep have been quoted simply as given by many authorities, but it may be stated that if properly investigated a much more exact and stable number would probably be found in each. 2) See appendices n and o. — IOI — Reverting to Tessier's observations. In the cow he found, that out of 577 cases no fewer than 544 had a gestation of 270 — 299 days. Ovulation-units of 20 — 21 days and critical units of 39 -42 days would give, with the multiple of 7, 273 — 294 days for the gestation, or, in other words, a variation of one day in the ovulation- and of 3 days in the critical unit account for rather more than 90% of his examples. An ovulation-unit of 22 days, with the critical one in correspondence, covers all Tessier's highest cases but one, and his single very exceptional instance of a gestation of 321 days may be disposed of by the assumption of an ovulation unit of 23 days. In the above it has, thus, not been necessary to introduce a further critical unit. Similarly, in the horse almost all of Tessier's cases would be embraced in ovulation-units of 20— 23 days with corresponding critical units, and by far the great majority by an ovulation-unit of 21 — 22 days. In the sheep Tessier's records do not show so great a divergence as in the cow and horse, and the slight variation in the table suffices. (Compare also appendix z.) Finally, it may once more be insisted, that these statistics give no clue at all as to the moment of fertilisation. In the horse and cow the interval between copulation and fertilisation may frequently be one of several days, and these would fall to be deducted from the gestation. In this connection it is significant to note, that the average of mares served several times, given by Tessier, is, as might have been expected, smaller, than in those served once only; and, whereas the latter exceeds 346 days, the former is little over 341. Even this is probably too high. c) Antithetic versus homologous alternation of generations. More than one morphologist of standing has expressed to me his inability to understand, in what sense the term "antithetic alternation" is used. As elsewhere remarked, botanists, never having been pestered with the recapitulation night- mare, have made far greater advances with their (simpler) embryo- logical problems, than have zoologists. The distinction between the two kinds of alternation is a botanical one, but, none the less, it is demonstrably applicable to animals. It was first made in 1874 by Celakovsky (Sitz. d. Gesell. d. Wiss. Prag 1874, p. 30), and the Annals of Botany (Vol. 4, 1889 — 91) contains an able exposition of it from the botanical side by F. O. B o w e r. Homologous alternation is what has long been known in zoology, it is, in fact, what Chamisso originally described in Salpa. — 102 — In it the two, or more, generations from egg to egg, though only one of them is sexual, are homologous organisms. The sexless chain- producing Salpa is just as much a Tunicate and a Salpa, as is the sexual form. Where, however, two distinct generations, sexually and asexu- ally-produced respectively, occur in a life-history, and, where the two are neither homologous in their parts nor in their entirety, there we deal with a case of antithetic alternation. To treat of the subject adequately a book would need to be written. But, as an instance, comparing together an Annelid and its Trochophore, it is noted, that the two organisms are not homologous, for, though the one is a segmented worm, the other is not. If only so much could be said, the thing might seem to many embryologists to be a mere quibble; for, although they look upon the Trochophore as an ancestral form, it is to them at the same time a young Annelid. At one and the same time it is an ancestor and also a descendant of an ancestor — a contradiction in terms only to be explained away by Okenian metaphysical reasoning !iBut, looking at the organisation of the two, it is clear, that no homology subsists between their parts ; for the nervous, excretory, locomotor, and sensory organs of the Trochophore are not those of an Annelid, neither do they become converted into those of the sexual form. If they be new formations, not in their origin parts of the Annelid, they must have arisen as temporary structures out of nothing ! d) The critical phase or period. In my work of last year, "Certain Problems of Vertebrate Embryology", the terms, critical stage, phase, and period, were used somewhat indiscriminately, in the present writing only the two latter are employed. The grounds for this course of action cannot be detailed here, since^they have arisen out of a very extensive and minute series of observations on the development at this epoch in Scyllium canicula. A full account of the facts, obtained from the study of a great number of embryos, will probably be published, as soon as the material has been exhausted. On the present occasion it only remains to state, that this epoch of the development, termed the critical one, is in reality a phase, lasting some little time, and not a stage. In Scyllium cani- cula it begins with embryos of 32 mm (measured after preserv- ation in alcohol), and it is ended by the time the embryo is 34 mm in length. Very similar conditions have been noted in the sheep and pig, and in these also the critical phase persists, whilst the . embryo increases somewhere about 2 mm in length. It may also be mentioned here, that, as nearly as one can — 103 -— judge, the critical phase has in mammals a longer duration than any other phase of the development, for it is far easier, by trusting to chance, to obtain embryos of this period than of any other. Thus, if a dozen pregnant uteri of pig or sheep be obtained from the slaughter-house, two or three of them are almost sure to contain embryos of the critical period. It may be well to note briefly the external characters by which one can detect an embryo of this period. They are as follows : — the embryo is first beginning to look like the form, whose offspring it is. The lachrymal groove has disappeared, the eyeballs are no longer rounded but somewhat oval, the fore-limbs are flexed, the digits of the hand are just indicated, the mammary line has just completely disappeared (if one be at any time present), and the "milk-points" are quite distinct and separate. Hair-follicles are usually visible on cheeks, snouts, or over eyes. Internally, there are many characters, but the best and surest guide is the condition of the spinal cord. At the beginning of the phase the formation of the posterior fissure or septum of the cord is just initiated, at the end of the phase it has progressed a very slight distance. e)Eutherian versus Meta therian gestation-arrange- ments. The advantages of a gestation even of but two critical units over that of a single unit are curiously exemplified by the facts resulting from the introduction of the rabbit into Australia, into the midst of a fauna, almost all of which belongs to the one-unit-series. These rodents have done far more than hold their own against marsupials in the struggle for existence, not only against food-com- petitors, but also against enemies. Mr. Begg, a resident for 31 years in Victoria, recently informed my friend and colleague, Dr. GreggWilson, that in his experience the rabbit had mainly been answerable for clearing out "kangaroos" in this colony, and others have assured me, that, where rabbits flourish in Australia , there herbivorous marsupials are now wanting. It is easy to perceive the great advantages these lowly Eutheria would have in this struggle. A rabbit will doubtless breed all the year round in such a warm climate as that of Victoria, will pro- duce many young at a birth, and suckle for less than a month. Marsupials breed seldomer — they cannot help themselves in the matter — many bring forth only one or two young1), and, accord- ing to Owen, Macro pus major suckles for 10 months! By 1) In Dr. Broom's rich collection of marsupial embryos of some ten or more genera there were not more than a couple of cases, where the minimum did not obtain. *- io4 — > the time this animal, with its one offspring, is ready to begin a second gestation, the number of young rabbits, offspring of one mother, and able to feed themselves, might well be a hundred. The rabbit, with a gestation of two units and a short lactation- period, has an immense advantage1) over a kangaroo (M. major) with a single rather long critical unit and a lactation- time of 10 months. If a Macropus major bred continuously, it must at the expiration of every new critical unit turn out the pouch-young, and this or these would be in a very ill-developed condition. Against this the rabbit, with a short gestation, can place in the scale its two critical units and continuous breeding, such that at the end of a new gestation-period the young of a preceding one are able to take care of themselves. This fatal conservatism of the majority of marsupials, in clinging to a gestation of one critical unit, would appear to have had in past times, and to have at present, more influence in leading to their extermination in various parts of the world than any other causes, not excluding that usually assigned, the evolution of the Carnivora. But it must not be forgotten, that the tendency of recent research, that of Semon, Klaatsch, O. Schultze, Fleischmann and mine2), goes to prove a mar- supial ancestry for the Eutheria. Therefore, in past times, when some forms attained the Eutherian stand, those still Metatherian had, prob- ably, a "Hobson's choice" between following suit and extinction. f) Ovulation in the cavy. It may be admitted, that the non-establishment of the ovulation-period of this animal weakens any arguments based upon this particular case. For this reason the mouse has been used as an illustration, where my preference would have been the cavy, had the facts all been made clear regard- ing it. For a long time, though not without misgivings, the period mentioned by H e n s e n was accepted as correct. His opinion in its wording does not bear a ring of certainty about it, it runs : — "Meerschweinchen mit 66 Tage dauernder Trachtigkeit scheinen eine Brunstperiode alle I772 Tage zu haben". If this be taken to mean, that rut, and with it ovulation, recurs in the non-pregnant cavy every 17V2 days, it requires little consideration to see, that this is impossible. During my recent experiments with cavies it only became certain, that there must be some different period of rut, when it was too late to establish this during the present summer. 1) Rabbits are said to crop the herbage so closely, that no herbivorous marsupial can find food, where they are numerous. And an allantoic placenta is a far more efficient nutritive organ than a mammary apparatus. 2) One might add also the placental work of Duval, Hubrecht, Selenka, and others. - 105 - But it is intended, that the actual ovulation-unit should be found out, as soon as the necessary experiments can be made. Reasons may be given for doubting the correctness of Hens en's statement. I believe the probabilities to be, that usually the critical unit of the cavy is rather less than 22 days and the gestation-period, as given by Hens en, 66 days, and these numbers will be used in the following discussion. If 171/, days, or any number very near this, be the ovulation- unit of the cavy, there can be no correspondence, such as there is in the mouse, between it and the critical unit, or the total gestation. By no amount of ingenuit}1' can I772 be divided into 66, or 67, or even 68 without remainder. Now, it is quite certain, that Guinea- pigs ovulate within a few hours of the birth of a litter (compare Bis ch off, Entwickelungsgesch. des Meerschweinchens, p. 12), and, unless the ovulation-unit correspond to the gestation-length, it must, in conformity with what we know of the mouse and other cases, when multiplied by some simple number, almost equal the gestation- length. If the latter be divided by 4, we get a number i61/2, which is too small, for it is considerably smaller than the critical unit. In the same way, half the gestation-length is too large, and the number, giving a relationship between critical- and ovulation-units, is 3, which yields 22. Information has been sought on this subject from all sorts of sources, scientific and "practical". Two experienced breeders of prize-cavies assured me, that these animals could become pregnant at any time. In need hardly be said, that I declined to believe them. The only figures found are given by Bischoff. They are as follows: — Birth of Litter Next copulation Interval May 27 July 6 41 days June 17 Aug. 7 50 „ June 17 Aug. 6 48 „ June 25 Aug. 15 50 „ The first of these numbers points to an interval of 22 days, or thereabouts, assuming one period to have been missed either by the cavy or by the observer, while the others indicate a much larger unit and one too large for the facts. It must, however, be pointed out, that, as the cavy, and the same, unfortunately, is true of the other forms mentioned in this work, lives under the artificial con- ditions of domestication, the ovulation rule may be sometimes more honoured in the breach than in the observance. Cases of mice, — 106 — which apparently failed to ovulate until 2 or 3 days after the birth of a litter, have come under my notice. Whatever else they establish, Bischoff's figures1) show this, that cavies do ovulate oftener than every 66 days, and than every 41—50 days, for otherwise, if the latter held good, there would be no correspondence between birth and ovulation. Founding on the facts relating to the development of the Guinea-pig and its critical unit, on those concerning the ovulation in the mouse and other forms, as set forth in this work, and on the above discussion, it may be stated that the normal ovulation-unit of the cavy is probably somewhere about 22 days. He, who would impugn the correctness of this conclusion, ought, if logical, to deny entirely the periodicity of ovulation in mammals. At any rate, to such a critic the words of Pouchet would find an apt application, for he would undoubtedly be one "de ceux qui, domines par leurs anciennes etudes, preferent nier l'evidence plutot que de s'adonner a de nouveaux travaux". g) The critical period of the horse. A year ago this was fixed, though not with certainty, as lying towards the end of the seventh week of gestation. A re-examination of the evidence as yet available has led me to alter this conclusion. In the first instance the result was arrived at from a misconception of Bonnet's statements, regarding the formation of the placenta and the change in shape of the egg. These were understood to happen towards the end of the seventh week; whereas, as a matter of fact, in his text-book he assigns them to the end of the sixth week. 1) Hensen's well-known researches could only be referred to long after the above was written. He states (Beobachtungen iiber die Befruchtung und Entwickelung des Kaninchens und Meerschweinchens, Zeitschr. f. Entwicklungsgesch., Bd. I p. 217), that if, after the birth of a litter, the female cavy escape fertilisation, the period, from the birth to the next ovulation, was found to be 17, 18, 35, and 37 days. The only observation to be made regarding these figures is, that they show at most the time of a new rut, and not, of a fresh ovulation. The date of this could be best determined by obtaining and estimating the age of the embryos, from comparison with known cases, at some period of the subsequent gestation. It may be added that the average of Bischoff's and Hensen's figures gives a probable ovulation-unit of almost exactly 21 days, or allowing for a days interval between copulation and fertilisation, one of 22 days. Hens en also writes on p. 215: — "bei den Meerschweinchen erfolgt (die Copulation) in der Regel o bis 3 Stunden nach dem Gebaren, und die Ovulation richtet sich nach letzterem". Regarding the latter it may be insisted, that just the very opposite is the case, i. e. that parturition adapts itself to the ovulation and not vice-versa. — 107 — Figures of Ewart's horse- embryos have now been published (]. C. Ewart, A critical Period in the Development of the Horse, London, A. & C. Black, 1897), and from them the following points can be made out. By the kindness of Prof. Ewart I have also had more than one opportunity of inspecting the original specimens of 4, 5, 6, and 7 weeks from copulation. The embryo of 49 days is decidedly beyond the critical phase ; for it is not merely beginning to resemble a horse, it is one, as the author notes. The 42 days embryo just borders on the critical phase, for the lachrymal groove is still present, and the head is not much raised. But its fore-limbs are flexed, the eye is oval, the snout is just commencing to lengthen and one or two hair-follicles are visible above the eyes. It is, however, so near it, that a few hours further development, and certainly not more than 24, would accomplish what is lacking. As mentioned in the text, this would give 43 days at most but only on the assumption, that copulation and fertilisation were iden- tical. If to the time given the interval between copulation and fer- tilisation be added, a sufficient span of time would be obtained to bring the embryo to the critical period. In fine, the embryo falls short of the critical period by some hours, but this deficiency is made good, if the interval between copulation and fertilisation be allowed for. It may be added, that neglect to make this allowance, which is usually an unknown quantity and often of some dimensions, accounts for many of the differences between embryos from different uteri, but apparently of the same age. h) The critical unit of the pig. I had hopes of being in a position, through the kindness of Mr. J. P. Glend inning of Overshiell, Mid-Calder, N. B., to settle this question by actual observ- ation. This gentleman placed a sow at my disposal for the purpose, but unfortunately, when the killing-time arrived, it turned out, that she was not in young. As there is no likelihood of my being able to test my theoretical conclusions for some time to come, it may be well to indicate here the grounds, on which they are based. In Edinburgh, and probably in most large towns, it is quite easy to obtain pig-embryos of many stages in a perfectly fresh condition from the slaughter-house. During the past winter and spring Mr. J. A. Murray and myself got together a very representative series in this way, but, though we have three or four different sets of critical embryos, the age of none of them is known. In estimating the probable length of the critical unit of the pig the data available were : — I) the average length of the gest- -*- io8 — ation, 2) somewhat wide statements of the rut-interval, 3) Keibel's valuable researches on the development, with ages of embryos until the 22nd day, 4) Bonnet's researches on the pig and sheep, but especially on the latter, and 5) his statements on the correspondence in the rate of development in the two animals. The average gestation is 112— 120 days, and the only numbers of possible critical units, which need be considered, are 3, 4, and 5. The last of these (5) is decidedly too high an estimate; for, when divided into the gestation-length, it gives 22 — 24 days as the critical unit. A glance at K e i b e 1 's figures of 22 day-embryos is sufficient to disprove this number, for such embryos are far younger than critical ones. The number 3 gives a critical unit of 37 — 40 days. From what follows, as also from the circumstance, that 37 — 40 days is more than double an ovulation-unit, this possibility must be rejected. We are left with 4 as the remaining possible number of units in the gestation, and with this a critical unit of 28 — 30 days is obtained. I believe this to be the correct one. With 28 — 30 days as the critical unit this latter is somewhat less than two ovulation- units. Bonnet (Entwickelungsgeschichte, p. 247) remarks, that pig and sheep show "so grosse Uebereinstimmung" in their early de- velopment, that they can be treated of together. A comparison of Bonnet's and Keibel's figures demon- strates the truth of this, for until the 22nd day stages of the two, almost corresponding in age and characters, can be found. Where differences are noted, the advance may be in either form, i. e. at a certain precritical stage there may be a day or so of difference in either of them, sometimes the pig is ahead, at others the sheep1). I conclude, therefore, that this correspondence lasts until the critical period, i. e. for 6 — 8 days longer than it is known to obtain, and that the critical unit of the pig is, as nearly as can be judged without further experiment, about equal to that of the sheep. i) The critical units of dog and cat. As no embryos of this period have as yet fallen into my possession, it is needful, that the reader should be informed, upon what data the estimates given in the table are based. Much time has been spent in seeking definite statements in the literature, and the only ones of service have been encountered in Duval's series of memoirs on "Le 1) Due, probably, to differences in the actual time of ovulation and fertilisation. — iog — placenta des Carnassiers" (Journ. de l'Anatomie, Vols. 29, 30, and 31). If in these any description of the embryos had been given, the conclusions to be drawn would have been perfectly clear; for very definite and precise statements could then have been made. Regarding the dog Duval remarks, that the replacement of the ectoplacenta or "l'angioplasmode" by the allantoic placenta commences towards the 30* to the 32nd day — "c'est-a-dire vers le milieu de la gestation, qui est, chez la chienne, de cinquante- huit a soixante-quatre jours" (Journ. de PAnat, Vol. 29, p. 663). Similar, though not so precise, statements are made concerning the cat in other papers, and other evidence, supporting the estimate of 28 days as the critical unit of the cat, will be found in O. Schultze's "Entwickelungsgeschichte", 1896, p. 84. It will be noticed, that, particularly in the dog, Duval com- ments upon the fact, that the allantoic placenta begins to replace the trophoblast at about the middle of the gestation. If any part of the argument of the work were really based on our knowledge of the development of dog and cat, it might be admitted, that the conclusions rested on insufficient data. But the results of the present writing stand quite independently of what we know of these animals, and they have only be introduced to show, that, so far as our knowledge extends, these two forms exhibit only evidences of conforming with the rule. k) The gestation of the hare. In many works consulted it is stated, that hare and rabbit have equal gestation-lengths. Ac- cording to Flower and Lydekker, "the rabbit is sharply separ- ated from the hare by the fact, that it brings forth its young naked, blind, and helpless; to compensate for this, it digs a deep burrow in the earth in which they are born and reared, while the young of the hare are born fully clothed with fur, and able to take care of themselves in the form' in which they are born" (Mam- malia, 1 89 1, p. 494). They also give the gestation of the hare as about 30 days, i. e. like that of the rabbit. There is, of course, no doubt about the condition of hare and rabbit at birth. But, either the gestation of the hare is longer than that of the rabbit, or its critical unit is shorter. From our knowledge of the state of the young at birth in mouse, rabbit, and cavy, it may be taken as certain, that the hare has at least three critical units in its gestation, and this would point either to a longer gestation, or, what is quite as likely, to a shorter critical unit than in the rabbit. If current statements be correct, the latter alternative must be the true one. — no — I have felt bound to cite this case, because throughout the research difficulties have always been sought for under the idea, that from these more was to be learnt than from simple facts. The instance of the hare cannot be regarded as a crucial one, unless the facts, when better known, should turn out to be at variance with the argument. This contingency is highly improbable, even impossible. 1) Smith, W. Tyler, Parturition and the Principles and Practice of Obstetrics, London 1849. p. 85. "The catamenial fluid is most probably, I may say, certainly, intended as an envelope for the ovule escaping from the ovarium, but unless impregnation takes place, no relation ever occurs between the uterus and ovule." p. 85. "Menstruation is, in tact, the unimpregnated oviposition, while conception is the impregnated oviposition of the human subject." p. 85. "Menstruation — — is the parturition of the ovule, instead of the parturition of the ovum ; and it represents, with the exception I have already dwelt upon, the whole of parturition in birds, and in many fishes and amphibia." p. 114. "In those animals of which we know the order of the oestrual periods, as the rabbit, the horse, and cow, the duration of pregnancy is a multiple of an oestrual period. Doubtless this law is as extensive as periodic oestruation itself. Not only is gest- ation a multiple of the oestrual period, but the time of parturition is positively an oestrual period. The maturation of ova, which has ceased during utero-gestation, is resumed, and the sexual instinct is predominant, just as though the uterus did not contain the pro- duct of a former ovulation and conception." p. 115. "There is, in fact, in all women, a tendency to abort at the times represented by catamenial periods." p. 118. Parturition is essentially a menstrual period. The author then goes on to state that "the sanguineous appearance called the 'show', which indicates the coming on of labour", is not due to rupture of certain vessels, but is really the beginning of a menstruation. So far as can be gathered, he regards the foetus as the ripe egg belonging to this menstruation, which only differs from any other flow as indicated above. p. 119. "I have observed, in cases of abortion occurring at what would have been a catamenial period, that menstruation has appeared at the end of a lunar month from the abortion, as if it were dating itself from a simple menstruation." — Ill — p. 122. "When the ovum is expelled as a mature embryo, it is still ovarian excitement, at the return of an ovarian period, which brings about its delivery from the uterus. Under this point of view, the dehiscence of an ovule from the most simple ovarium is ana- logous to the complex function of human parturition." A Course of Lectures on the Theory and Practice of Obste- trics by W. Tyler Smith, Lancet, 1856, Vol. I. Also as a separate work: — A Manual of Obstetrics, London 1858. • — The paging below is from the "Lancet". p. 86. "In the human female, it is believed that in ordinary cases the ovule escapes towards or shortly after the termination of the menstrual flow, through the pore or fissure found in the peri- toneal surface of the ovary." p. 113. "In women who menstruate two or three days only, the interval, between the cessation of one period and the beginning of another, is longer, and in those who are many days unwell, shorter, the length or brevity of the discharge does not usually affect the time of its return." p. 114. "The cause of menstruation must be referred to the ovaria." p. 276. "Many years ago I drew a comparison between men- struation, abortion, and parturition" — to the effect that all three were the equivalents of rutting-periods. p. 276. "The tendency of abortion to occur at the catamenial dates has long been a matter of observation." p. 232. "In 1851 Mr. Coleman, of Surbiton, drew my atten- tion to the following circumstance: — Two ladies, patients of his, were married on the same day, and both were delivered within ten hours of each other. The marriages took place on the 7th of May, 1850, and one was confined on the afternoon of Febr. 8th, 1851, and the other at 2 a. m. on the 9th, or 276 days from the date of the marriage." The author proceeds to support the table of Dr. R e i d , accord- ing to which the average duration of pregnancy is 275 days. On p. 332 the question, why parturition should occur at the tenth, rather than at any other, catamenial period, is raised, and he asks why the menstrual period itself should consist of twenty- eight-days. "The only answer is, in fact, that these particular periodicities do occur." He also states that he has noted, that in women with longer or shorter menstrual periods "the duration of pregnancy is more irregular than usnal". p. 333. "I have thus advanced, and I trust impartially, the argu- ments and facts for and against the theory of the ovarian cause of — 112 — labour, which I believe I was the first to propound. I have for many years taught that the ovaria, acting at or near the tenth period from the time of the ovulation which has ended in impreg- nation, excite in the uterus those changes which lead to the expulsion of the ovum. I have also compared the show which accompanies parturition, and the lochial discharge which follows it, to the menstrual discharge. It has appeared to me, that the changes in the uterine portion of the membranes are similar to the changes which occur in menstruation, and that the contractions of the uterus resemble those more imperfect contractions which occur in many women at the catamenial periods, and which we do not hesitate to refer to the ovaria. I believe there are some facts, which do not admit of explanation upon the supposition, that the cause of labour depends on the maturity of the ovum; as for instance, the occur- rence of uterine contractions at the end of gestation in cases of extra-uterine foetation. I believe, on the contrary, that the facts which appear to militate against the ovarian theory, most of them admit of explanation." Tyler Smith's conclusions have been quoted at some length, because, on a superficial reading, they seem to resemble some of the views advanced in the present writing. A more careful study of his works will, however, lead to a different conclusion. He regarded heat and menstruation as the same thing, and he looked upon parturition as a heat-period, in which, instead of the passage of an unfertilised ovum, that of a fully-developed one took place. Although he noted, that in many animals an ovulation ensues soon after parturition1), one searches in vain for a statement that the approach of this is the cause of birth. In spite of his mistakes, all honour to his memory; for, like Pouchet and Bischoff, he was a pioneer-worker at a very difficult question, which could not at that time be solved, because the data were insufficient. m) Length of pregnancy in worn an. It is quite beyond the scope of this work to consider at length the conflicting testi- mony on this subject. It is discussed in detail from the points of view of the anatomist, physiologist, and gynecologist in the works of His, Hensen, Spiegelberg, Tanner, Tyler Smith and others. The only course open to the writer was to examine their testimony from his own standpoint, and so endeavour to come to a decision. The conclusions drawn as to its average duration are given in the table as 276 — 280 days, and after much consideration of the I) With Sobotta I hold that this happens in all mammals, — H3 — matter there seems no good ground for altering this result. Many striking cases and more than one list of instances are on record supporting this view. The two births cited by Tyler Smith (see p. in) are very significant, and Hens en's citations and arguments appear to me to carry conviction. It would, as more than once admitted, be very easy to bring forward numbers of exceptional instances totally at variance with the law, but what would they prove ? Take the Gardner case, with a suggested gestation of 304 — 311 days, as an example. The only thing known about this is the date of birth; of the other factors, date of last menstruation, fertilisation, and length of menstrual cycle, there is not a particle of information available. If most of the factors of supposed abnormal cases were revealed, possibly the abnormality would disappear. In the lower mammals, where the date of fertilisation can be approximately gathered from that of copulation, the length of gestation varies only within the narrowest limits ; more especially in those forms, which, as a rule, produce a number of young at a birth. The apparent variations are largest (1) where the rut-period is long, (2) where the normal gestation-period is a prolonged one, and (3) where the number of young at a birth is usually the minimum. I am inclined to lay much emphasis on the latter point, for it is in the horse, cow, and human subject, where the variations are most pronounced ; whereas in rabbit, sheep, and pig, in which as a rule two or more eggs are fertilised, they are of a trifling character. (Compare also latter part of appendix z.) If a single egg be fertilised at a certain time, it by no means follows that it will develop. In almost every gestation of rabbit, cavy, and pig some of the fertilised eggs quickly abort, while others of the same batch live through the gestation. In horse, cow, and the human subject such an early death and absorption of the "egg" must be of very frequent occurrence, while living spermatozoa may be left in possession for some considerable time. As Burns sings: "The best laid schemes o' mice and men gang aft a-gley." Although, as a rule, the ovulation-unit of any fonn is a fixed one, the possibility of an unusual ovulation must not be lost sight of. In the higher forms (where alone such irregularities seem to occur and, perhaps, frequently) it would be an exception of a kind proving the rule, instead of controverting it. As the horse and human subject are those, in which these apparent difficulties crop up, our study of the matter may be confined to theni. Beard, Span of Gestation. g — ii4 — It has been shown, that in these at the present day the critical unit corresponds to two ovulation-units instead of to one only, as must have been the case in past times. If such a duplication of the one unit can take place, a change in the other is rendered a priori possible, more especially, when it is noted, that the lower mammals, and especially certain marsupials, appear to have very small units. In both man and the horse it appears highly probable, that a doubling, or even a trebling, of the ovulation- unit has happened in the ancestral history, for it is more than double that of the mouse in both cases, and, if one may guage the ovulation-unit of Didelphys from its critical unit, the ovulation-period of the two Eutheria is nearly threefold this. Thus, in the horse and man we may occasionally have to deal with irregular reversions to a former ovulation-unit of 7 or 14 days in the former, and, in round numbers to 8, 12, or 16 days in the latter. The reader may also be reminded, that, just as certain women possess the .peculiarity of regularly - occurring short menstrual periods, and thus necessarily of briefer ovulation-units than usual, so also there are mares, which "come into season" every 14 days, instead of every 21. To illustrate the matter by examples. A mare may have ovulated at a certain date and the egg may have been fertilised. If this egg were to quickly succumb — and my experiences of the past ten years would not lead me from analogy to regard this as unlikely to happen *) — an irregular ovulation at some ancient ovul- ation-period might conceivably lead to a new fertilisation without a new copulation, and, as likely, this would apparently result in a gestation of unusual type. In the human subject one might have to deal with a reversion to an old ovulation-unit of 8, 12, or 16 days2); and, if the egg given off at the normal period had, in the meantime, died or missed ferti- lisation, the egg of the irregular and unusual ovulation might be fertilised, before there had been time to form a menstrual decidua, and thus the next menstruation would be suppressed. But the embryo of such an egg would be bound to adapt itself to the rules of the development, or have these modified to suit it. 1 ) It is a much more likely thing, that a gestation after its initiation should be suppressed, than that ovulation should fail to happen at the proper time. 2) It is impossible at present to decide with certainty, if in man's ancestry former ovulation-units were 8 and 16 days, or 12 days. — H5 — If they were not altered, its early development until the critical period would require to be abnormally slow, otherwise it would reach the critical period at too early a date to admit of agreement with the normal ovulation-rhythm. If harmony were absent, the embryo would certainly be aborted, and this probably very frequently happens : if all were brought to chime in, its development would proceed. But — and this is the significant point — the gestation would probably be apparently longer than usual, and might, by some three weeks, or even more, exceed the normal time. This matter has been discussed, not only because of its bearings on the length of gestation, but also with reference to those human embryos, whose probable ages date not from the last menstruation but from the first missed menstrual period. Several instances of such are given by H i s *). In dealing with these we are bound to conclude, that ovul- ation and fertilisation took place, before the missed menstrual period was due; otherwise, of course, it would not have failed to appear. As others have remarked, there would seem to be no grounds for assuming, that an ovulation immediately prior to a menstruation could arrest the latter. We must, therefore, conclude, that the ovul- ation probably occurred from 7 — 14 days before the menstruation was due, that is, before the menstrual decidua was formed. I incline very strongly to the conviction, that the most likely time for such an unusual ovulation would be half way between the end of one menstruation and the beginning of the next, and, in fact, at a ancient ovulation-period, which obtained before the present one was acquired. As stated above, the egg of such an ovulation would probably act, as if it belonged to the normally -succeeding one, on account of the necessity of obedience to certain rules. And, thus, if later on aborted, its apparent age would date from the latter. His' cases can, I believe, be best explained in this way, assuming that they really relate to embryos dating from suppres- sed menstruations. Moreover, it must be noted that they are all abortions, and this indicates that there was something wrong about the develop- ment. As the embryos themselves are normal, the abnormality must lie either in the placental changes, or with the mother. In most of them the cause is to be sought in the maternal organism, and this is in agreement with the conclusion we have arrived at. If no other cause can be assigned for the abortion, something connected 1) His, loc. cit. II. p. 74 etc. 8< — n6 - with the suppressed menstruation may possibly underlie it. So far as I can see, the thing must likely to bring about the suppression of a menstruation, apart from sickness or disease, would be an unusual ovulation in the middle of the menstrual cycle, before a decidua had been formed, and followed by fertilisation. The uterus is then in a state of rest, and its subsequent changes may be influenced by the existence of the fertilised egg. n) The critical unit of man in its medico-legal aspects. According to the law of England, and possibly of other countries, the embryo in utero is not alive until "quickening" appears in the sixteenth to seventeenth week of pregnancy. The importance attached in law to this sign has relation to the procuring of abortion and to pregnant women condemned to death. It is not necessary to cite here, what the law lays down regarding these points : every-one interested in the matter knows in what works to seek for information. But the only person, whose right it is to decide, at what period of its development the child may be said to live, is the embryologist. Now, it may be admitted at once, that, if embryological opinion were asked for on this subject, the majority would hold to the view, that the child was alive and in existence from the moment of fertilisation of the egg. Such is not the conclusion, which anyone, who has read this memoir and its predecessor, has the right to draw. One thing, which stands out clearly as the result of these studies of Vertebrate, mammalian, and human development, is, that, until the critical period is reached, the embryo, as that of a certain species of animal, [has no complete existence. It is only coming into being and its parts are not all present until the critical period. Thence onwards its indi- viduality is established, and, whereas its early development was in its essence an epigenesis, from the critical period this becomes an evolution or unfolding of parts already in existence. Thus, to be consistent and to be in accord with the results of science, the law ought to consider the child to be alive, from the moment the critical period is reached ; and, certainly, from the end of the seventh week of intra-uterine life. o) Medico- legal aspects of the length of gestation. From time to time, but naturally somewhat rarely, cases involving the question of prolonged pregnancy come before the courts. A celebrated one is that of the Gardner peerage, cited by Tanner, and another is referred to by Tessier (p. 176). In the former the main point to be decided was whether or not a certain pregnancy could have been as prolonged as 304 to 311 days. Seventeen medical men were examined, of whom "five supported the view, — li7 — that the period of human utero-gestation was limited to nine cal- endar months, or from 270 to 280 days. The remaining twelve seemed to maintain the possibility, that pregnancy might be pro- tracted to nine and a half, ten, or even eleven calendar months ; and so, of course, to 311 days, the alleged term of gestation". Looking at the matter from the point of view afforded by the results of the present work, it is readily seen, that without the addition of a further critical unit — and the assumption of this ought in any case to be our last resort — an ovulation-unit of 26 days, with the critical unit in correspondence, would give in the human subject a period of pregnancy of 312 days. In such an instance the only justification for the adoption of such a unit would, so it appears to me, depend on the existence in the particular individual of a regular menstrual cycle of 30 days or more. If this obtained, the abnormality of so prolonged a gestation would appear to be explicable: without it and without other data no certainty on the matter could be arrived at. It must be mentioned, that Ceders- c h j 6' 1 d "has tried to show that the duration of gestation is in- fluenced by the length of the intervals between the catamenial periods". (Tanner, loc. cit., p. 193.) p) Pouch et, F. A., Theorie positive de l'ovulation spon- tanee et de la fecondation des mammiferes et de l'espece humaine, Paris 1847. The book is apparently a revised edition of an earlier work published in 1842. The latter has not been available for reference on the present occasion. Pouchet's work is a very important one, and to realise this one has to remember, what was held re- garding the ovulation of mammals prior to 1840. It was considered, that ovulation only occurred as the result of sexual congress, and to have disproved this is one of the chief merits of Pouchet's work. He showed conclusively, that in various mammals, but more particularly in the pig, rabbit, and in man, ovulation takes place at characteristic periodic intervals quite unconnected with copul- ation. Thus, Pouchet was the pioneer, upon whose foundation more than upon any other the slight advances contained in the present writing are laid. It seems to us so natural now to recognise the regular peri- odicity of mammalian ovulation and its independence of copulation, that it is difficult to appreciate the great advance, which, in the first instance, we owe to him. His results were confirmed, in- dependently of him and of each other and within a short period after their first publication, by Bischoff and Raciborski. Pouchet, and not Tyler Smith, would appear to have — ii8 — been the founder of the view of the correspondence of rut and menstruation. That the times of occurrence of the two, i. e. just prior to ovulation, are alike there can be no doubt, but that the two things themselves are identical is quite out of question. It need only be repeated here that the sole similarity between the two is that mentioned above. But they may both in their first origin have arisen ont of the same phenomena. (Compare H e a p e ap- pendix w.) q) The corpus luteum. Bonnet, in his excellent "Entwicklungsgeschichte der Haussaugetiere", p 19, remarks that during gestation, as he repeatedly observed, the ovary becomes "blutarm", and that the corpus hcteum persists till shortly before the birth. The indifferent blood supply of the ovary and the growth and nourishment of the corpus luteum during the greater part of the gestation, such that in the mare it may come to equal in size the rest of the ovary (Bonnet), are factors which must result in either the suppression or the rendering abortive of ovulation during gestation. If the growth of the corpus luteum be not a contrivance to this end, why does it not go on growing to the very finish of the gestation? Why does it stop short some time before the close, unless it be to render an ovulation just after birth possible? It must be noted here, that P. Mingazzini and E. Gia- comini have described what they consider to be true corpora lutea in reptiles, amphibians, and selachians. This is a satisfactory result, for it removes any difficulties about a first and sudden appearance of this structure in mammals. Whether it possesses any functional importance in lower Vertebrates, and whether its functions in these, if any, are everywhere similar, remains to be seen. For recent work on the corpus luteum in the mouse and rabbit the reader may be referred to Sobotta's memoirs in the Arch. f. mikrosk. Anat., Bd. 45, 1895, and in Anat. Hefte, Heft 26, 1897. r) Changes in the mammary glands at the critical period. The reviewer of my "Problems, of Vertebrate Embryology" in "Natural Science" for January 1897 first directed my attention to the fact, that changes in the mammae are among the earliest symptoms of pregnancy. It became of interest to learn whether, like other things, these were in association with a certain phase of the gestation; whether, in fact, the mammae still retained their ancient sympathetic connection with the critical period, at which, as shown last year, they begin to function in non-placental Marsupials. It is very clear from works on midwifery, such as Spiegelberg's, that this is the case in the human subject, where changes in them begin from the seventh week. — iig — Tanner (Diseases of Pregnancy, 1867, p. 75) remarks: — "and, finally, it may be laid down as a law, that, when a woman is gravid for the first time, and has missed two monthly periods a drop or more of milk may be expressed from the breasts." And in Dalziel's "British Dogs" p. 468 the author writes : — "it is not easy to tell whether the bitch is in pup before the fourth week has passed ; by that time the teats begin to enlarge, and there is a ridge-like swelling between them." The critical period is given in the table as 29—32 days, and it probably varies with the breed of dog. s) Is the o vulation-unit ever approximately equal to the gestation-period in Eutheria? For some time during the course of the experiments and investigations of the present work the possibility of this being the normal rule in certain cases had to be considered. If current statements were trust- worthy, it appeared undoubtedly to hold good for mouse, rabbit, and cavy. But, although the periodicity of ovulation was established by Pouchet and Bischoff more-than 50 years ago, since then no appreciable advance towards an accurate knowledge of the subject has been made. In more than one point I found current beliefs either vague or utterly at sea, and thus began to doubt the existence of exact information even regarding the true ovulation-units of the three common animals mentioned above, all of which, by the way, are bred by fanciers in countless numbers every year, whilst books on breeding agree in complete silence regarding the periods of ovulation. Concerning the case of the cavy, as mentioned in another appendix, time has not been available, in which to clear the matter up. But the true facts have, I think, been established regarding the mouse : an exceedingly satisfactory result, because of its lowly position in the Eutherian scale. Fleischmann has commented upon the agreement of rodents and marsupials in respect of certain developmental arrangements. The mouse, especially, exhibits this to a great degree. Even its new-born young are exceedingly like young Metatheria, of which Dr. R. Broom has shown me a large number from different species. As in marsupials the yolk-sac of the mouse plays a consider- able role in the early development (Robinson). And the gestation is a short one of only two critical units, i. e. but one more than in non-placental marsupials. *- 126 -* Now, with all these agreements it might have been anticipated, that the mouse would exhibit a correspondence and agreement between ovulation- and critical units. As detailed elsewhere, this has turned out to be the case. Exceptions are, however, met with, and in some white mice, when not pregnant, there may be a failure to ovulate some ten days after a birth, or the ovulation may be abortive. Which of these is the correct explanation can only be settled by observation. Thus, the mouse must be removed from the list of those placental forms, in which, if current belief be correct, the ovulation- unit corresponds to the length of gestation. Such an arrangement cannot hold good for the cavy either. A correspondence of the ovulation in this case to two critical units would be disastrous, for it would throw the arrangements for inducing the birth out of gear, an ovulation not being due until 22 days after the event. As described on p. 104 — 106, it is quite out of question that the ovulation-unit can equal the gestation in the cavy. It cannot do anything other than rather exceed one third of the latter. There remains, then, the rabbit, and here there are no experi- ments to offer. None the less, from the known lengths of critical unit and of gestation there can, to my mind, be no particle of doubt, that the ovulation-unit here has a length of 15% to 16 days. In fact, if one except cases, like cat and dog, in which breeding has become restricted to certain periods of the year, wheresoever there is a periodic rut with ovulation, these recur at intervals approximately equal to, or half the length of, the critical unit of the species. t) Sizes of embryos of the critical period. The measurements given below are taken from embryos killed with corrosive sublimate and brought up to 90 % alcohol, i. e. they are of embryos measured when preserved. 1 have made a considerable number of measurements of embryos before, during, and after preservation, and have found, that the main shrinkage with sublimate preservation takes place in the alcohol, and even in a weak solution of it. The figures will be published elsewhere in another connection. The most surprising result was the trifling extent of the shrinkage in corrosive sublimate both in mammals and fishes. The total shrinkage, the final measurement being made when the embryo had reached 90 °/0 alcohol, was on a fairly constant average rather under n1,, % °f the live-length. In five critical embryos of one rabbit the following are very exact measurements of the lengths in the preserved condition: — 121 Greatest length in mm Nacken-Lange in mm 12 II I2£ "1 12 ii 13 12 12 II Thus far three pig-uteri with embryos of the critical period have been obtained. They are lettered F, FA and FB. The embryos of F and FA are in very much the same posi- tion in the critical phase, whilst those of FB are almost emerging from it. The lengths are given in millimetres : Series F Series FA Series FB G. L. N. L. G. L. N. L. G. L. N. L. 20.5 17 19 17 20.5 18 19-5 17 17 15 21 19 19-5 17 19.25 17 20 17-5 18 15-3 18.5 17 21-5 19 20 17 17 15 20.5 18 20 18 18 17 20.5 18 19-5 16.5 18 16.5 20 18 19 17 17-5 15 20.5 18.5 21 18 17-5 15-5 and three monsters. 19 17 18 16 and two monsters. 18.5 17 18.5 18 16.5 15 16.5 16 no monsters. Of critical embryos of the sheep but two sets have as yet been obtained, they are both cases of twins. The measurements are G. L. 16, 16, 16 ■75, and 16 mm. Guinea-pig embryos of the critical period are rather smaller than c< jrresponding rabbit embryos. u) M i n o t , C. S., "The changes of the uterus during men- struation and gestation are homologous, the menstrual cycle being prolonged and modified by pregnancy." Uterus and Embryo. Journ. of Morph., Vol. II, p. 437. With almost prophetic instinct M i n o t writes concerning the cause or causes of parturition: — "we evidently have to do with a progressive maturation of the uterus — a series of changes we cannot explain, but which is, as already pointed out, closely similar to the series of changes during menstruation. Hence it is probable that there is a common cause for the ending of the series (the casting off of the superficial part of the mucosa in both cases) etc." Human Embryology, p. 27. "The menstrual and gravidital changes follow the same cycle, and differ from one another essentially only in two points : 1) the — 122 time occupied, and 2) the extent of the changes. In fact the alterations, though of the same character, are greater in extent and occupy a longer period during gestation than during men- struation. These considerations force us to the conclusion that the gravid uterus is passing through the menstrual cycle prolonged and intensified. The function of gestation is a direct modification of the function of menstruation, and the two are physiologically homologous. The deduction is so evident that I have been sur- prised not to have yet encountered it clearly enunciated in any of the authors I have consulted." Human Embryology, p. 25. The above extracts from the writings of M i n o t appear to me to indicate a certain amount of agreement between his view of menstruation and that adopted in the text. How much further one may go is not very clear; for the meaning of more than one of his phrases is anything but obvious. No attempt is made to explain the cause either of birth or of menstruation, and the opinion, that "the function of gestation is a direct modification of the function of menstruation", looks very much like an inversion of matters. We know what gestation and parturition are, and why certain phenomena accompany them. From these, and other facts, as we have seen, menstruation and the menstrual cycle can be explained. But how gestation and parturition can be obtained as direct, or indirect, or any sort of modification of a menstrual cycle and of menstruation, passes comprehension. If parturition and the gestation-cycle be explained in terms of men- struation and the menstrual cycle, we must first know what the latter really are, and regarding their meaning Mi not supplies no information whatever. It is, of course, quite true, that there is an association between the two, as Mi not insisted, and it is such that, if an embryo were present in connection with the decidua formed before a menstru- ation, the menstrual cycle would be "prolonged and intensified" through the ensuing gestation. But menstruation is a modification of gestation and not vice-versa. In stating, that "the changes in the uterus during menstruation and gestation are homologous", Minot really goes no further than did Tyler Smith, for the latter distinctly recognised this, and only differed from Minot in assigning an erroneous cause to the two, whereas the latter attempts to explain neither. He merely states, that "it is probable there is a common cause for the ending of the series", but does not say what this is, though the cause of the casting-off of the superficial part of the mucosa at parturition — 123 — is not far to seek: it has played its part, and is no longer required. A view quite similar to Minot's is "clearly enunciated" in Burdach's Die Physiologie als Erfahrungswissenschaft *), where it is written : — "Die Menstruation ist in ihrem Beginnen durch Blutandrang, Auf lockerung, und Herabsenkung ein Vorbild der Be- fruchtung und Schwangerschaft, in ihrem Ausgange aber durch Zusammenziehung und Blutergiessung ein Vorbild des Gebarens." Undoubtedly to some extent agreement subsists between Minot's views on menstruation and the menstrual cycle and those adopted in the text, and as these, so far as they are similar, date back to Burdach in 1830, if not to an earlier writer, there is not much novelty about them. The one new point is the recognition in the present writing, that menstruation is an abortion of the decidua prepared for a missing embryo, which ought to have arisen from an egg given off after the preceding menstruation, and that this abortion happens at a time corresponding to the end of a former critical unit, and prior to a new ovulation. v) Sobotta, J., Die Befruchtung und Furchung des Eies der Maus, Taf. II— VI, p. 15—93. Arch. f. mikrosk. Anat, Bd. XLV, 1895. p. 27. "Die Brunst tritt bei der Maus wie beim Kaninchen, Meerschweinchen , Hund und wahrscheinlich alien Saugern un- mittelbar nach dem Wurf auf, wie von den verschiedensten Unter- suchern gefunden wurde. Auch ist von Bischoff sicher fest- gestellt worden, dass Eier unabhangig von der Copulation aus- gestossen werden." p. 28. Trachtigkeitsdauer fast 20 Tage. Ovulation (apart from that soon after birth i. e. on the day of birth) ensues 21 days later. "Dass zwischen dem 1. und 21. Tage post partum noch ein oder mehrere Ovulationstermine liegen, kann ich nicht mit Be- stimmtheit bestreiten, mochte es indess auf Grund einiger daraufhin angestellter Untersuchungen vorlaufig in Abrede stellen." Copul- ation takes place 3 hours after birth, and may be repeated 24 hours later, p. 35. Lactation lasts nearly 3 weeks, p. 28. As mentioned in the text the only point, in which my ex- periments have led to a result different from the above, relates to the frequency of ovulation. The ovulation-unit of the mouse is not 20 or 21 days, as hitherto supposed, but it corresponds to the crit- ical unit, and, as nearly as can be guaged, it is 95/6 to 10 days. A "normal" mouse, which has given birth to a litter, does not first 1) 1830, Bd. Ill, p. 6. — 124 — ovulate 20 — 21 days later, if not allowed to become pregnant at once, but under this latter condition after the ovulation following the birth, it ovulates in g5/6 to io days. Thus ovulation- and crit- ical units are approximately equal, the latter being very slightly less than the former. w) Heape, Walter, The Menstruation of Semnopithecus entellus. Phil. Trans. London, Vol. CLXXXV, Part I B, pp. 411— 471, with 7 plates. — The Menstruation and Ovulation of Macacus rhesus. Proc. Roy. Soc. Lond., Vol. LX, 1896, pp. 202 — 205. Since the results of the two works are, as Heape himself states, very much the same, only the first will be cited. The work contains a list of literature treating of menstruation and a discussion of various views, which have been held. p. 462. Heat or rut and menstruation are concluded to be analogous processes. p. 462. "A substantial periodic growth of the (uterine) mucosa is arrested by degenerative changes when a fertilised ovum is not present". This is a pithy summary of what the phenomena of menstruation are. Ovulation was not often encountered in menstruating apes, and (p. 443) the ripening of the ovum was found to be independent of the process of menstruation. "Ovulation is neither the cause nor the necessary result of menstruation." „It is possible, however, that the increased blood supply to the generative organs during menstruation may induce ovulation, when a sufficiently ripe ovum is present." p. 461. "The primary cause of menstruation remains unex- plained." x) Selenka, E. Studien iiber Entwickelungsgesch. d. Tiere, Heft IV, I, 1886: Das Opossum. The interval from copulation to birth in Didelphys virginiana is given by Selenka as not quite 13 days, and that from fertilis- ation to the end of gestation as 7% days. p. 105. "Die Zahl der in den beiden Uterushornern aufge- fundenen Embryonen schwankte zwischen 7— 27, betrug aber meistens 12 — 16; da sich im Beutel des Weibchens aber nur 8 — 15 Zitzen vor- finden, so mochte ich glauben, dafi das reichliche Futter und der Mangel an Bewegung die Veranlassung werden zu dieser iiber- raschenden Fruchtbarkeit." p. 106. — "Alle Eier im Uterus des trachtigen Weibchen stehen immer auf gleicher Entwickelungsstufe ; nur zuweilen findet man — 125 — einige Eier in ihrer Entwickelung den iibrigen vorausgeeilt oder hinter ihnen zuriickgeblieben, aber das Alter differirt doch kaum um mehr als 1—2, sehr selten bis 8 Stunden." — Studien uber Entwickelungsgesch. d. Tiere, Heft V, I 1891 : Beuteltiere u. Kangaruratte. p. 173. From copulation to fertilisation the following intervals are given: — Hypsiprymnus II days Phalangista 13 „ Dasyurus 14 ,, p. 175. In Hypsiprymnus 8 days elapse from fertilisation to birth, i. e. almost the same as in Didelphys. y) Owen, R., The length of gestation in the kangaroo, Macropus -major, was determined as 38 days from copulation to birth. If an allowance of 14 days, as the possible interval between copulation and fertilisation, were made, as in Dasyurus, we should obtain an actual critical unit of 24 days. This, however, would be merely a surmise. But the actual gap between the two is hardly likely to be as small as in the opossum (5 days) and it may be equal to or exceed that in Dasyurus (14 days). Owen's results are given in, among other places, Todd's Cyclopaedia of Anatomy and Physiology, 1839-47, Vol. Ill, p. 322. z) Bonnet, R. , Beitrage zur Embryologie der Wiederkauer, gewonnen am Schafei. Arch. f. Anat. u. Entwickl., 1884, pp. 170—230, 3 Plates. p. 171. The "Brunst", on which Bonnet made no investig- ations himself, is stated to be short, lasting for 24, 16, or 20—30 hours. p. 172. The gestation of the merino-sheep is given as 150 days, rut returning 7 months later and then recurring at intervals of 17 days (Rueff). The rut-periods happen in spring and autumn. While this work was in the press, an opportunity of con- sulting the following important communication arose, but too late to permit of reference in earlier pages. Nathusius, H. von, Ueber einen auffallenden Racenunter- schied in der Trachtigkeitsdauer der Schafe. Zool. Garten, Jahrg. 3, 1862, p. 102 — 105. This short, but exceedingly interesting and important, paper records observations, extending over 4 years, on the average gest- ation-lengths of over 200 merino and about 100 Southdown sheep, and the former are stated to have been imported originally from Spain, the latter from England. On p. 103 the author remarks 126 — "trotzdem tragen die Merinos constant die Frucht durchschnittlich sechs Tage langer als die Southdowns". On p. 104 there is a table of the results of the experiments. In the 4 years the gestation-length varied in the merinos from 149.3— 151 days, and 872 cases gave 150.3 days as the average gestation-length. In the same period it varied in the Southdowns between 143— 144.3 days, and the average of 442 cases was 144.2 days. In crosses between the two breeds remarkable results were obtained, thus: J merino, \ Southdown = 1 46.3 days average gestation \ » I 5) = !45-5 y> » » 1 5) s » == *44 '> >j » Here no allowance is made for the interval between copulation and fertilisation. Even without allowing for this, if we calculate theoretical gestation-lengths for the crosses on the basis of 150 days average gestation in the merino and 144 in the Southdown, with the proper fractions of these numbers we obtain : \ merino, \ Southdown = 147 days, i ■>■> 4 » = I45o » \ „ I ,, = 144-75 „ As Nathusius distinctly denies any attempt at accuracy in noting the gestation-lengths within half a day or more in many of the cases, this result is a very remarkable one, and the observed gestation -length so closely approaches the calculated one (the discrepancies being probably due to "errors of experiment") as to warrant the conclusion, that it is a fixed quantity for the variety, and that it is determined in the cross by the relative percentages of the two races contained. To summarise the matter, Nathusius 's figures serve to support the contention, that the span of gestation is, as a normal thing, practically a constant quantity for the individual and for the variety, but that it may differ within certain limits in different varieties of a species; and, moreover, that in crosses between vari- eties the gestation-length tends to approach the mathematical mean. Index. page Abnormal development . . 20 Abortions 79 — connected with unusual ovulations 115 — and critical period . . 81 — and menstrual periods . 80 — in sheep 84 — Klein and Burdach on 80 — periods of 81 — periodicity of many . .81 — tables on 80 — Tyler Smith on .79. 83 — Whitehead on . . . 79 Alteration of critical unit . 41 — of ovulation-unit ... 41 Alternation of generations . 101 — antithetic 101 — homologous 101 Atrophy senile of phorozoon 21 Birth cause of . . . .84. 89 — condition of young at 27. 47. 54 • inhibition at critical period 23 Birth-period of Marsupials . 22 of Perameles ... 26 — — of Trichosurus . . 23 Birth problem of . . . .44 Bischoff on ovulation . .117 — on cavy . . • ... • 105 page Bonnet on corpus luteum 1 18 — on horse -placenta ... 49 — on pig and sheep . . . 108 — on placenta 24 — on sheep 53- I25 Bower on alternation of generations 101 Burdachonmenstruation7l. 123 Cannabalism of larvae . .16 Cause of birth 84 Cat, critical unit of . . .108 Cavy, condition at birth of 27. 47 — gestation-period of . .48 — critical unit of . . 106. 120 — ovulation in 1 04 Cederschjold on preg- nancy 117 Celakovski on alternation 101 Chambers on influence of gestation 56 Climate influence of . . . 30 Cocoon, primary nature of . 14 Condition at birth how de- termined 60 — at birth of human foetus 54 — at birth of young ... 27 Cope on Phenacodus . . 62 Corpus luteum . . . 82. 118 — 128 — page Correspondence of embryo and placenta ... — of units 34 Corresponding phases pos- sible Crosses gestation in sheep Critical period . . II. 19, — — constant characters of — — characters of . . — — Duration of . — — in man .... — -- of horse .... — — and mammae — — placenta at . . . — — sizes of embryos at Critical phase . . . .11 — — duration of . . . — — pause at . . . . — — physiological Critical unit .... — — alteration of. — — ancient .... — — of cat and dog . constancy of — — and ovulation - unit 34- 41 definition of . — — differences in — — duplication of — — enlargement of — — of horse . increase of . influence of number of 54 — — of large animals . — — length in various mam- mals — — of man .... — — of marsupials . 49 medico-legal aspects of 1 16 new number in gestation — — of man, sheep, etc. 9 41 55 126 102 7 103 21 77 106 118 43 120 19 21 39 24 34 4i 114 108 40 60 35 40 56 59 52 46 55 62 36 77 40 Critical unit of pig — — of sheep . — — of small forms . Critical units, table of page . 107 • 49 . 61 Dalziel on mammary func tion Definition of critical unit Degeneration of phorozoon — of yolk-sac .... Development of placenta — regularity of the . Dog, critical unit of . Duplication of critical unit 49 Duration of pregnancy . Duval on Carnivora . — on yolk-sac of rabbit . 119 35 19 19 43 37 108 56 112 109 19 100 53 Ectoplacenta 18 Egg-purse of Elasmobranchs 14 Egg-white, nature and history of 15 Embryos destroyed by lact- ation 65 — sacrifice of . . . . .47 — sizes at critical period . 120 Epigenesis 18. 116 Eutheria v. Metatheria . . 103 Eutherian gestation-length . 42 Evolution 116 — of egg-case 14 Ewart on horse-embryos 49. 107 Extinction of "kangaroo" . 103 Fleischmann on rodents 119 Flower and Lydekker on hare 109 40 24 85 66 Food-yolk, influence of . Formation of placenta Foster on parturition . Franck on the "Ong-te" — i2g page Gardner case . . 113. 116 Gastrulation, meaning of G e d d e s and Thomson on size and gestation . Gestation abnormal . 100 — of cavy .... 48 — correspondence with men- strual cycles — of cow 51 — duration in man — Eutherian v. Metatherian — fixed nature of . — frequency in mouse etc — of hare — Harvey on — of horse .... 50 — Kielmeyer on . — length of, in Eutheria — lengthening of . — in man . . . • • 53 — in mouse 32 — medico-legal aspects of — non-influence of length — of pig . . . — prolongation of — prolonged — of rabbit . — of sheep . — supposed irregularity of — table of lengths of — Tessieron — of two ovulation-units 53- 99 Hare, gestation of . H e a p e on menstruation 69 — on ovulation . . .74 Heat in mares .... -• and menstruation . — Pouchet on . — Tyler Smith on Hensen on cavy. . 104 — on pregnancy . Hill on Perameles . Beard, Span of Gestation. 16 62 113 I05 112 I03 44 31 109 78 99 62 42 59 112 123 117 48 99 46 113 99 125 6 99 46 109 124 124 51 118 117 no 106 "3 26 page His on human embryos 53- 74-H5 Horse, Bonnet on placenta of 49. 106 — critical unit of . . . -49 — critical period of . . . 106 — ovulation-unit of . . .49 — E wart on 107 — gestation of. . 51. 99. 114 — small size of ancestors of 62 Human critical unit 49. 51- 77 — embryos, ages of . . . 115 — ovulation-unit . . .50. 77 — pregnancy and the law . 116 Influence of climate . . 30. 66 — of food-yolk 40 — of lactation 63 Keibel on the pig . . . 108 Kielmeyer on gestation- length 62 Lactation in cat and dog . 65 — effect on embryos ... 65 — influence of 63 — in kangaroo 104 — in man 66 — in marsupials .... 104 — in mouse 123 — in pig 65 — prolonged in marsupials . 104 — Ryder on 66 Length of gestation in Eu- theria 42 Loewenhardton menstru- ation 75 Macropus, Owen on 40. 125 Mammae and critical period 118 — limitations of . . . .28 Mammals, critical period in 22 — original 30 — ovulation in 27 9 13° — Mammals regularity of de velopment in ... — rhythmic development in 37 Man, special case of . Mares heat in . . . .51 Marsupials, birth-period of 22 — critical units of . — lactation in . — precritical nutrition of Marsupium how lost . Menstruation an abortion 71 — Burdach on . — H e a p e on . . . .69 — Loewenhardt on . — meaning of . . . .68 — M ino t on . page — nature of 71 — and ovulation . — phenomena of . — relation to rut of . .69 — Tyler Smith on — Webster on. — unusual effects of . Merino sheep, gestation of Mi not on menstruation . Metamorphosis abrupt o Phoronis Monstrosities Mouse, gestation of — and marsupials . — ovulation in . — ovulation-unit of — Sobottaon . .32 37 67 91 9i 40 104 22 47 95 123 124 69 95 121 95 7i 69 118 no 69 115 125 121 20 39 123 119 32 123 123 Nathusius on sheep-gest- ation 125 — on crossing 126 Opossum, critical unit of . .36 — Selenka on . . .40. 124 Origin of large-yolked egg . 16 Ovary, rhythm connected with 82. 97 page Ovulation, Bischoff on . 1 17 — and birth 89. 95 — in cavy 120 — effect of gestation on 28. 30 — following birth . 30. 89. 95 — Hensenon . . . .73 — lunar correspondence of . 3 — in mammals 27 — in man 73 — and menstruation . .71. 115 — in mouse 32. 120 — Pouchet on ... . 117 — suspension in gestation 28. 30. 94 — unusual . . . . 113. 115 — interval in cavy . . . 104 — — of man 78 — — of mouse 32 — unit . 41 — — alteration of. . . 41. 42 ■ ancient 114 — — and critical unit . 33. 41 — — gestation of two . . 46 — — table of 98 Owen on Macropus 40. 57. 125 Parturition, cause of . . 89. 95 — Fo ster on 85 — Harvey on . . . .86 — Spiegelbergon . . 86 — Tanner on . . . -85 — Werth on 85 Perameles, birth-period of . 26 — placenta of 44 Phenacodus 62 Phorozoon 26 — degeneration of ... 19 — life-span of . . . .21. 22 — motive of existence of .12 — as a parasite .... 17 — organisation of . . . . 13 Pig, gestation of . 53. 99 — critical unit of . . . . 107 — sizes of embryos . . .121 i3i page Placenta, Bonnet on . . 24 — consequence of . 25. 44. 91 — development of ... 43 — and its embryo .... 9 — formation of .... 24 — functional period of . .26 — inadequate at critical per- iod 43 — period of formation of . 25 — supposed degeneration of 44 — variations in functional period 27 Placental - span fraction of total gestation .... 26 Pouchet on ovulation . -117 — on rut 117 Pregnancy, length of, in wo- man . . . . . 112. 117 Purse, evolution of ... 14 Rabbit, critical unit of .98. 120 — and hare 109 — and kangaroo .... 103 — ovulation in 120 — .sizes of embryos of . .121 Racib o rs ki on ovulation 117 Reproduction, rhythm of 89. 97 — limitations of 28. 29 Rhythm of reproduction Robinson on degeneration of yolk-sac 89. 97 19. 119 Sacrifice of embryos . .47. 113 Selenka on marsupial de- velopment 124 — on opossum . . . 40. 124 Semnopithecus, menstruation of 124 Senile atrophy of phorozoon 21. 90 Sheep, Bonnet on . .53. 125 — critical period in . . .84 — critical unit of . . . 49. 98 page Sheep, gestation of 125 — Nathusius on . . .125 — gestation of merino . . 126 — ovulation-unit of . . 49. 98 Smith, W. Tyler, on men- struation . . . . no. 122 — — views of no — - works of no Sobotta on corpus luteum 118 — on mouse 123 — on ovulation . . . .123 Southdown sheep, gestation of 126 Table of gestation-lengths . 98 — of critical units . . 36. 57. 98 — of number of critical units in development . — of ovulation-units — remarks on . Tables of Tessier Tanner on mammae 57- 98 • 99 • 99 . 118. — on parturition .... 85 Teats in Didelphys . . 29. 124 — number of 29 — in pig 29 — reservation of . . . .29 Temperature in development 38 Tessier on gestation .50. 99 — — of cow . . . .53. 101 of horse . . . .53. 101 of pig .... 53. 101 Tessier's tables .... 99 Trickbsurus, birth-period of 23 Trochophore 102 Trophoblast 18. 90 — in mammals . . . . 22. 90 Unfolding or evolution . .116 Yariations in development and number of critical units 51 — in mammalian develop- ment 37. 100 9* 132 — page Webster, J. C., on men- struation 69 Woman, length of pregnancy in 112 Yolk, acquisition of . . .16 — loss of 17 page Yolk-sac, degeneration of 19. 119 — — of mouse . . . . 19. 1 19 placenta . . . . 18. 22 Young, condition at birth of 27 — excessive number of . 28. 29 — limitations in number . . 29 6 line 12 line 25 last 33 line 36 line 44 line 48 line 88 line 125 line Errata. 16 for being read 13 line 7 5 10 20 30 5 an Par am eles marsupial Parameles Parameles Parameles Marsupial Beuteltiere bring. a transient. Perameles. mammal. Perameles. Perameles. Perameles. non-placental. Beutelfuchs. p. 127 2nd column for cannabalism read cannibalism. 2 line 18 following 8 insert after. 12 line 12 delete a transient. 15 line 3 „ the comma. 16 line 4 „ the first comma. 18 line 27 before phorozoon insert the. 18 line 29 delete the comma. 49 line 9 „ the comma after "with". 71 line 10 ,, the comma after "point". 71 line 15 „ the comma. Jena, printed by Hermann Pohle. — 1717