M '^, ¥ I # $ rS\i <^ . 22. Head of a Chick at the End of the Second Day viewed from below as a Transparent Object ........ 76 23. Diagram of the Circulation of the Yolk-Sac at the end 'of the Third Day of Incubation 85 24. Chick of the Third Day (54 hours) viewed from underneath as a Transparent Object 88 25. Head of a Chick of the Third Day viewed sideways as a Transparent Object ........... 90 26. Section through the Hind-Brain of a Chick at the end of the Third Day of Incubation . . . . . . . . -95 27. Diagrammatic Sections illustrating the Formation of the Eye. . 97 28. Diagrammatic Section of the Eye and the Optic Nerve at an early stage 98 LIST OF ILLUSTRATIONS. XVli FIG. PAGE 29. Diagrammatic representation of the Eye of the Chick of about the Third Day as seen when the head is viewed from underneath as a transparent object ......... 99 30. D, E, F. Diagrammatic Sections of the Eye of the Chick of about the Third Day . . . . . . . . . .100 31. Section of the Eye of Chick at the Eourth Day .... 104 32. Section through the Hind-Brain of a Chick at the end of the Third Day of Incubation . . . . . . . . . 1 io 33. Two Yiews of the membranous Labyrinth of Columba domestica. A from the exterior, B from the interior . . . . .112 34. Transverse Section of the Head of a Foetal Sheep (16 mm. in length) in the region of the Hind-Brain . . . . . 113 35. Section of the Head of a Foetal Sheep (20 mm. in length) . . 114 36. Section through the internal Ear of an Embryonic Sheep (28 mm. in length) . . ... . . . . . . .116 37. Head of an Embryo Chick of the Third Day viewed sideways as an Opaque Object . . . . . . . . . .118 3S. The same Head as shewn in Fig. 37, seen from the Front . 121 39 A. Diagram of the Arterial Circulation on the Third Day . . 122 39 B. Diagram of the Venous Circulation on the Third Day . .124 40. Section of the Tail-end of an Embryo (Chick) of the Third Day . 125 41. Section through the Dorsal Region of an Embryo at the commence- ment of the Third Day . .126 42. Diagram of a portion of the Digestive Tract of a Chick upon the Fourth Day. . . . . . . . . . .128 43. Four diagrams illustrating the Formation of the Lungs . . .129 44. Section through the Dorsal Region of an Embryo at the end of the Third Day .......... 135 45. Head of an Embryo Chick of the Third Day (seventy-five hours) viewed sideways as a transparent object . . . . .137 46. Embryo at the end of the Fourth Day seen as a transparent object . . . . . . . . . , . .142 • • • XV111 LIST OF ILLUSTRATIONS. FIG. PAGE 47. Section through the Lumbar Region of an Embryo at tbe End of the Fourth Day . . -144 48. A. Head of an Embryo Chick of the Fourth Day viewed from below as an opaque object. B. The same seen sideways . .146 49. Longitudinal Section of the Tail-end of an Embryo Chick at the commencement of the Third Day . . „ . . .148 50. Longitudinal Section of the Tail-end of an Embryo Chick at the middle of the Third Day . . . . . . . .149 51. Section of the intermediate Cell-mass on the Fourth Day . . 165 52. State of Arterial Circulation on the Fifth or Sixth Day . . . 169 53. Diagram of the Venous Circulation at the Commencement of the Fifth Day 170 54. Heart of a Chick on the Fourth Day of Incubation viewed from the Ventral Surface . . . . . . . . .172 55. View from above of the Investing Mass and of the Trabecuke on the Fourth Day of Incubation . . . . . . .177 56. A. Head of an Embryo Chick of the Fourth Day viewed from below as an opaque object. B. The same seen sideways. . 180 57. Head of a Chick at the Sixth Day from below . . . 181 58. Head of a Chick of the Seventh Day from below .... 182 59. Section through the Spinal Corel of a Seven Days Chick . . 18S 60. Two views of the Heart of a Chick upon the Fifth Day of Incuba- tion ..... ....... 192 61. Heart of a Chick upon the Sixth Day of Incubation, from the Ventral Surface . . . . . . . . . .193 O2. Diagram of the Venous Circulation at the Commencement of the Fifth Day 206 6$. Diagram of the Venous Circulation during the later days of Incu- bation ............ 20S 64. Diagram of the Venous Circulation of the Chick after the com- mencement of Respiration by means of the Lungs . . .211 65. State of Arterial Circulation on the Fifth or Sixth Day . . .212 LIST OF ILLUSTRATIONS. XIX FIG. TAGE 66. Diagram of the Condition of the Arches of the Aorta towards the Close of Incubation 216 67. Diagram of the Arterial System of the Adult Fowl . . .219 68. View from above of the Investing Mass and of the Trabecule on the Fourth Day of Incubation ....... 226 69. View from below of the Paired Appendages of the Skull of a Fowl on the Fourth Day of Incubation . . . . • .229 70. Side view of the Cartilaginous Cranium of a Fowl on the Seventh Day of Incubation ......••• 231 71. Embryonic Skull of a Fowl during the Second "Week of Incubation (third stage) from below . . . . . . • -234 INTRODUCTION. Every living being passes in the course of its life through a series of changes of shape and structure. These changes may, in their completest form, be considered as constituting a morphological cycle, beginning with the ovum and ending with the ovum again. Among many living beings and especially among verte- brate animals by far by the greater part of the life of the individual is spent in one particular phase, which is not only of longer duration than the rest, but also of much more importance, inasmuch as during it the greater part of the ' work ' of the living being is done. This is generally spoken of as the adult stage, and in most cases immediately precedes, or is peculiarly associated with, the completion of the morpho- logical cycle in the appearance of a new ovum. The word embryology may be generally taken to mean the study of the successive morphological phases through which a living being passes from the ovum to the adult stage, or the study of the gradual 'development' of the ovum to the adult form ; though, especially among some of the so- called lower forms of life, its meaning must be so extended as to embrace all the morphological phases of an individual life. Embryology is thus a part of and a necessary intro- duction to the wider study of ' Generation.' As a matter of E. 1 2 ON EMBRYOLOGY. history we find that the study of it sprang out of the various attempts to solve the problems of why and how living beings come into existence. It would be beyond the scope of this work to enter at all fully into any account of the earlier of these inquiries from those of Aristotle downwards ; but it may be of some use to point out the chief steps by which in modern times embryology has been established as a distinct branch of knowledge. From the very first, incubated bird's eggs, and especially hen's eggs, owing to their abundance at all seasons, and the ease with which they could be examined, became special objects of study. Aristotle examined the growing chick within the egg, and gave the name of punctum saliens to the 'bloody palpitating point,' which marks the growing heart in the early days of incubation. Since his time all observers have had recourse to the hen's egg; and though it may be urged that the highly specialised characters of the avian type unfit it for so general a purpose as that of serving as the foundation of embryology, the practical advantages of the bird's egg over either the mammalian or any other ovum, are so many, that it must always continue to be, as it has been, a chief object of study. From the time of Aristotle down to that of Fabricius of Aquapendente so little progress in real observation of facts had been made, that we find the latter anatomist [De Forma- tione Ovi et Pulli, 1 621) describing the chick as being formed out of the chalazaa of the white of the egg; a view which lived long afterwards, and whose influence may still be recognized in the names 'tread' or 'treadle' which the housewife sometimes gives to those portions of thickened albumen. Harvey was the first to clearly establish that the essential part of the hen's egg} that out of which the embryo pro- INTRODUCTION. 3 ceeded, was the cicatricula. This Fabricius had looked upon as a blemish, a scar left by a broken peduncle. In his Anatomical Exercises on the Generation of Animals (1651), Harvey describes the little cicatricula as expanding under the influence of incubation into a wider structure, which he calls the eye of the egg; and at the same time sepa- rating into a colliquamentum. In this colliquamentum, according to him, there appears, as the first rudiment of the embryo, the heart or punctum saliens, together with the blood-vessels. These gradually gather round them the solid parts of the body of the chick. Harvey clearly was of opinion that the embryo arose, by the successive formation of parts, out of the homogeneous nearly liquid colliquamentum. He was an early advocate of the doctrine of epigenesis. 'Notwithstanding the weight of Harvey's authority, the doctrine of epigenesis subsequently gave way to that of evolution, according to which the embryo pre-existed, even though invisible, in the ovum, and the changes which took place during incubation consisted not in a formation of parts, but in a growth, i. e. in an expansion with concomitant changes, of the already existing germ. Of this theory Malpighi is frequently said to have been the founder. In a limited sense this is true. In his letter to the Royal Society of London, Be Formatione Pulli in Ovo (1672), he confesses himself compelled to admit that even in unincu- bated eggs an embryo was present (Quare pulli stamina in ovo pre-existere, altioremque originem nacta esse fateri convenit). Yet he evidently struggled against such a con- clusion, and instead of developing a consistent theory of evolution, left the earliest stages of the embryo as too mysterious to be profitable objects of study, and contented himself with tracing out the events of later days. From his descriptions it is clear that his so-called unincubated eggs 1—2 4 ON EMBRYOLOGY. had under the warmth of summer already made considerable progress in development. The man who first logically worked out a theory of evolution and became its most distinguished and zealous advocate was Haller (Sur la Formation du Coeur dans le Poulet, 1758, and Elementa Physiologies, Liber xxix. 1766). This great anatomist insisted that the embryo existed even in the unincubated egg though in a rudimentary form, and indeed invisible. He supposed that it was a vermiform structure composed of all the essential parts of a full-grown animal in an undeveloped state, and that the effect of incu- bation was to educe or evolve these undeveloped organs into an adult condition. The same views were urged with cha- racteristic extravagance by Bonnet (Considerations sur les corps organises, 1762). This doctrine of evolution or prsedilineation, as it was called at the time, was doomed to be overthrown even in Haller's own day. In an inaugural dissertation entitled Theoria Generationis, published 1759, Gasper Frederick Wolff laid the foundations of not only modern Embryology, but modern Histology. He shewed that the cicatricula of the unincubated hen's egg con- sisted of a congeries of particles (such as we now call cells) all alike, or divisible into groups only, and that anything like distinct rudiments of an embryo were wholly absent. Out of these particles the embryo was built up by means of a series of successive changes (several of which he described in detail, especially in his work on the Formation of the Alimentary ( anal, 1768), part being added to part, and parts once formed b«-ing modified into fresh parts. Thus the old imperfect theory of evolution was supplanted by a view, which, under the term of epigenesis, was in reality a more complete and truer theory of evolution. Wolff also shewed that all the parts as well of plants as of animals could be conceived of INTRODUCTION. o as being arrangements of these particles or cells variously modified, and that all the phenomena of the form and structure of living beings were to be regarded as the results of a variable nutritive energy, to which he gave the name vis essentialis. Haller complained of Wolff, that he had attempted to make a great leap instead of being contented with small on- ward steps. Wolff's leap proved too great for his time. While his insight into the fundamental doctrines of histology re- mained for the most part without fruit till the next century, so also the way he opened up in embryology was successfully followed by no one for many years after. In 1816 that admirable teacher Dollinger, of Wiirzburg, induced Pander to take up the study of the incubated hen's egg. We owe to Pander (Dissertatio Inauguralis sistens Historiam Metamorphoseos quam Ovum Incubatum prioribus quinque diebus subit, and Beitriige zur Entwichelungsge- schichte des Hiihnchens im Eie) a clear and excellent descrip- tion of many of the changes which take place during the early days of incubation. It was he who introduced the term blastoderm. He too first drew attention to the distinction of the three layers, serous, mucous, and vascular. But his greatest merit perhaps consisted in the fact of his studies having been the exciting cause of those of Von Baer. Coming to Wiirzburg to study under Dollinger, and finding Pander busily engaged in his embryological work, Von Baer enthusiastically took up the same subject, and thenceforward devoted the greater part of his life to it. Of the results of his labours, which are embodied in his Entwichelungsgeschichte der Thieve, 1828, 1837, this simply may be said. Von Baer found the true line of inquiry already marked out by Wolff. He followed up that line so sedulously and with such success, that nearly all the work which has been done since his day up to the present time, in Vertebrate fi ON EMBRYOLOGY. Embryology, may be regarded as little more than an ex- tension, with corrections, of his observations. Were it de- sirable to re-publish Von Baer's work, the corrections and expansions of matters of fact necessary to bring it up to the present time, as the phrase goes, would, with some few exceptions, be of minor importance, though they might be many. The theoretical considerations embodied in his Scholia through which he interprets the morphological sig- nificance of embryological facts are of great and lasting importance, though they need some modifications in order to bring them into harmony with the theory of natural selection. Since Von Baer's time, the advances made in Vertebrate Em- bryology, through the elaborate work of Remak, the labours of Kathke, Allen Thomson and others, the admirable lectures of Kolliker, and the researches of more recent inquirers, though many and varied, cannot be said to constitute any epochs in the history of the subject, such as that which was marked by Von Baer, and before him by Wolff. We may perhaps make an exception in favour of the discovery by Purkinje, of the germinal vesicle in the fowl's ovarian ovum (1825). This led to Von Baer's discovery of the mammalian ovum (1827), which first rendered possible a consistent view of mammalian gene- ration. The study of invertebrate embryology has, on the other hand, during the last few years produced the most striking results. In the following pages we propose to follow in the path thus marked out by the history of the subject. We begin with the chick as being the animal which has been most studied, and the study of which is easiest, and most fruitful for the beginner. The first part accordingly will be devoted to a description of the changes undergone by an incubated hen's egg, especially during the early clays of incubation. We shall endeavour to explain, with such details as are necessary, INTRODUCTION. 7 the manner in which the embryo is formed, and the way in which the rudiments of the most important organs of the chick arise. We shall follow a chronological order, tracing out the changes day by da}7 (or with even shorter periods), during the first few days. We are convinced that this method (adopted by Von Baer) is on the whole the one which most commends itself to the learner. It has of course its disad- vantages ; and in several instances we have found it desirable when describing, at its appropriate date, the most striking phase in the development of an organ, at once to follow up the subsequent history, instead of giving it piecemeal after- wards. But the general advantages of the chronological method, especially when the reading of such a book as this is rendered really useful by an accompanying actual exami- nation of incubated eggs, are so great that they far outweigh the evil of any such slight irregularities. After tracing out the history of the several organs, no farther than is necessary to give a clear idea of the general course of events in each case, we propose to treat the changes and incidents of the latter days of incubation with great brevity, not attempting any special account of avian development, except in the case of the skull. And even this will be treated summarily. The First Part will therefore really be an introduction to the general facts of vertebrate embryology, the chick being taken as an example. In the Second Part we purpose to consider the embryonic histories of other vertebrates, in so far as these differ from that of the bird ; and then to treat of the development of special organs in a more complete manner. The Third Part will be devoted to an exposition of the main facts of invertebrate embryology, and to the discussion of general morphological considerations. S ON EMBRYOLOGY. The reader will scarcely fail to notice that the First Part especially is entirely confined to a simple description of observed facts, no attempt whatever being made to interpret their meanings. We have purposely pursued this course, because any interpretation of the facts of the bird's develop- ment is impossible, or at least illusory, till the history of other animals, vertebrate and invertebrate, has been studied. When all the facts are before him the reader will be in a position to judge of the interpretations offered. PART I. THE HISTOKY OF THE CHICK. CHAPTER I. THE STRUCTUKE OF THE HEX'S EGG, AND THE CHANGES WHICH TAKE PLACE UP TO THE BEGINNING OF INCUBATION. 1. In a hen's egg quite newly laid we meet with the following structures. Most external is the shell (Fig. 1, s.), composed of an organic basis, impregnated with calcic salts. It is sufficiently porous to allow of the interchange of gases between its interior and the external air, and thus the chemical processes of respiration, feeble at first, but gradually increasing in intensity, are carried on during the whole period of incubation. According to Nathusius, Zeitsch. f. Wiss. Zool. Vol. xvili. p. 225 — 270, xix. 322 — 348, XX. 106 — 120, xxi. 330 — 355, the egg-shell of birds consists of an outer thinner and an inner thicker layer. The outer layer varies con- siderably in its consistency in different species. It is soft and pliant in the hen, but in many other birds, as for instance the ostrich, is hard and friable. It is frequently striated both vertically and transversely. Pigment when present is confined to this layer. The inner layer is thicker ; and its internal surface is marked with rounded processes more or less separated from one another, whose blunt extremities are sunk into the shell-membrane. The presence of these pro- cesses must be considered as universal amongst birds. Vertical sections shew that this layer is composed of alternating horizontal laminae of transparent and opaque material, the opaque laminae being composed of exceedingly minute par- ticles of an organic nature imbedded in a matrix impregnated with calcic salts. Both layers of the shell are pierced by vertical canals, which are simple in Carinate but ramified in "Ratite birds. These canals open freely on the exterior surface and also on the interior surface in the pits between the blunt processes of the inner layer. It is probable that the outer openings of these canals become closed by the presence of moisture, so that when the shell is wet neither air nor water can pass through it. If the shell is dry, air will penetrate easily ; and if the upper layer with the free ends of the tubes be rubbed off, both water and air will pass through it without difficulty. In eggs with coloured shells the colouring matter frequently passes into the canals. 12 THE HEN S EGG. [CHAP. 2. Lining the shell, is the shell-membrane, which is double, being made up of two layers; an outer thicker (Fig. 1, s. m.), and an inner thinner one (i. s. m.). Both of these layers consist of several laminae of felted fibres of various sizes, intermediate in nature between connective and elastic fibres. Fig. i. ch.l '. Diagrammatic Section of an Untncubated Fowl's Egg (modified from Allen Thomson). hi. blastoderm, w. y. white yolk. This consists of a central flask-shaped mass and a number of layers concentrically arranged around this. y. y. yellow yolk. v. t. vitelline membrane. x. layer of more fluid albumen immediately surrounding the yolk. w. albumen consisting of alternate denser and more fluid layers, ch. I. chalaza. a. ch. air-chamber at the broad end of the egg. This chamber is merely a space left between the two layers of the shell-membrane, i. s. m. internal layer of shell- membrane, s. m. external layer of shell-membrane, s. shell. Over the greater part of the egg the two layers of the shell-membrane remain permanently in close apposition to each other; but at the broad end they tend to separate, and thus to develope between them a space into which air finds its way. This air-chamber, as it is called, is not to be found in perfectly fresh eggs, but makes its appearance in I.] THE WHITE OF THE EGG. 13 eggs which have been kept for some time, whether incubated or not, and gradually increases in size, as the white of the egg shrinks in bulk by evaporation. 3. Immediatelv beneath the shell-membrane is the white of the egg or albumen (Fig. 1, iu.), which is, chemically speaking, a mixture of various forms of proteid material, with fatty, extractive, and saline bodies. Its average composition may be taken as i2*o p. c. proteid matter, 1*5 p. c. tat and extractives, '= p. c. saline matter, chiefly sodic and potassic chlorides, with phos- phates and sulphates, S6'0 p. c. water. The white of the egg when boiled shews in section alternate concentric layers of a transparent and of a finely granular opaque material. In the natural condition, the layers corresponding to these opaque layers are composed of more fluid albumen, while those corresponding to the transparent layers are less fluid, and consist of networks of fibres, con- taining fluid in their meshes. The outer part of the white, especially in eggs which are not perfectly fresh, is more fluid than that nearer the yolk. The innermost layer, however, immediately surrounding the yolk (Fig. 1, x.), is of the more fluid finely granular kind. In eggs which have been hardened a spiral arrangement of the white may be observed, and it is possible to tear orY laminae in a spiral direction from left to right, from the broad to the narrow end of the egg-. Two twisted cords called the Chalazce (Fig. 1, ch. L), com- posed of coiled membranous layers of the less fluid albumen, run from the two extremities of the egg to the opposite portions of the yolk. Their inner extremities expand and merge into the layer of denser albumen surrounding the fluid layer next the yolk. Their outer extremities are free, and do not quite reach the outer layer of the white. Thus they cannot serve to suspend the yolk, although they may help to keep it in position, by acting as elastic pads. The interior of each chalaza presents the appearance of a suc- cession of opaque white knots ; hence the name chalaza?, grandines (hailstones). 4. The yolk is enclosed in the vitelline membrane (Fig. 1, v. t.)} a transparent somewhat elastic membrane easily 14 THE HEN'S EGG. [CHAP. thrown into creases and wrinkles. It misdit almost be called structureless, but under a high power a fine fibrillation is visible, and a transverse section has a dotted or punctated appearance ; it is probably therefore composed of fibres. Its affinities are with elastic rather than connective tissue. The vitelline membrane of most vertebrates is perforated by fine pores. These are largest in osseous fishes and much finer in mammals; they have not been found in the vitelline membrane of birds. 5. The whole space within the vitelline membrane is occupied by the yolk. To the naked eye this appears toler- ably uniform throughout, except at one particular point of its surface, at which may be seen, lying immediately under the vitelline membrane, a small white disc, about 4 mm. in diameter. This is the blastoderm, or cicatricida. A tolerably typical cicatricula in a fecundated egg will shew an outer white rim of some little breadth, and within that a circular transparent area, in the centre of which, again, there is an opacity, varying in appearance, sometimes uniform, and sometimes clotted. The^ disc is always found to be uppermost whatever be the position of the egg, provided there is no restraint to the rotation of the yolk. The explanation of this is to be sought for in the lighter specific gravity of that portion of the yolk which is in the neighbourhood of the disc, and the phenomenon is not in any way due to the action of the chalazse. A section of the yolk of a hard-boiled egg will shew that it is not perfectly uniform throughout, but that there is a portion of it having the form of a flask, with a funnel- shaped neck, which, when the egg is boiled, does not become so solid as the rest of the yolk, but remains more or less fluid. The expanded neck of this flask-shaped space is situated immediately underneath the disc, while its bulbous enlarge- ment is about the middle of the yolk. We shall return to it directly. 6. The great mass of the yolk is composed of what is known as the yellow yolk (Fig. 1, y. y.). This consists of spheres (Fig. 2, A.) of from 25//, to 100/a1 in diameter, never containing a nucleus, but filled with numerous minute highly refractive granules; these spheres are very delicate and easily 1 /x— ooi mm. I-] THE WHITE YOLK. 15 destroyed by crushing. When boiled or otherwise hardened in situ, they assume a polyhedral form, from mutual pressure. The granules they contain seem to be of an albuminous nature, as they are insoluble in ether or alcohol. Chemically speaking the yolk is characterized by the presence in large quantities of a proteid matter, having many affinities with globulin, and called vitellin. This exists in peculiar association with the remarkable body Lecithin. (Compare Hoppe-Seyler, Hdb. Phijs. Chem. Anal.) Other fatty bodies, colouring matters, extractives (and, according to Dareste, starch in small quan- tities), &c. are also present. Miescher (Hoppe-Seyler, Chem. Untersuch. p. 502) states that a considerable quantity of nuclein may be obtained from the yolk, probably from the spherules of the white yolk. Fig. 2. A. Yellow yolk-sphere filled with fine granules. The outline of the sphere has been rendered too bold. B. White yolk-spheres and spherules of various sizes and presenting different appearances. (It is very difficult in a woodcut to give a satisfactory repre- sentation of these peculiar structures.) 7. The yellow yolk thus forming the great mass of the entire yolk is clothed externally by a thin layer of a different material, known as the white yolk, which at the edge of the blastoderm passes underneath the disc, and becoming thicker at this spot forms, as it were, a bed on which the blastoderm rests. Immediately under the middle of the blastoderm this bed of white yolk is connected, by a narrow neck, with a central mass of similar material, lying in the middle of the yolk (Fig. 1, w. y.). When boiled, or otherwise hardened, the white yolk does not become so solid as the yellow yolk; hence the appearances to be seen in sections of the hardened yolk. The upper expanded extremity of this neck- of white yolk is generally known as the "nucleus of Pander." Concentric to the outer enveloping layer of white yolk there are within the yolk other inner layers of the same substance, which cause sections of the hardened yolk to 1G THE HEN'S EGG. [CHAP. appear to be composed of alternate concentric thicker laminae of darker (yellow) yolk, and thinner laminae of lighter (white) yolk (Fig. 1, w, y). 8. The microscopical characters of the white yolk are very different from those of the yellow yolk. It is composed of spheres (Fig. 2, B.) for the most part smaller than those of the yellow yolk (4yu, — 75//.) , with a highly refractive nucleus- like body often as small as 1/x in the interior of each; and also of larger spheres, each of which contains a number of spherules, similar to the smaller spheres ; these latter appear- ing to have passed into the larger spheres, by a process of inclusion. There has been a considerable amount of controversy as to whether these elements possess a membrane ; there is little doubt however that there is no membrane present. It has also been disputed as to whether they should be considered as true cells or not. If by definition a cell must contain a nucleus, they can hardly be considered as such, since the characters of the highly refractive bodies con- tained in them have nothing in common with nuclei. We shall give later on reasons for thinking that they may however, as a result of incubation, become veritable cells. Another feature of the white yolk, according to His, is that in the region of the blastoderm it contains numerous large vacuoles filled with fluid ; they are sufficiently large to be seen with the naked eye, but do not seem to be present in the ripe ovarian ovum. 9. It is now necessary to return to the blastoderm. In this, as we have already said, the naked eye can distinguish an opaque white rim surrounding a more transparent central area, in the middle of which again is a white spot of variable appearance. In an unfecundated cicatricula the white disc is simply marked with a number of irregular clear spaces, there being no proper division into a transparent centre and an opaque rim. The opaque rim is the commencement of what we shall henceforward speak of as the area opaca ; the central trans- parent portion is in the same way the beginning of the area pellucida. At this stage the distinction between these two areas depends entirely on the disposition of the white yolk beneath them, for the blastoderm when lifted up from the white yolk on which it rests appears uniform throughout. In the part corresponding to the area opaca the blastoderm rests immediately on the white yolk, which here forms a I.] THE BLASTODERM. 17 somewhat raised ring, often spoken of as the germinal wall ; underneath the area pellucida is a shallow space containing a nearly clear fluid, to the presence of which the central transparency seems to be due. The white spot in the middle of the area pellucida appears to be the nucleus of Pander shining through. Vertical sections of the blastoderm shew that it is formed of two layers. The upper of these two layers is com- posed, see Fig. 3, ep, of a single layer of cells, with their long axes arranged vertically, adhering together so as to form a distinct membrane, the edge of which rests upon the white volk. After staining with silver nitrate, this membrane viewed from above shews a mosaic of uniform polygonal cells. Each cell is composed of granular protoplasm filled with highly refractive globules; in most of the cells an oval nucleus may be distinguished, and is most probably present in all. They are of a uniform size (about 9 fx) over the opaque and the pellucid areas. The under layer (Fig. 3, I), is composed of cells which vary considerably in diameter ; but even the smaller cells of this layer are larger than the cells of the upper layer. They are spherical, and so filled with granules and highly refractive globules, that a nucleus can rarely be seen in them: in the larger cells these globules contain a highly refractive body very similar to that present in the white yolk spheres, from the smaller kinds of which indeed they are scarcely distinguishable. The cells of this layer do not form a distinct membrane like the cells of the upper layer, but lie as a somewhat irregular network of cells between the upper layer and the bed of white yolk on which the blastoderm rests. The lowest are generally the largest ; in addition w7e find a few still larger cells generally separated by a small interval from the remainder of the cells of the lower layer, and resting directly upon the white yolk (Fig. 3, b). These are frequently spoken of as formative cells; they are however similar in character and' indeed connected by gradations with the larger cells of the lower layer. Their mode of formation during segmentation will be subsequently de- scribed. E. 2 18 THE HENS EGG. [CHAP. «- — M^telM <&- O Fig. 3. Section of a Blastoderm of a Fowl's Egg at the commencement of incubation. The thin but complete upper layer ep composed of columnar cells rests on the incomplete lower layer I, composed of larger and more granular bodies. The lower layer is thicker in some places than in others, and is especially thick at the periphery. The line below the under layer marks the upper surface of the white yolk. The larger so-called formative cells are seen at b, lying on the white yolk. The figure does not take in quite the whole breadth of the blastoderm ; but the reader must understand that both to the right hand and to the left ep is continued farther than I, so that at the extreme edge it rests directly on the white }olk. Over nearly the whole of the blastoderm the upper layer rests on the under layer. At the circumference however the upper layer stretches for a short distance beyond the under layer, and here consequently rests directly on the white yolk, and forms that part of the blastoderm known as the area opaca. 10. To recapitulate: — In the normal unincubated hen's egg we recognize the blastoderm, consisting of a complete upper layer of smaller nucleated granular cells and a more or less incomplete under layer of larger cells, filled with larger granules ; in these lower cells nuclei are rarely visible. The thin flat disc so formed rests, at the uppermost part of the entire yolk, on a bed of white yolk so disposed as to give rise to the appearance in the blastodermic disc it- self of an area opaca and an area pellucida. The great mass of the entire yolk consists of the so-called yellow yolk composed of gra- nular spheres. The white yolk is composed of smaller spheres of peculiar structure, and exists, in small part, as a thin coating around, and as thin concentric lamina? in the sub- *• .§ stance of the yellow yolk, but chiefly in the form of a flask-shaped mass in the interior of the yolk, the upper somewhat expanded top of the neck m m m I.] THE OVARIAN OVUM. 19 of which forms the bed on which the blastoderm rests. The whole yolk is invested with the vitelline membrane, this again with the white ; and the whole is covered with two shell-membranes and a shell. 11. Such an egg has however undergone most important changes while still within the body of the hen ; and in order to understand the nature of the structures which have just been described, it will be necessary to trace briefly the history of the egg from the stage when it exists as a so-called ripe ovarian ovum in the ovary of a hen up to the time when it is laid. If one of the largest capsules of the ovary of a hen which is laying regularly be opened, it will be found to contain a nearly spherical (or more correctly, ellipsoidal with but slightly unequal axes) yellow body enclosed in a delicate membrane. This is the ovarian ovum or esf2f. Examiued with care the ovum, which is tolerably uniform in appearance, will be found to be marked at one spot (generally lacing the stalk of the capsule and forming the pole of the shorter axis of the ovum) by a small disc differing in appearance from the rest of the ovum. This disc is known as the germinal disc or discus proligerus. It consists of a lenticular mass of Fig. 4, Section through the Germinal Disc of the ripe Ovarian Ovuji op a Fowl while yet enclosed in its Capsule. a. Connective-tissue capsule of the ovum. b. epithelium of the capsule, at the surface of which nearest the ovum lies the vitelline membrane, c. granular material of the germinal disc, which becomes converted into the blastoderm. (This is not very well represented in the woodcut. In sections which have been hardened in chromic acid it consists of fine granules.) w. y, white yolk, which passes insensibly into the fine granular material of the disc, x, germinal vesicle enclosed in a distinct membrane, but shrivelled up by the action of the chromic acid. The material enclosed in the membrane of the vesicle is in the hardened specimens finely granular. y, space originally completely filled up by the germinal vesicle, before the latter was shrivelled up by the action of the chromic acid. 20 THE HEN'S EGG. [CHAP. protoplasm (Fig. 4, r), imbedded in which is a highly refrac- tive globular or ellipsoidal body (Fig. 4, x), about 310/-1 in diameter, called the germinal vesicle, in the interior of which again is a small body, the germinal spot. The rest of the ovum is known as the yolk. This consists of two elements, the white yolk- and the yellow yolk-spheres, which are distributed respectively very much in the same way as in the laid egg, the yellow yolk forming the mass of the ovum, and the white yolk being gathered underneath and around the disc (Fig. 4, iv. y), and also forming a flask- shaped mass in the interior of the ovum. The delicate membrane surrounding the whole is the vitelline membrane. Oellaclier's (Untersuckung fiber die Furclmng unci Blatterbildung in Huh- vereie. Studien aus dem Institute fur expcrimentale pathologie in Wien aus dem Jahre 1869, P1- l) account of the ovarian ovum differs considerably from that given above. He finds in the neighbourhood of the blastoderm a finely granular material, within which lies a body appearing circular when viewed from above, but having in section a somewhat quadrilateral shape; its side-walls, however, are curved, with their convexity turned inwards. At the bottom of it lies an oval cavity with doubly contoured walls, and at its upper surface placed somewhat excentrically a semieircular space filled with clear material. Oellacher believes that the quadrilateral body which he thus describes is the germinal vesicle which has commenced to undergo a retrogressive meta- morphosis. For the further stages in the metamorphosis, and for further par- ticulars, vide Section 13. The circular hole beneath the vesicle is probably merely filled with fluid and is due to the contractions of the germ. 12. When the ovarian ovum is ripe and about to be dis- charged from the ovary, its capsule is clasped by the dilated termination of the oviduct. The capsule then bursts, and the ovum escapes into the oviduct, its longer axis corre- sponding with the long axis of the oviduct, the germinal disc therefore being to one side. At the time of the bursting of the capsule the germinal vesicle disappears. In describing the changes which take place in the oviduct, it will be convenient, following the order previously adopted, to treat first of all of the formation of the accessory parts of the egg. These are secreted by the glandular walls of the oviduct. This organ therefore requires some descrip- tion. It may be said to consist of four parts ; — 1st. The dilated proximal extremity. 2nd. A long tubular portion, opening by a narrow neck or isthmus into the 3rd portion, which is much dilated, and has been called the uterus; the 4th part is somewhat narrow, and leads from the uterus I.] DESCENT OF THE OVUM. 21 into the cloaca. The whole of the mucous membrane lining the oviduct is largely ciliated. The accessory parts of the egg are entirely formed in the 2nd and 3rd portions. The layer of albumen which imme- diately surrounds the yolk is first deposited; the chalazas are next formed. Their spiral character and the less distinctly marked spiral arrangement of the whole albumen is brought about by the motion of the egg along the spiral ridges into which the interior of the second or tubular portion of the oviduct is thrown. The spirals of the two chalazse are in different directions. This is probably produced by their peripheral ends remaining fixed while the yolk to which their central ends are attached is caused to rotate by the contractions of the oviduct. During the formation of the chalazse the rest of the albumen is also deposited ; and finally the shell-membrane is formed in the narrow neck of the 2nd portion, by the fibrillation of the most external layer of albumen. The egg passes through the 2nd portion in little more than 3 hours. In the 3rd portion the shell is formed. The mucous membrane of this part is raised into numerous flattened folds, like large villi, containing follicu- lar glands. From these a thick white fluid is poured out, which soon forms a kind of covering to the egg, in which the inorganic particles are deposited. In this portion of the oviduct the egg remains from 12 to 18 hours, during which time the shell acquires its normal consistency. At the time of laying it is expelled from the uterus by violent muscular contractions, and passes with its narrow end downwards alono- the remainder of the oviduct, to reach the exterior. 13. We have now to trace out the changes which take place in the germinal disc, during the passage of the egg down the oviduct. By the time when the egg becomes clasped by the expanded extremity of the oviduct the germinal vesicle has, according to Oellaci.er (loc cit. and also Archiv. fiir Micr. Anat. Vol. vnr. 1872. p. 18), undergone still further retrogressive changes. It has now become very much flattened and closely applied to the vitelline membrane. Both this and former stages, if we may judge from the analogy of osseous fishes, are preparatory to the whole germinal vesicle being bodily ejected from the germinal disc. For further particulars vide Oellacher, Arckiv.fiir Micr. Anat. Vol. viir. pp. 1 — 26. Impregnation occurs in the upper portion of the oviduct ; the spermatozoa being found actively moving in a fluid which is there contained. 22 THE HENS EGG. [CHAP. It is not certain whether impregnation takes place previous to the deposition of the albumen, or whetber the spermatozoa bore their way through tbe albumen. The former would appear to be the more probable view, though the fact that Oellacher has found spermatozoa in the albumen, speaks in favour of their being involved in the depositing albumen, and so being brought in contact with tue blastoderm. According to Coste, Ilistoire du developpement des corps organizes, the access of the cock to the hen once in seven days is sufficient. We have no positive evidence that the spermatozoa make their way through the vitelline membrane and so gain access to the germinal, disc; but, as will be seen in a later part of this work, analogy renders such an event probable. 14. At about the time when the shell is being formed round the esfg, the germinal disc undergoes a remarkable change, known as segmentation. We shall have occasion to treat more fully of the nature of segmentation when we come to consider the amphibian ovum in which the various steps of the process may be more easily and satisfactorily traced. Meanwhile, inasmuch as the segmentation of the ABC Surface Views of the early Stages of the Segmentation in a Fowl's Egg. (After Coste.) A represents the earliest stage. The first furrow (b) has begun to make its appearance in the centre of the germinal disc, whose periphery is marked by the line a. In B, the first furrow is completed right across the disc, and a second similar furrow at nearly right angles to the first has appeared. The disc thus becomes divided somewhat irregularly into quadrants by four (half) furrows. In a later stage (C) the meridian furrows b have increased in number, from four, as in B, to nine, and cross furrows have also made their appearance. The disc is thus cut up into small central (c) and larger peripheral (d) segments. Several new cross furrows are seen just beginning, as ex. (jr. close to the end of the line of reference d. I-] SEGMENTATION. 23 germinal disc of a hen's egg differs materially from the segmentation of the entire ovum of an amphibian, the former may briefly be described here. Viewed from above, a furrow is seen to make its appear- ance, running across the germinal disc and dividing it into two halves (Fig. 5, A). This primary furrow is succeeded by a second at right angles to itself. The surface thus becomes divided into four segments or quadrants (Fig. 5, B). Each of these is again bisected by radiating furrows, and thus the number of segments is increased from four to eight (it may be seven or nine). The central portion of each segment is then, by a cross furrow, cut off from the peripheral portion, giving rise to the appearance of a number of central smaller segments, surrounded by more external elongated segments (Fig. 5, C). Division of the segments now proceeds rapidly by means of furrows running apparently in all directions. And it is Fig. 6. Surface View op the Germinal Disc or a Hen's Egg during the later Stages of Segmentation. (Chromic Acid Preparation.) At c in the centre of the disc the segmentation masses are very small and numerous. At b. nearer the edge, they are larger and fewer; while those at the extreme margin a are largest and fewest of all. It will be noticed that the radiating furrows marking off the segments a do not as yet reach to the extreme margin e of the disc. The drawing is completed in one quadrant only ; it wall of course be under- stood that the whole circle ought to be filled up in a precisely similar manner. 24 -'