Cornell University Library introduction to vertebrate embryology CORNELL UNIVERSITY MEDICAL LIBRARY THACA DIVISIO: 4G ip a Cornell University Library The original of this book is in the Cornell University Library. There are no known copyright restrictions in the United States on the use of the text. http://www. archive.org/details/cu31924002907602 “spyog efnozado ay Aq uMoISI9A0 Buraq are spt ayy YOryM ur a8eys ‘CQ ‘s|[TS [eussyxe podoyaaap-]jom Wau aSeis ‘ay “syuauudas apasnur pedeys-< ay} pur si[is peuzayxo 943 Jo sSutuutGeq ayy Surmoys oArquia ‘yy “oAIquia pasyesuoja alow [[Ns e ‘7 = “SplOF [EIIISIA JY} JO pur [1e3 ay3 Jo Suruurgaq aya Surmoys ‘apis ay3 Wosy Uses se OAIQUIa payeSUOTS APYSys Ly “pus Joris" ayy WoIZ Ud—as SE Spjoj [EINOU OY) va ‘ga0o1s pUe Sprof [eINau ayy jo Suruutsaq ay3 pue ssodoyseyq pesojo ayy Surmoys ‘mora sopeysod ‘7 ‘aseis Snjd-yJoA ‘zy “‘uoEuaUlsas Joe] ‘9 ‘aBeys ][90-FQ ynoqe ‘y ‘aBejs [Jeo-gl ‘ZF ‘aseIs [poxg ‘Gg ‘eseas []s0-p ‘D ‘aeys TJoo-g ‘gy “88a pejueudesun ‘py (‘sjapour xem sJo[8arz yo ydeisojoyd ew worg) ‘OAUMWA SOYA AHL AO INIWAOTAAAC AHL NI SHDVIS ATYVA—'! ‘SIT € An Introduction to Vertebrate Embryology Based on the Study of the Frog and the Chick By Albert Moore Reese, Ph.D. (Johns Hopkins) Associate Professor of Histology in Syracuse University, and Lecturer on Histology and Embryology in the College of Medicine With 84 Illustrations G. P. Putnam’s Sons Jon tess QL 955 ere mMe d. $76 CopyRIGHT, 1904 BY ALBERT MOORE REESE Published, April, 1904 The thnickerbocker Press, Hew Work ‘PREFACE HIS small volume is the result of a need that the author has felt, for some years, for a concise text-book of embryology that described the development of both the chick and the frog. The only other single book, with which the author is acquainted, that describes the development of both these commonly studied forms is the large volume of Marshall, which is too cumbersome and expen- sive for a general, class text-book. The present volume is intended as an out- line, from which the student may learn the main facts about the embryology of the two animals in question ; and the instructor is sup- posed in his lectures to enlarge upon this out- line to any extent that he may see fit. Since the needs of the medical student have been largely considered in compiling the text, very little space has been given to theoretical dis- cussions ; these may be given by the instructor, at whatever length may seem desirable. Vv vi Preface For purely pedagogical reasons, the develop- ment of the chick has been described by periods ; that is, all the changes that take place during a certain period are described in one section, instead of describing at one time, the complete development of any one organ. The author does not lay claim to any great originality in the compilation of this volume. He has sought simply to collect, into conven- ient form, the more important facts of the sub- ject under discussion, together with a series of figures that will suitably illustrate these facts. Marshall and Morgan have been quoted at length, in several instances. Nearly all the figures have been taken from well-known text-books of embryology, the au- thor being stated in every case. I am especially indebted to Dr. Charles S. Minot and his publishers, P. Blakiston’s Son & Co., for the loan of the electros of the numer- ous figures that have been taken from Professor Minot’s recent Laboratory Text of Embryology. ALBERT M. REESE SYRACUSE UNIVERSITY, March 15, 1904. THE THE THE THE THE THE DEVELOPMENT DEVELOPMENT DEVELOPMENT DEVELOPMENT DEVELOPMENT DEVELOPMENT CONTENTS CHAPTER I OF THE FROG CHAPTER II OF THE CHICK CHAPTER III OF THE FIRST DAY CHAPTER IV OF THE SECOND DAY CHAPTER V OF THE THIRD DAY CHAPTER VI OF THE FOURTH DAY vii PAGE go 120 139 222 Vill Contents CHAPTER VII THE DEVELOPMENT OF THE FIFTH DAY CHAPTER VIII THE DEVELOPMENT FROM THE SIXTH DAY TO THE TIME OF HATCHING INDEX PAGE 263 279 285 ILLUSTRATIONS THE DEVELOPMENT OF THE FROG FIGURE 1. Early Stages in the Development of the Frog 10. Embryo. (From a photograph of Ziegler’s wax models) . : : Frontispiece . Egg of Starfish. (After Gegenbaur) . Ovarian Egg of Frog ; . 4. Various Stages in the Development of the Frog. (After Brehm from Marshall) . Vertical Sections of Segmenting Eggs Sections of Three Stages of Segmentation of the Frog’s Egg . . Longitudinal Vertical Section of a Frog Em- bryo, Showing Commencing Invagination. (After Marshall) . Longitudinal Vertical Section of a jigs Em- bryo at a Later Stage in the Formation of the Mesenteron. (After Marshall) Longitudinal Vertical Section through a Frog Embryo, Showing the Completion of the Mesenteron. (After Marshall) . Transverse Section through the Middle of a Frog Embryo, at about the Stage Repre- sented in Fig. 9. (After Marshall) ix PAGE 19 20 22 23 26 ».¢ Illustrations FIGURE II, 12. 13. 14. 1S. 16. 17. 18. 19. 20. Stages in the Early Development of the Frog, Seen Obliquely from the Posterior End. (After Ziegler from Marshall) A—Sagittal Section of Embryo Shown in Fig. 11, D. B—Sagittal Section of the Embryo Shown in Fig. 11, E (After Marshall) Transverse Section of a Frog Embryo during the Formation of the Neural Canat. (After Marshall) . Sagittal Section of a Tadpole at the Time ot Hatching. (After Marshall) Transverse Section across the Middle of the Embryo Shown in Fig. rr, D. (After Mar- shall) The Brain of the Frog. (After Marshall) Longitudinal Vertical Section through the Anterior End of a Tadpole Shortly after Hatching. (After Marshall) Longitudinal Vertical Section through the Anterior Part of a Tadpole about the Time of Appearance of the Hind Legs. (After Marshall) . : Half Sections in the Transverse Plane of a Tadpole 1o mm. Long (left half) and of a Tadpole 12 mm. pene (right half), (After Marshall) . : : Transverse Section ee the Head of a Tadpole 64% mm. in Length, about the Time of Hatching. (After Marshall) PAGE 30 33 35 39 4I 44 47 49 52 Illustrations FIGURE 21. 22. 233 5 24. 25. 26. 27. 28. 29. 30. Transverse Section through the Region of the Hind-Brain of a Young Tadpole Horizontal Section of a Tadpole at the Time of Hatching. (After Marshall) . Diagrammatic Figures of a 7-mm. Tadpole, Shortly after Hatching, Showing the Ar- rangement of the Blood Vessels: A—from Below; B—from the Side. (After Marshall) A 12-mm. Tadpole, Dissected from the Ventral Side. (After Marshall) Diagrams to Illustrate the Mode of Develop- ment of the Heart A Diagrammatic Figure of the Head and Neck of a 12-mm. Tadpole, from the Right Side, to Show the Heart and Branchial Vessels. Gills and Gill Capillaries not Represented. (After Marshall) Diagrammatic Figure of the Arterial System of the Male Frog, from Right Side. (After Marshall) . 5 ; . A 4o-mm. Tadpole Dissected from the Ventral Surface. (After Marshall). A Tailed Frog, near the Close of Metamor- phosis, Dissected from the Ventral Surface. (After Marshall) Diagrams to Illustrate the Development of the Head-Kidney. (Somewhat altered from Morgan) xi PAGE 57 59 63 65 68 vas 73 76 80 82 xii Illustrations FIGURE ; PAGE 31. Transverse Section through the Body of a Tadpole at the Time of Hatching. (After Marshall) . : : ‘ ; é . 84 32. The Skull of ar2-mm. Tadpole: A—seen from the Right Side; B—seen from Dorsal Side; C—seen from the Ventral Surface. (After Marshall) . j P ; : : . 86 Tue DEVELOPMENT OF THE CHICK 33. Semi-Diagrammatic View of the Egg of the Fowl, at the Time of Laying. (After Parker and Haswell, slightly altered from Mar- shall) . a. ; P : : . 92 34. A—Yolk with the Blastoderm in the Centre, the Latter Showing the First Two Cleavage Planes. B—The Blastoderm on a Larger Scale and at a Later Stage of Segmentation. (After Duval) . é ‘ : , 96 35. Surface View of the Blastoderm at a Later Stage of Segmentation than that Shown in Fig. 34, B. (After Duval) : : . 98 36. Longitudinal Section of the Blastoderm after the Completion of Segmentation. (After Duval). : : : ‘ : 104 37. Series of Diagrams to Illustrate the Formation of the Chick Embryo, Especially the Rela- tions of the Embryo, Yolk-Sac, Amnion, and Allantois. (After Foster and Balfour), 106, 109, 112 Illustrations FIGURE 38. A—B—C—D—Diagrams Illustrating the De- velopment of the Foetal Membranes of a Bird. (After Parker and Haswell) . 117, 39. Part of a Section through the Blastoderm after the Formation of the Definite Entoderm or Hypoblast. (After Duval) 40. Surface View of the Embryo at about the Six- teenth Hour of Incubation. (After Duval) 41. Median Portion of a Transverse Section of an Embryo at the Time of Formation of the Primitive Streak. (After Duval) 42. Transverse Section of an Embryo of about Twenty-one Hours, through the Anterior Part of the Medullary Folds. (After Duval) 43. Three Transverse Sections across the Caudal End of the Medullary Groove of a Chick Embryo with Seven ee sae Minot) 44. Surface View of Embryo at the Peees -third Hour of Incubation. (After Duval) . 44A. Anterior End of the Preceding Figure, More Highly Magnified to Show the Details. (After Duval) 45. Sagittal Section of an Embryo of neem: six Hours. (After Duval) 46. Diagrammatic Representations of Chick Em- bryos: A—after Twenty Hours’ Incuba- tion; B—after Twenty-four Hours’ Incuba- tion. (After Parker and Haswell from Marshall) . I21 122 124 126 128 129 131 133 XIV Illustrations FIGURE 2 47. Surface View (dorsal) of an Embryo of Thirty- three Hours. (After Duval) : 48. Transverse Section of a Chick Embryo with Seven Segments, to Show the Beginning of the Formation of the Heart. (After Minot) 49. Ventral View of the Anterior Region of the Embryo Shown in Fig. 47. (After Duval) 50. Sagittal Section of the Embryo Shown in Fig. 47. (After Duval) 50A. Diagrammatic Representation of Fig. 50. (After Foster and Balfour) ‘ : 51. Sagittal Section of an Embryo Chick of Eighty- two Hours. (After Duval) ; 52. Transverse Section of a Chick Embryo with about Twenty-eight Segments. (After Minot) 53. Transverse Section through the Heart Region of an Embryo of Thirty-three Hours. (After Duval) ‘ ‘ ; 54. Transverse Section through the Dorsal Region of an Embryo of Forty-six Hours. (After Duval) ; 55. Transverse Section through a Chick Embryo to Show a Slightly Later Stage in the De- velopment of the Heart than is Shown in Fig. 48. (After Minot) : 56. Diagram of the Circulation of a Chick bie at the End of the Third Day of Incubation, as Seen from Below. (After Minot) . 57. Surface View of an Embryo of Fifty-two Hours. (After Duval) PAGE 153 156 159 161 166 175 Illustrations FIGURE 58. 59- 60. 61. 62. 63. 64. 65. 66. 67. 68. Transverse Section through the Anterior Region of a Chick Embryo with about Twenty-eight Segments. (After Minot) Transverse Section through the Anterior Region of a Chick Embryo with about Twenty-eight Segments. (After Minot) Transverse Section through the Fore-Brain of a Chick of Fifty to Sixty Hours’ Incubation The Eye of a Bird: A—Sagittal Section; B— Surface View of Entire Organ. (After Par- ker and Haswell from Vogt and Yung) Transverse Section through the Anterior Region of a Chick Embryo of about Twenty- eight Segments. (After Minot) . Transverse Section through the Anterior Part of an Embryo of Sixty-eight Hours. (After Duval) : : ‘ : ‘ ; : The Head of an Embryo Chick at the End of the Fifth Day of Incubation, Seen from Below. (After Marshall) . Diagram of the Arterial Circulation on the Third Day. (After Foster and Balfour) Diagram of the Venous Circulation of the Third Day. (After Foster and Balfour) Diagram of a Portion of the Digestive Tract of a Chick Embryo during the Fourth Day. (Foster and Balfour from Gétte) ; Transverse Section of an Embryo of the Fourth Day. (After Duval) XV PAGE 177 180 182 184 186 192 200 201 205 208 XVI Illustrations FIGURE 69. 70. 7X. 72. 73- he 75- 76, 77- 78. Transverse Section just Posterior to the Pre- ceding. (After Duval) Transverse Section through the Dorsal Region of an Embryo of Sixty-eight Hours. (After Duval) ‘ = ‘ Transverse Section Anterior to the Preceding. (After Duval) Two Stages in the Development of the Chick Embryo: A—at about the Fifth Day; B— at about the Ninth Day. (After Parker and Haswell from Duval) . Transverse Section through the Dorsal Region of a Chick Embryo of Ninety-six Hours. (After Duval) Transverse Section through the Wolffian Body. (After Duval) ; : ala Heart of a Chick on the Fourth Day, Ventral View. (After Foster and Balfour) Diagrammatic Figure Showing the Arrange- ment of the Blood Vessels in a Chick Em- bryo at the End of the Fifth Day. (After Marshall) . ; : , Diagram of the Venous Circulation at the Com- mencement of the Fifth Day. (After Foster and Balfour) Diagram of the Venous Circulation during the Later Days of Incubation. (After Foster and Balfour) PAGE 213 215 219 225 231 239 242 247 252 Illustrations FIGURE 79: 80. 81. 82. 83. 84. Diagram of the Venous Circulation after the Commencement of Respiration by Means of the Lungs. (After Foster and Balfour) Egg of Chick with Embryo and Feetal Appen- dages. (After Parker and Haswell from Duval) Two Views of the Heart of a Chick on the Fifth Day of Incubation: A—Ventral; B— Dorsal View. (After Foster and Balfour) . Heart of a Chick on the Sixth Day, Ventral View. (After Foster and Balfour) Transverse Section through the Dorsal Region of a Chick Embryo with about Twenty-eight Segments. (After Minot) . Section through an Advanced Embryo of a Rabbit, to Show how the Pericardial Cavity Becomes Surrounded by the Pleural Cavi- ties. (After Foster and Balfour) XVii PAGE 257 266 268 271 274 AN INTRODUCTION TO VERTEBRATE EMBRYOLOGY CHAPTER I THE DEVELOPMENT OF THE FROG INTRODUCTION HE eggs of the common frogs (Rana tem- porarza and allied species) may usually be found without difficulty in small ponds and pools of water, where they are laid, in the early spring, soon after the melting of the ice. They may easily be kept alive, in the laboratory, in shallow dishes of water, and their entire development thus observed. Their rate of development varies greatly with the tem- perature, so that if it be desirable, for any reason, to hurry the development, all that is necessary is to place the aquarium in a warm place ; or if it be desired to keep a lot of eggs I 2 Vertebrate Embryology in a certain state of development, this may be accomplished by keeping the aquarium in a cool place or by putting lumps of ice in the water. During the act of spawning, which may last several days, the male clasps the female firmly with his fore legs and fertilizes the eggs as they leave the cloaca. With the act of spawn- ing the parental instinct ceases, and the eggs are left to themselves, to develop or perish as the case may be. As in the laboratory, so in nature, the rate of development depends upon the temperature of the water, but in a few days or a week the eggs have lost their spherical form and have be- come ovoid in shape; and in about ten days the head, body, and tail are marked off from each other by slight constrictions. The embryo now elongates rapidly and by the end of the second week is provided with three pairs of tiny ex- ternal gills, and is able to work its way out of the jelly-like mass with which it is surrounded, and to swim freely in the water. At this time it has no true mouth, and so is dependent, for growth, upon the granules of yolk which were contained in the egg. For several days, until the appearance of the mouth, the tadpole is pro- vided with a horseshoe-shaped sucker on the The Development of the Frog 3 lower side of the head, by means of which it attaches itself to any solid body that may be in the water. As the mouth is being formed, the digestive tract becomes greatly elongated, so that the abdominal region of the body becomes rounded and swollen by the coiled mass of the intestine lying within. Being now provided with horny jaws, the young tadpole feeds actively upon the plants of its habitat, and is, therefore, no longer dependent upon the yolk for growth. The gill-clefts make their appearance, at about this time, as four pairs of slit-like open- ings which connect the pharynx with the ex- terior. The edges of these slits become folded to form the internal gills, and as the internal gills increase in size, the external gills gradu- ally diminish and are covered by two folds of skin which grow back over them from in front. These two opercular folds fuse to- gether along the mid-ventral line, and their posterior edges fuse with the body-wall behind the gills, so that the latter are completely en- closed except for a sort of spout on the left side, through which water, taken into the gill- chamber through the mouth, to bathe the gills, passes again to the exterior. 4 Vertebrate Embryology The young tadpole has now practically the structure and habits of a fish, but very soon the rudiments of the hind legs appear as small protuberances at the base of the tail, one on each side of the cloaca ; and by the eighth week the joints and toes are formed, and the legs have about the same structure as in the adult. The fore legs are formed at about the same time as are the hind legs, but they are hidden, for some time, by the operculum. The left fore leg, in the course of two or three weeks, projects through the above-mentioned opercu- lar spout, while the leg of the opposite side has to force its way directly through the oper- cular fold. The lungs, in the meantime, have become functional and the tadpole frequently comes to the surface to breathe, although, for a time, respiration takes place by means of both lungs and gills. Before this remarkable metamorphosis is complete and the frog is ready to begin life as an air-breathing animal, the tail must be com- pletely absorbed, the mouth, eyes, and other structures must be greatly changed, and the gills, gill-clefts, and other fish-like structures must diminish or entirely disappear. The Development of the Frog 5 Tue Ecc Since every animal begins its individual ex- istence as an ovum or egg, it may be well, before taking up the study of the frog’s egg, to examine some egg that will more easily show the different structures of a typical ovum, the frog’s egg being so large and so full of yolk that it is difficult for the beginner to distin- guish its different parts. For this purpose the eggs of the starfish, or of the sea-urchin, are very convenient, and if some of these eggs be properly stained and mounted, the main fea- tures of their structure may be made out with- out difficulty. The ovum, whether it be of microscopic size or 30 mm. in diameter, as is the yolk of the hen’s egg, is always a single cell. Al- though the egg of the common starfish is only as large as a small grain of sand, yet if it be examined under a moderate magnification of the microscope, it will be found to be made up of several distinct parts. Like most ova it is spherical in shape, and is enclosed in a thin cell-wall or vztelline membrane (Fig. 2). In the granular, protoplasmic contents of the egg two regions may be distinguished: a 6 Vertebrate Embryology lighter portion which makes up the greater part of the egg and which is known as the cytoplasm, and a darker, spherical portion which lies in the cytoplasm and is known as the nucleus (germinal vesicle). Inside of the nu- cleus may be seen one or more small areas, the zucleol’ (germinal spots) Fic. 2.—Ecc oF sTARFISH. (After Gegenbaur.) a, Granular protoplasm. é. Nucleus (germinal vesicle). ¢. Nucleolus (germinal spot). By the use of more refined methods and higher magnification many other details of structure might be brought out, but for these the reader is referred to more extensive text- books. The frog’s egg is several thousand times the bulk of the egg of the starfish, being about I.7 mm. in diameter. This increase in size is The Development of the Frog 7 chiefly due to the large amount of food-yolk contained in the frog’s egg (Fig. 3.) As the eggs ripen and are set free from the ovary, they fall into the body cavity and pass forward into the abdominal openings of the oviducts. Passing slowly along these ducts, the eggs at last collect in large numbers in the Fic. 3.—OVARIAN EGG OF FROG. thin-walled, dilated posterior parts of the ovi- ducts, where they remain until they are forced out into the water at the time of spawning. Owing to the opacity of the frog’s egg, the nucleus is only visible in sections, although it is very large, sometimes as much as one-third to one-half the diameter of the egg (Fig. 3). Before it is set free from the ovary, the egg has secreted around it, in some way that is not well understood, a thin vitelline membrane; 8 Vertebrate Embryology and as it passes through the anterior, thick- walled part of the oviduct a gelatinous envel- ope is deposited around it. This envelope, on coming in contact with the water, swells enor- mously, and forms a mass of colorless jelly, characteristic of frog’s spawn (Fig. 4). The Fic. 4.—VARIOUS STAGES IN THE DEVELOPMENT OF THE FROG. (After Brehm from Marshall.) 1. Eggs just laid. 2, Eegs shortly after eying oe Tadpole shortly before . Ta hatching. 4. Tadpoles just hatched. 5 and poles with external gills, 7 and 8. Tadpoles with fully-formed opercular folds. 9 and 10. Tadpoles with well-developed hind legs, shortly before the metamorphosis. 11. Tadpole during the metamorphosis, 12. Young frog with tail only partially absorbed. thin, gelatinous envelope is said to begin to swell about one minute after it comes in con- tact with the water, and to reach its greatest expansion in three hours. Its purpose is to protect the soft eggs from being injured by The Development of the Frog 9 contact with surrounding objects, and also to hasten development by storing up heat, this latter power being due to the fact that it per- mits the heat of the sun to pass inward more rapidly than it permits the reflected heat to pass out again, with the result that the mass of spawn, in the sunlight, is warmer than the surrounding water. The jelly may also protect the eggs from being eaten by other animals. The egg itself consists of a black and a white pole of approximately equal sizes, the dark color of the black hemisphere being due to the presence of a superficial layer of pigment in that region. The origin of this pigment is not clearly understood, but its purpose seems to be that of absorbing the heat of the sun, and in furtherance of this object the pigmented half of the egg is of less specific gravity than the other half, with the result that the dark pole is always the upper one and the one in which the segmentation takes place the more rapidly. This automatic orientation is made possible by the fact that the egg shrinks away from its vitelline membrane, between which and itself is secreted a small quantity of fluid in which the egg may easily rotate. 10 Vertebrate Embryology A mass of frog’s spawn may be compared to a number of hen’s eggs which have been carefully broken into a dish, so that the yolks are all unbroken. The yolks of the hen’s eggs correspond to the true eggs of the frog’s spawn, and the white of the hen’s egg to the jelly mass of the spawn. The white of the hen’s egg, however, serves as food for the develop- ing chick, while the jelly of the spawn probably serves no such purpose. Maturation of the Egg Asa rule, before an egg may begin its de- velopment, it must be fertilized, and before it can be fertilized it must undergo a process of ripening or maturation. The details of this maturation vary in different eggs, but the es- sential processes are about the same in all. As has been stated above, the egg of the frog, when just set free from the ovary, con- tains a very large nucleus. It is the nucleus that is chiefly concerned in the maturation changes, and the first change that is noticed is a shrinkage of this large nucleus and a loss of the nuclear membrane. After passing through other changes, a description of which cannot be given here, the nucleus divides into two The Development of the Frog 11 equal parts, and one of these halves is extruaed from the egg and lies under the vitelline mem- brane at the upper or black pole. This ex- truded half of the nucleus is known as the frs¢ polar body, and its formation takes place while the egg is still in the oviduct of the frog. Shortly after the egg is laid and the sperm has entered it, the half of the nucleus that remained after the formation of the first polar body again divides into equal parts, and the second polar body is extruded. The part of the nucleus that remains after the formation of the polar bodies is known as the female pronucleus, and con- tains, as may easily be understood, just one- fourth of the material of the original egg nucleus. The two small, round polar bodies, lying side by side under the vitelline membrane at the dark pole of the egg, take no further part in the development of the egg, and eventu- ally disappear. What the purpose of the maturation of the egg-cell may be it is not possible at the present time to say; and so many theories on the sub- ject have been advanced that it is very difficult to give any simple statement of the case. The formation of the polar bodies or glob- ules is the result of a form of cell division 12 Vertebrate Embryology known as “reduction division.” When the first globule is formed, it is by the division of the egg-cell into two cells, a large one and a very small one, the small one being the polar globule. In like manner the second globule is formed by the very unequal division of the larger of the first two cells, the larger cell of the latter division being the true female element which is capable of being fertilized. The essential element of the nucleus, that part which is especially concerned in heredity, or the transmission of parental characteristics, seems to be the chromosome. Vhe number of chromosomes in the nucleus of any given species is normally constant. It has been found that the number of chromosomes in the egg after maturation is just half what it was before that process, and the amount of chromatin is reduced to one quarter of the original quantity. O. Hertwig thinks that the reduction divi- sions taking place in maturation are for the purpose of increasing the relative amount of cytoplasm, rather than for reducing the quan- tity of nuclear material. The cell that is left after the extrusion of the polar bodies, although containing only one fourth of the original The Development of the Frog 13 chromatin, has retained practically all of the cytoplasm ; so that the result of the formation of the polar bodies is practically to increase the cytoplasm fourfold. The objections to this theory cannot be given here. One of the oldest and most celebrated the- ories in regard to the formation of the polar bodies is that of Minot. According to this theory the fertilized egg-cell is said to be hermaphrodite, that is, it is both male and female, since it is formed by the fusion of the male and female elements. When the fertilized egg divides for the first time the nuclear ma- terial is equally divided between the two blas- tomeres that are formed, so that each of these blastomeres must be hermaphrodite. If this be true of the first two blastomeres, it must be equally true of all the cells that are formed by the repeated division of the original egg-cell : hence the unfertilized egg-cell developed from the original egg must also be hermaphrodite ; and before it can receive additional male chro- matin, in the act of fertilization, it must get rid of the male chromatin that it already possesses, by extruding the polar bodies. If this theory were true, it is evident that a child could not inherit the characters of its 14 Vertebrate Embryology mother’s father, nor the characters of any of its father’s ancestors. Delage and Herouard have elaborated a theory that is, briefly, as follows: The simplest organisms are capable of reproducing them- selves indefinitely by a process of repeated division. More highly organized beings are not possessed of this indefinite power of di- vision, and must occasionally undergo a process of fusion or conjugation. This process con- sists of two parts, the elimination of chromatic material (maturation), and the conjugation proper, or fertilization. The maturation is usu- ally considered as an accessory phenomenon, whose object is simply to make the fertiliza- tion possible, but, according to this hypothesis, “The essential phenomenon is the chromatic reduction, and the fecundation is an addition which is advantageous but not indispensable.” In the simplest organisms metabolism is a closed cycle; but in more complicated beings this is not the case, and there is gradually accumulated a substance which is injurious and which affects all the functions of life, especially those concerned in cell division. Unless this substance is gotten rid of, the cell will die. The Development of the Frog 15 In the case of the tissue cells of the higher organisms there is no method of removing this substance, so that these cells must eventually perish; but in the case of the simplest uni- cellular organisms, and in the reproductive cells of the higher forms, this substance is removed in a single operation, and the cell thus enabled to begin a new series of di- visions. Labbé found among certain insects a reduc- tion of chromatin, not followed by fecundation, that was followed by cell division and develop- ment; and other cases of parthenogenesis are known. The expulsion of chromatic material is rep- resented in higher organisms by the extrusion of the first polar body, and “if one could prevent the extrusion of the second polar body, all beings would develop parthenogen- etically.” The formation of the second polar body reduces the amount of chromatin and the num- ber of chromosomes, and makes impossible further development, until fresh chromatic material has been added by the process of fecundation. It will be seen from the above examples 16 Vertebrate Embryology that the processes of maturation and fertiliza- tion offer a wide field for speculation, but one into which it is not within the province of this book to enter. Fertilization of the Egg As the eggs are extruded from the cloaca of the female, which process may take place in a few minutes or may be prolonged over several days, the spermatozoa are spread over them by the male and at once begin to bore their way through the jelly towards the eggs. The exact nature of the changes that take place after the sperm enters the egg has not been entirely determined, but the essential points will be given. A few minutes after coming in contact with the vitelline membrane, the head of the spermatozoon works its way into the egg and moves towards the female pronucleus, with which it fuses to form the so-called segmentation nucleus. The head of the spermatozoon, after it has entered the egg, is known as the male pronucleus, and the essential act of fertilization is the fusion of the male and female pronuclei. The tail of the spermatozoon remains outside of the egg and apparently takes no part in the process The Development of the Frog 17 of fertilization; the fate of the middle-piece, in the frog, is not well understood, but it is possible that it may effect segmentation in some way. Asa rule, only one spermatozoon enters the egg, but it is likely that if two or more spermatozoa reach the vitelline mem- brane at the same time, they may all enter the egg, although only one male pronucleus will fuse with the female pronucleus. In some other animals the entrance of two or more spermatozoa into the egg (polyspermy) pro- duces serious results, causing irregularities in segmentation; but in the frog the extra pro- nuclei probably disappear without producing any unusual effect. Segmentation of the Egg About two or three hours (depending on the temperature) after fertilization, the first in- dication of segmentation is seen as furrow on the dark pole of the egg. This furrow gradu- ally extends around towards the white pole until it completely encircles the egg (Fig. 1, B). By the time this has taken place, the contents of the egg have been separated into two parts by a plane corresponding to the superficial furrow, so that the egg is now 18 Vertebrate Embryology completely separated into two dlastomeres of approximately equal size, which, at first, tend to become spherical in shape, but which are soon flattened against each other to form hemispheres. Before the formation of this first cleavage plane, the segmentation nucleus has divided into two equal parts, one of which is found in each of the two blastomeres. In dividing, the nucleus passes through a com- plicated series of changes known as £aryokz- nests, for a description of which the reader is referred to more extensive text-books. The first cleavage plane corresponds to the medio- longitudinal (sagittal) plane of the future frog : this, however, is not true of all animals. After a short resting period, the second cleavage plane is formed, preceded, as in the former and as in all subsequent cases, by the division of the nucleus of each blastomere. The second plane is also a vertical one begin- ning in the dark pole, and is at right angles to the first plane. The egg now consists of four more or less equal blastomeres (Fig. 1, C). The third plane is normally a horizontal one, at right angles to the first two, but not in the equatorial plane of the egg, so that the egg is divided into eight cells (Fig. 1, D), four The Development of the Frog 19 small dark cells at the upper pole, and four larger white cells at the lower pole; that is to say, the third plane is horizontal but nearer the upper than the lower pole (Fig. 5, 2). The next division is by two vertical planes, at right angles to each other and _ half-way between the first two planes. Thus we have FIG, §5.—VERTICAL SECTIONS OF SEGMENTING EGGS, A, 2-cell stage. B, &-cell stage. the egg made up of sixteen blastomeres (Fig. i, £2). The thirty-two-cell stage is formed by two horizontal planes, one above and one below the first horizontal plane (Fig. 1, 7). After the thirty-two-cell stage the segmenta- tion proceeds so rapidly and so irregularly that it cannot be followed with certainty,—indeed it is seldom that the processes above described 20 Vertebrate Embryology can be followed as far as the thirty-two-cell stage, irregularities often being seen as early as the four-cell stage. As early as the eight-cell stage, sections of CoBotoOwn, eee oe Fie, 6. A and B, early and late stages in the segmentation of the egg. C, beginning of the archenteron, A>. (gastrulation). S C, segmentation- cavity (camera lucida; C slightly altered). the egg (Fig. 5, 2) show a small central cavity, the segmentation cavity, which becomes larger as segmentation proceeds, and is filled with an albuminous fluid. This cavity, as will be seen, The Development of the Frog 21 eventually disappears and forms no part of the adult structure. After the thirty-two-cell stage, a series of concentric segmentation planes are formed, dividing the blastomeres into several layers of cells. By the continuation of this process of cell-division the egg is eventually divided into several hundred cells (Figs. 1, 6), those of the dark pole being much smaller, more sharply defined, and more numerous than those of the light pole. As is seen in the figures, the seg- mentation cavity lies nearer the dark pole, so that its roof is composed of a few layers of small, dark cells, while its floor is made up of many layers of ill-defined, yolk-filled cells. There is, however, no sharp dividing line be- tween the pigmented and unpigmented cells, any more than there was between the dark and light poles of the unsegmented egg. Formation of the Germ-Layers The egg, at the close of segmentation, has been converted into a hollow sphere, with the cavity nearer the upper, or dark pole (Fig. 6). The cells of the dark hemisphere are arranged in two more or less distinct layers, while the large, unpigmented cells have no such regular 22 Vertebrate Embryology arrangement. At this time a crescentic groove appears on one side of the egg at the bor- der between the white and dark cells. This groove, whose convex side is upward, is the dorsal lip of the d/astopore, and is the begin- Fic. 7.—LONGITUDINAL VERTICAL SECTION OF A FROG EMBRYO, SHOWING COMMENCING INVAGINATION. X28. (After Marshall). , &, blastopore. E£, outer or epidermic layer of ectoblast. ZV, inner or nervous layer of ectoblast. SC, segmentation cavity. FY, yolk-cells. ning of the process of zzvagznation (Figs. 6, 7, and 8). The horns of the crescent extend towards each other until they meet to form a circle (Fig. 1, H), the blastopore, bounded on the outside by pigmented cells, and filled inside The Development of the Frog 23 with white cells. The mass of white cells which fills the blastopore is known as the yolk-plug (Fig. 8, YP). By a rapid division of the black cells around : Tie eens ne am ee MN 4 an Tues , ee oe < A ae i San Sica on ls a vel ie YP—é ey te ii e#/, een as ‘ ie a us ( . ie Ad = os 4 - Fic, 8,—LONGITUDINAL VERTICAL SECTION THROUGH A FROG EMBRYO AT A LATER STAGE IN THE FORMATION OF THE MESENTERON, (After Marshall.) #7, invaginate entoblast. 47, mesoblast. A7N, mesenteron, N, notochord. SC, segmentation cavity. VP, yolk-plug, filling up the blastopore. the rim of the blastopore, especially at the dor- sal lip, the exact nature of which process is in some dispute, the diameter of the blastopore is gradually reduced and the yolk-plug is with- drawn into the egg (Figs. 8 and 9). This overgrowth of the black cells continues 24 Vertebrate Embryology until the yolk-plug entirely disappears from the surface, and the blastopore is reduced to a narrow slit. The layer of black cells, which now completely surround the egg or embryo, is the upper germ-layer or ectoblast (Fig. 9). Carefully prepared sections through the embryo at the time of the appearance of the dorsal lip of the blastopore may show, in the region where the white and black cells meet, a more or less clearly defined zone of cells extending equatorially around the em- bryo. This band is several cells deep, the inner cells passing insensibly into the yolk- cells, the peripheral cells being indistinguish- able from the ectoblast. “This ring of cells, as subsequent develop- ment shows, is the beginning of the embryo, and the ring itself is composed of the material which subsequently forms the central nervous system, the mesoderm, the notochord, and a part of the entoderm.”? By a process of concrescence, which is closely related to the closure of the blastopore, de- scribed above, this band of cells shifts towards one side of the embryo, and its right and left halves fuse to form a broad meridional band 1 Morgan, The Development of the Frog 25 extending into the dorsal lip of the blastopore. This process may be roughly illustrated, per- haps, by placing a rubber band around the equator of a ball, and then gradually slipping two opposite sides of the band towards one pole of the ball, until they meet and form a single broad band lying in a meridional instead of in an equatorial position. This process of concrescence is difficult of determination, and it will probably not be practicable for students to work it out in the laboratory. As has been said, the growth of the ectoblast over the yolk-cells takes place much more rapidly from the dorsal lip of the circular groove, which we have called the blastopore, so that, while this groove never becomes very deep on the lower side, on the upper side it becomes a long, narrow slit extending nearly to the opposite side of the embryo (Fig. 9, MN). This slit, whatever may be the exact method of its formation, is the primitive di- gestive tract of the frog, and is known as the mesenteron or archenteron. It is much wider from side to side than it is in a dorso-ventral direction ; and while its roof is made up of a more or less clearly defined layer of closely 26 Vertebrate Embryology packed cells, its floor is a mass of undiffer- entiated yolk-cells. The cells forming the roof of the mesenteron are the beginning of the lower germ-layer, or extoblast (Fig. 9). The mesenteron, opening to the exterior by akc Sze. se Fic. 12, A.—SAGITTAL SECTION OF EMBRYO SHOWN IN FIG, II, D, SHORTLY BEFORE THE CLOSURE OF THE BLASTOPORE, (After Marshall.) B, blastopore. BF, fore-brain. BH, hind-brain. 2M, mid-brain. 4, entoblast. Z, liver. JZ, mesoblast. 47, mesenteron. JV, notochord. NVC, neurenteric canal. , beginning of pituitary body as in growth of ectoblast. PD, proctodeum. &, rectal diverticulum of mesenteron. S, central canal of spinal cord. Y, yolk-cells. B.—SAGITTAL SECTION OF EMBRYO SHOWN IN FIG, 11, E, SHORTLY AFTER THE CLOSURE OF THE BLASTOPORE. (After Marshall.) BF, fore-brain, BA, hind-brain. BM, mid-brain. CH, notochord. AZ, mesoblast. MC, neuraltube. MW 7, neurenteric canal. PW, pineal body. P7, pituitary body. 7 /, intestinal region of mesenteron. 7, pharyngeal region of mesenteron. U/, proctodzal or cloacal opening. HW, liver. 1’, yolk-celis. Y 33 34 Vertebrate Embryology folds begins in what will be the neck region of the future tadpole, and extends forward and backward from that point, the extreme anterior end being the last to close in completely. As the neural folds, or medullary folds as they are often called, fuse together to form the. - neural or medullary canal, the epidermal layer of ectoblast fuses over the top, and forms, once more, a smooth, continuous layer (Figs. 12 and 15). It will be seen by an examination of the figures that the chief thickness of the wall of the neural tube is derived from the nervous layer of ectoblast, but that the layer of cells which lines the inside of the tube is from the epidermal layer (Fig. 13). The extreme posterior ends of the neural folds extend on each side of the blastopore, so that, as they come together, they cover the blastopore, which persists for a time as a nar- row passage, the zeurenterzc canal, connecting the mesenteron with the neural canal (Fig. 12, VC and VT). The neural tube, formed as above described, becomes converted into the central nervous system, the anterior end forming the brain, the rest of the tube forming the spinal cord. The Development of the Frog 35 Of the changes that convert the simple tube into the adult cord little need be said. The walls thicken rapidly, and the originally large, circular canal is reduced to a narrow, vertical slit (Fig.15, MS). As development proceeds, Fic. 13.—TRANSVERSE SECTION THROUGH A FROG EMBRYO DURING THE FORMATION OF THE NEURAL CANAL, (After Marshall.) C, celom. £ F, epidermic layer of ectoblast. £N, nervous layer of ectoblast. A, entoblast, J, mesoblast. J £, somato- leuric layer of mesoblast. 4H, splanchnopleuric layer of meso- Biase. MN, mesenteron. WV, notochord. MN/, neural fold, NG, neural groove. Y, yolk-cells. the relative size of the canal becomes smaller and smaller until, in the adult, it is seen as a tiny tube in the centre of the cord, lined with a thin layer of epithelial cells, derived 36 Vertebrate Embryology from the epidermal layer of the original ecto- blast. The changes that convert the anterior end of the neural tube into the complicated struc- tures of the adult brain are much more exten- sive, and must be described in more detail, although the limits of this work will not per- mit a full description. The first indication of a separation of the brain from the rest of the neural tube is seen at about the time of the appearance of what is known as cranzal flexure. As the name would indicate, cranial flexure is the bending of the brain, at about its middle region, so that the anterior end is pushed down, and comes to lie almost at right angles with the posterior part (Fig. 12). This cranial flexure takes place around the anterior end of the notochord, and persists throughout life, its apparent rectification being due to inequalities in the growth of the differ- ent parts of the brain. At this time the cen- tral nervous system has somewhat the shape of a retort, the bulb of the retort corresponding to the brain, and the neck of the retort cor- responding to the spinal cord. Two transverse folds appear very early in The Development of the Frog 37 the brain region, dividing it into three more or less distinctly marked portions, the fore-brazn, mia-brain, and hznd-brain (Fig. 12.) The de- velopment of these regions of the brain will be taken up, briefly, in turn. The hind-brain forms the medulla and cere- bellum of the adult, its cavity remaining as the fourth ventricle. The floor and sides of the hind-brain become thickened, while the roof becomes very thin, except at the part next to the mid-brain, where the cerebellum is devel- oped (Fig. 16). The floor of the mid-brain thickens to form the crura cerebrz, while from the roof grow out two hollow, ovoid bodies, the oftzc lobes. The cavity of the mid-brain persists as the Sylvian agueduct or zter. The walls of the fore-brain (¢halamenceph- alon of the adult) thicken to form the oftzc thalam? and its cavity, the ¢thzrd ventrecle, is thus reduced to a narrow, vertical slit. From the floor of the fore-brain the zzfundzbulum is formed as a hollow pouch, pushed out in a ventro-posterior direction (Figs. 14, 7, 17, and 18, 7). Since the pztuztary body is so closely associated with the infundibulum, its origin will be spoken of at this time. 38 Vertebrate Embryology At a very early period, a thickening of the nervous layer of the ectoblast is formed just below the anterior end of the neural canal. This collection of cells grows inward as a tongue of ectoblast tissue between the anterior end of the brain and the digestive tract (Figs. 14, PZ; 17, and 18, P). The inner end of this tongue of cells becomes broader and hollow, and even- tually separates from its stalk to form the pituitary body, which lies just under the in- fundibulum (Fig. 13, 7). From the roof of the fore-brain, at the point where the neural tube finally closed, a small, hollow diverticu- lum is pushed out and becomes enlarged at the end (Figs. 14 and 17, PV’). This is the pineal body, and when the skull is formed it cuts off the enlarged knob from its hollow stalk, the knob, which has become solid, re- maining outside of the skull, and the stalk retaining its connection with the fore-brain. The cerebral hemispheres, which form the larger part of the fore-brain, do not form until a comparatively late period. They begin as a large, median diverticulum from the front of the fore-brain (Figs. 17, CV, and 18), which diverticulum is at first unpaired, but later be- comes divided into the two hemispheres. ‘raat Vy *eoeoya 40 snue ‘77 ‘“piordyy ‘477 “UoIajUasoUL LZ “ueay Jo qed repnomuea ‘4 y ‘s9yons ‘G *Apog Areynyd ‘7 g *Apoq yeourd ‘7g ‘Jeueo yeutds *9 Ay ‘3onp otydauoad jo pua Jorsa4 -sod ‘yoy ‘wnynqrpungur ‘yy ‘ud jeepoutors ‘gq *Awava JeIpreopied ‘yD “ployooiou ‘yD ‘‘urerq-prm yy g “urerq-pury ‘yg (Treysie Jax;V) "“SNIHOLVH dO AWIL AHL LY AIOdGVI V HOMOVHL NOILOGS TVLLIOVS—'FI ‘OIG OE FINA \ wa HO Ha a ON wt 40 Vertebrate Embryology The olfactory lobe is formed by the fusion of an outgrowth from the anterior end of each cerebral hemisphere. The cerebral hem- ispheres are at first unpaired and later become double, while the olfactory lobe is at first double and afterwards becomes single. There are many details in the development of the peripheral nervous system that are not yet clearly understood. The dorsal roots of the spinal nerves arise very early as outgrowths from the sides of the neural plates before the latter fuse together to form the neural canal. They grow down between the neural canal and the myotomes (Fig. 15, V2) and become slightly enlarged, a short distance from their points of origin, to form the spinal ganglia. The ventral roots of the spinal nerves arise later as independent outgrowths from the ven- tral side of the neural canal, and fuse with the dorsal roots distal to the spinal ganglia. DEVELOPMENT OF THE SENSE ORGANS Since the organs of special sense are chiefly derived from the ectoblast, and since they are all closely connected with the brain, it is well to speak of their development at this point. The Development of the Frog 41 Tue Eve The development of the eye can be more easily made out in the chick than in the frog, Fic. I5.—TRANSVERSE SECTION ACROSS THE MIDDLE OF THE EMBRYO SHOWN IN Fic. 11, D. (After Marshall.) CH, notochord. C/, subnotochordal rod. C7, myocoel. CS, body cav- ity. , ectoblast. AB, archinephric duct. /, mesoblast. 47 S, mesoblastc somite. MD, dorsal root of spinal nerve. MS, spinal cord. SO, somatopleure. SP, splanchnopleure. 7, intestinal region of mesenteron. Y, yolk-cells. so that many of the details will be left until the latter part of the book, since the process is essentially the same in both animals. The first rudiment of the eye is seen in 42 Vertebrate Embryology very young tadpoles as an evagination from each side of the fore-brain. This evagination, which is known as the of¢ze veszcle, reaches a considerable size and becomes constricted off from the brain, so that it forms a large, hollow bulb connected with the brain by a very nar- row stalk. The walls of the optic vesicle are at first comparatively thin, but as the vesicle enlarges and approaches the superficial ectoblast, the wall of the vesicle that is next to the ectoblast begins to thicken and at the same time to be pushed in on itself (Fig. 19, OC); this invagi- nation of the optic vesicle continues until the two walls are in contact, and the cavity of the original optic vesicle is obliterated. The cup- shaped structure which is thus formed, and which is still connected with the fore-brain by the narrow stalk, is known as the oftzc cup. The thick inner wall of the optic cup forms the essential part of the retina, while the thin outer wall forms the pigmented layer that sur- rounds the retina. The rim of the optic cup is not complete, like the rim of an invaginated hollow rubber ball, but is interrupted, at one place, as a narrow slit known as the chorord fissure. Through the choroid fissure the sur- The Development of the Frog 43 rounding mesoblast enters the optic cup and forms the vitreous humor. The outer layers of the eye are formed from the mesoblast. As the optic vesicles approach the superficial ectoblast, the inner layer of the latter becomes thickened, and eventually separates from the outer layer and lies at the opening of the optic cup as a hollow spherical, or ovoid body, the Zens veszcle (Figs. 19, OL, and 20 OL). By the growth of its walls, chiefly the inner, the cavity of the lens vesicle is obliterated and the adult lens is formed. Tue Ear The ear begins as a thickening, followed by an invagination of the inner or nervous layer of the ectoblast. This invagination begins very early and, in the frog, never opens to the ex- terior. It is, almost from the first, connected with the brain by the auditory nerve. The invagination gradually closes to form a com- paratively thin-walled cavity, lying in the re- gion of the hind-brain, known as the audztory vesicle. This vesicle, whose walls are com- posed of a single layer of cells (Fig. 21, £), forms the lining of the middle ear. A more complete description of the development of the ear will be given in connection with the chick. 44 Vertebrate Embryology Tue Nose The nose, like the eye and ear, begins at a very early period, and is first seen as two thickenings of the inner layer of the ectoblast, Fic, 16.—THE BRAIN OF THE FROG. (After Marshall.) Figure on left is a dorsal view ; figure on right is a ventral view. C, cerebellum. CA, cerebral hemisphere. CP, choroid plexus of third ven- tricle. #, fourth ventricle. 7, tuber cinereum. AZ, medulla oblongata. O, olfactory lobe. OC, optic chiasma. OZ, optic lobe. /, stalk of pineal body. PB, pituitary body. 7, thalamencephalon. /, olfactory nerve, //, optic nerve. ///, third or motor oculi nerve. JV, fourth nerve, /’, fifth or trigeminal nerve. /7/, sixth nerve. V// and I///, combined root of facial and auditory nerves. /.V and .\, combined root of glosso- pharyngeal and pneumogastric nerves. one on each side of the anterior end of the head. Aninvagination of both layers of theecto- blast now takes place (Fig. 22, OF), forming the xasal pits whose openings to the exterior The Development of the Frog 45 will form the anterzor nares. The lining of the nasal pits will become connected with the brain by the olfactory nerves, and will form the olfactory epithelium of the nose. From the inner side of the nasal pits a diverticulum grows down to open into the mouth cavity as the posterior nares. DEVELOPMENT OF THE ALIMENTARY TRACT The origin of the primitive digestive tract or archenteron has already been given: it now remains to describe the further changes that take place in the digestive tract itself, and to describe the development of the various struc- tures that are derived from it. The digestive tract may, for convenience, be divided into three regions: (1) the mesen- teron (whose formation has been described), which is lined with entoblast and from which are developed the liver, pancreas, lungs, gill clefts, etc.; (2) the stomodeum, which forms the mouth; (3) the proctodeum or cloacal re- gion. The first region is lined throughout with entoblast, while the latter two are both lined with ectoblast. The reason for this dif- ference will be seen when the development of the mouth and anus is described. 46 Vertebrate Embryology The anterior end of the digestive tract early becomes expanded into what may be called the pharynx, and there is a similar though smaller expansion at the posterior end (Figs. 12 ZPand 22). The entoblast at first forms a more distinct layer on the dorsal side of the mesenteron than it does on the ventral side, but it very soon extends entirely around the cavity as a distinct layer. In the frog the primitive mouth or blastopore closes entirely, so that the digestive tract may be for a short time a completely closed cavity, but in some other animals the blastopore per- sists as the permanent anus. Shortly before hatching, a depression of the ectoblast may be seen on the ventral side of the head (Fig. 14, DS); this is the beginning of the stomodzum. The depression becomes deeper and deeper until it is separated from the front of the pharynx by only a thin septum (Fig. 17, SD). Soon after hatching this sep- tum becomes perforated and the mouth open- ing is formed. The lips now grow forward, the jaws become formed, and the tadpole begins to take food from the surrounding water (Fig. 18). The proctodeeum or anal opening is formed The Development of the Frog 47 before the stomodeum. Before the neural canal has been completely closed a small de- N Fic. 17.— LONGITUDINAL VERTICAL SECTION THROUGH THE AN- TERIOR END OF A TADPOLE SHORTLY AFTER THE TIME OF HATCHING, LENGTH OF THE TADPOLE, 8 MM. (After Marshall.) A, auricle of heart. 8, fore-brain. 4H, hind-brain. 8B AZ, mid-brain. C’, pericardial cavity. CV, vesicle of cerebral hemispheres, /, infundibulum. L, liver.. N, notochord. O, depression of floor of fore-brain from which the optic vesicles arise, OF, cesophagus. /, pituitary body. PA, pineal body. S, central canal of spinal chord. SD,stomodeum. 7; truncus arteriosus. V, ventricle, 3°, yolk-cells, pression is seen below the blastopore (Fig. 12, PD); this is the beginning of the anal in- vagination. At the same time a diverticulum 48 Vertebrate Embryology grows backward from the posterior end of the digestive tract towards this invagination (Fig. 12), with which it finally fuses and thus puts the hind gut in communication with the ex- terior (Figs. 12 U,and14, UV). The formation of the proctodeum may be completed before the closure of the blastopore, so that the hinder end of the digestive tract may have two open- ings to the exterior. It will be understood, from the above de- scription, why it is that the oral and anal cavities are lined with ectoblast instead of with entoblast, as is the rest of the digestive canal. The “ver may be early recognized as a diver- ticulum pushed out from the front end of the digestive tract in a ventro-posterior direction (Figs. 12 and 14, W). The walls of this diverticulum thicken and become folded, and the mesoblast penetrates between these folds. The diverticulum, which is evidently lined with entoblast, persists as the bile duct, and from it an outgrowth arises to form the gall bladder. The pancreas arises as a pair of hollow out- growths from the mesenteron back of the liver. The Development of the Frog 49 The Zungs arise from the narrow part of the digestive tract which lies just back of the wide anterior end or pharynx. A longitudinal fold = = WY w SS S Rwy < ro Noo 774 nT AANADYSSS LE Fic. 18,—LONGITUDINAL VERTICAL SECTION THROUGH THE HEAD AND ANTERIOR PART OF THE BODY OF A TADPOLE ABOUT THE TIME OF APPEARANCE OF THE HIND LEGS. LENGTH OF TADPOLE, I2 MM. xX 14. (After Marshall.) A, auricle of heart. 4 D, dorsal aorta. B&B, basi-branchial cartilage. BF, fore-brain. BA, hind-brain. BM, mid-brain. C, coelom or body-cavity. C’, pericardial cavity. CH, cerebral hemisphere. CB, rudimentary cerebellum. CP, choroid plexus of fourth ventricle. CP’, choroid plexus of third ventricle, ¥, pharynx. G, stomach, H, lung. /, infundibulum. //, horny jaws. &, lip. L, liver. N, notochord. O, depression of floor of fore-brain from which the optic nerves arise. OZ, cesophagus. P, pituitary body. PA, pineal body. S, central canal of spinal chord. 7, truncus arteriosus. V, ventricle. appears in each side of the mesenteron, at this place, and by the meeting of these folds the ‘digestive tract is divided into a dorsal portion 4 50 Vertebrate Embryology or esophagus, and a ventral portion or darynx. From the laryngeal chamber the lungs arise as hollow lateral outgrowths, some time after hatching, when the tadpole is about 8 mm. long. Atabout the time of the formation of the lungs the tubular cesophagus becomes solid and remains closed until after the formation of the oral opening. What the significance of this curious fact may be is not known. The ¢hyrozd body begins, at about the time of hatching, asa small, median depression in the floor of the pharynx. The depression becomes deeper, especially at its posterior end, and finally loses its connection with the pharynx and lies as a solid rod of cells just in front of the pericardium. When the tadpole is about 12 mm. in length the thyroid becomes sepa- rated into right and left halves by the growth of a median longitudinal septum, and after considerable growth each of these halves is converted into the adult structure by the re- arrangement of its cells to form the round or oval vesicles that are characteristic of the thy- roid gland. The d/adder arises at about the time of metamorphosis as an outgrowth from the ven- tral wall of the hind gut. The Development of the Frog 51 DEVELOPMENT OF THE GILL CLEFTS AND Foups The gzll clefts are five pairs of narrow, vertical slits which connect the cavity of the pharynx with the exterior. The portions of the wall between the clefts, and also in front of the first and behind the last clefts are the gill folds or arches. The most anterior cleft is known as the hkyomandzbular cleft, the others, from before back, are the first, second, third, and fourth gill clefts. The arch in front of the hyomandibular cleft is called the mandibular arch, the arch between the hyomandibular and the first gill clefts is the Ayocd arch, and the other arches, like the clefts, are called the first, second, third, and fourth. The gill clefts, or, as they are often called, the visceral or branchial clefts or pouches, be- gin to develop before the tadpole hatches, and are best studied in horizontal sections. The first three pairs of pouches begin almost simul- taneously as evaginations of the entoblastic wall of the pharynx, which push outward towards the ectoblast. The third and fourth pouches are formed in succession behind the first three. All of the pouches grow outward until they come in contact with the ectoblast, 52 Vertebrate Embryology with whose inner layer they fuse (Fig. 22). The two lamellae of entoblast that make up the pouches are at first in contact with each other, so that there is no actual cleft between them (Fig. 22, WAZ, HC.4); but at about the time of the opening of the mouth the lamelle Q Fic. 19, HALF SECTIONS IN THE TRANSVERSE PLANE OF A TADPOLE, IO MM. LONG (LEFf HALF) AND OF A TADPOLE I2 MM, LONG (RIGHT HALF). X 35. (After Marshall.) BF, forebrain. OD, outer wall of optic cup (pigment layer of adult retina); OC, inner wall of optic cup (remainder of adult retina), OZ, lens, attached to epiblast in younger tadpole, but forming a hollow vesicle at the later stage. YZ, pharynx. Q, sucker. H. FJ separate from each other to form the actual clefts, all of which open to the exterior except the hyomandibular pair, which recede from the ectoblast and persist, for a time, as a pair of diverticule from the front part of the pharynx. | The Development of the Frog 53 The Eustachian tube and the tympanic cav- ity develop near the hyomandibular cleft, but it is doubtful if any such close relation exists between those structures and the hyomandib- ular cleft as exists in some other animals. The other visceral clefts persist for a con- siderable time, but towards the end of the tadpole stage they close up and disappear. The fate of the gill arches is of more im- portance, but as it is more easily studied in the chick, a very brief statement will suffice at this time. The mandibular arch, as its name would indicate, becomes converted into the essential part of the lower jaw. The hyoid arch, as its name indicates, forms the greater part of the hyoid apparatus, while the other four arches almost entirely disappear. Dur- ing the larval period there is present in each visceral arch a rod of cartilage, which is closely joined to its fellow of the opposite side ven- trally but is separated from it dorsally. Thus there is in each pair of arches a U-shaped bar of cartilage which serves to stiffen the walls of the pharynx. The gzlls, of which there are two sets, the external and the internal, are developed in connection with the gill arches. 54 Vertebrate Embryology Even before the tadpole is hatched there may be seen, on each side of the neck region, a series of vertical folds, or thickenings; these are the visceral folds, and it is upon them that We -BF DS Fic. 20.— TRANSVERSE SECTION THROUGH THE HEAD OF A TADPOLE 6% MM. IN LENGTH ABOUT THE TIME OF HATCHING, THE SECTION PASSING THROUGH THE FORE-BRAIN AND DEVELOPING EYES. (After Marshall.) al C, carotid artery. BF, fore-brain. DS, stomodza) pit. NL, cutaneous or lateral line branch of trigeminal nerve. OC, inner wall of optic cup. OD, outer wall of opticcup. OZ, lens. OS, optic stalk. #7, pituitary body. 7'P, pharynx, VY, jugular vein. the gills are developed (Figs. 1, A—O, and 22). The external gills appear first, reach their maximum development, and then are replaced by the internal gills. The external gills arise, shortly before hatching, upon the first and The Development of the Frog 55 second gill arches, and a little later a third pair is formed upon the third arch. The ex- ternal gills reach their greatest development at about the time of the opening of the mouth, and at that time each of the first two consists of from five to seven main lobes, with numer- ous secondary lobes along their posterior bor- ders (Fig. 23, 4). The gills on the third arch are much smaller than those of the arches in front, and are nearly covered by them. The course of the circulation in the external gills may be easily seen under the microscope, each main lobe and each minor lobe being supplied with an efferent and an afferent blood-vessel (Fig. 23, BF and AF ). The opercular folds arise, before the mouth opens, as two folds of skin from the hyoid arches ; they unite with each other in the ven- tral line, and grow backward as a sort of hood over the external gills (Figs. 1,O,and 24). The posterior border of this hood fuses with the body wall behind the gills, on the right and ventral sides, but remains open on the left side as a sort of spout (Fig. 24, OA), through which the gills of that side frequently protrude, and through which the water, taken into the gill chamber through the mouth, passes again to 56 Vertebrate Embryology the exterior. The opercular folds are not com- pleted until after the formation of the mouth. The internal gills arise quite early as a double row of papillz on the first, second, and third visceral arches, below the external gills, and as a single row on the fourth arch. They are very vascular, and when the external gills begin to shrivel, they take up the function of respiration. The inner borders of the gill arches develop a sort of straining apparatus, to prevent solid substances from passing from the pharynx into the gill chamber. At the end of the tadpole life, as the lungs begin to function, the gill chamber is filled and gradually obliterated by the growth of lym- phoid and epithelial tissue, the gill clefts are closed by the fusion of their edges, and the gills are almost entirely absorbed, small por- tions persisting in the adult as the so-called tonsils. Toe DeEvELOPpMENT OF THE HEART AND Bioop VESSELS As has been stated above, the heart and blood vessels, as well as the blood itself, are formed from mesoblast, the chief point of in- terest being the changes that take place at The Development of the Frog 57 metamorphosis, when the circulation changes from practically that of a fish to that of the adult frog. Since changes similar to these H.B. a at Fic. 21.--TRANSVERSE SECTION THROUGH THE RE- GION OF THE HIND-BRAIN OF A YOUNG TADPOLE, D, digestive tract. £, ear vesicle. HB, hind-brain. J, notochord. S, sucker. (Camera lucida. take place in even the highest animals, they are of more than passing interest. The development of the heart and pericar- dium, though difficult to follow out in detail in the laboratory, will be readily understood from the following description, studied in connection with Fig. 25: 58 Vertebrate Embryology “The heart appears at the time when the medullary folds have rolled in, and have met along the mid-dorsal line; it lies below the pharynx, and anterior to the liver (Fig. 12). The mesoderm in this region shows a tendency to split into two sheets, and, where the heart is about to develop, a cavity, a part of the ccelom, appears between the sheets. A cross-section of the larva (Fig. 25, 4) shows on each side of the mid-ventral line in the region of the heart the somatic and splanchnic layers widely separated from each other. The ccelomic cavities of the right and left sides are not continuous across the middle line, but anterior and posterior to this section the ccelomic cavity is found to be continuous before and behind with the general ccelomic space on each side. A few scattered cells lie in the middle line between the splanchnic layer and the wall of the pharynx (Fig. 25, Z). Thesecells have been described as originating from the ventral wall of the archenteron, and, if so, have had a different origin from the other cells of the heart. “At a somewhat later stage of development the walls of the coelomic cavities of the right and left sides sepa- rate further (Fig. 25, 8). The splanchnic layer thickens, and begins to surround the proliferation of scattered ‘endodermal cells.” These endodermal cells arrange themselves in a thin-walled tube stretching throughout the heart region (Fig. 25). Subsequent development shows that this tube becomes the endothelial lining of the heart. Around this endothelial tube the thickened splanchnic layers now begin to push in from the sides between the tube and the lower wall of the pharynx. The tube becomes finally entirely surrounded by meso- derm (Fig. 25). The mesoderm from the sides that has BR.z Fic, 22, HORIZONTAL SECTION OF A TADPOLE AT THE TIME OF HATCHING, (After Marshall.) AF, afferent branchial vessel of the first branchial arch. BF, fore-brain. BRA, BR.2, BRB, first, second, and third branchial arches. C, body-cavity or celom. £ F, efferent branchial vessel of first branchial arch. HA , hyomandi- bular gill-pouch. A’ ¥,hyoidarch. /N,infundibulum, XA, archinephric duct of right side. KA’, archinephrie duct of left side, seen in section. P, head- kidney or pronephros. A'S, third nephrostome of right head-kidney. AS’, same of left side, seen in section. OF, olfactory pit. OS, optic stalk. 7, pharyn- geal region of mesenteron. 7Z/, intestinal region of mesenteron, FY, yoik-cells, 59 60 Vertebrate Embryology met beneath the pharynx forms the dorsal mesentery of the heart. The mesoderm around the tube continues to thicken, and forms later the musculature of the heart. “ At first the heart has also a ventral mesentery formed by the union of the walls of the ccelomic cavities below it (Fig. 25), but later the mesentery is in part absorbed and the ccelomic cavities become continuous below from side to side, forming the pericardial chamber. The outer layer of somatic mesoderm gives rise to the peri- cardium itself. “The tubular heart is attached at its posterior end to the liver and anteriorly to the wall of the pharynx. It becomes free ventrally and also dorsally along the middle of its course, and owing to an increase in length is bent on itself into an w-shaped tube (Fig. 14).”"' A series of transverse constrictions now gives indication of the division of the heart into the various chambers, though there are, for a time, no actual partitions between the different regions. A septum is finally formed which divides the single auricle into right and left halves, and by the time of metamorphosis the heart has practically the adult structure. Without distinguishing between the exter- nal and internal gills, the larval circulation is, briefly, as follows: ‘ The venous blood, re- turned from the body at large, enters the pos- terior end of the heart, or szzus venosus ; from 1 Morgan, The Development of the Frog 61 this it passes into the second or auricular chamber, thence to the ventrzcle, and from that to the ¢runcus arteriosus (Fig. 17).” As there is, at first, no division into right and left sides, the blood passes in succession through these various chambers. “The truncus arteriosus divides distally into right and left branches, from each of which four afferent branchial vessels (Fig. 26, 4 /, 1-4) arise. The four vessels of each side run outwards along the hinder borders of the four branchial arches, giving off along their whole length numerous branches to the gill-tufts on these arches. From the gills the blood, now aérated, passes into the efferent branchial vessels (Figs. 23 and 26, EF, 1-4). These lie alongside of the afferent branchial vessels, and just in front of them, but do not communicate with them except through the capillary loops of the gills. The four efferent branchial vessels of each side unite in the dorsal wall of the pharynx to form the dorsal aorta: the two aortz are continued forwards to the head as the carotid arteries, while posteriorly they unite to form the single dorsal aorta, from which branches arise supplying all parts of the body (Figs. 24, 4, and 26, 4). The lungs arise at a very early stage, but are for a long time extremely small and of little functional importance. Each lung receives blood from a branch of the fourth ef- ferent branchial vessel (Fig. 26, 47), and returns it di- rectly to the auricle by the pu/monary vein. As the tadpole increases in size, and the lungs become of greater im- portance, a septum appears, dividing the auricle into 62 Vertebrate Embryology systemic or venous, and pulmonary or arterial cavities. Simultaneously with this, valves are formed in the trun- cus arteriosus, by which the streams of venous and arte- rial blood are kept apart to a certain extent. At the time of the metamorphosis the gill circulation is cut off, by the establishment of direct communications between the afferent and efferent branchial vessels (Fig. 26), and the pulmonary circulation becomes of much greater im- portance than before.’’? The branchial blood vessels, or aortic arches, are six in number, and lie in the visceral arches, the afferent vessel lying parallel and posterior to the efferent vessel (Fig. 26). Of the branchial vessels, only those lying in the first, second, third, and fourth arches are func- tional, the vessels of the mandibular and hyoid arches being in a rudimentary condition. Although the afferent and efferent vessels lie so close together, there is at first, as has been said, no communication between them except through the gill capillaries (Fig. 23) which are given off from their sides, first to the external and then to the internal gills. As the direct communication between the afferent and efferent vessels, which lies near the ven- tral end of the arch (Fig. 26), becomes larger it is evidently easier for the blood to flow 1 Marshall. The Development of the Frog 63 through that passage than to pass through the fine capillaries of the gills, so that the supply of blood to the gills is gradually cut off, and the amount of blood that goes to the lungs is correspondingly increased ; but for a time the tadpole breathes both by gills and by lungs. If the tadpole be prevented from coming to the surface to breathe, as by fastening wire netting just below the surface of the water, it is said that the change to the lung-breathing condition may be indefinitely postponed. The changes in the circulation that take place at metamorphosis are chiefly concerned with changes in the branchial blood vessels, or, as they are called after the disappearance of the capillaries and the establishment of the direct communication, the aortzc arches. Some of the details of these changes have not been made out as satisfactorily as is to be de- sired, but the main points are pretty definitely established. Since the branchial blood vessels in the man- dibular and hyoid arches are, from the first, rudimentary, they may be disregarded in this discussion. After the establishment of the di- rect communication between the afferent and efferent branchial vessels, the blood passes Gow y vD VH a VM RA RV ARS are any VK i 64 FIG, 23. Fic, 23. A.—DIAGRAMMATIC FIGURE OF THE HEAD AND ANTE- RIOR PART OF THE BODY OF A 7-MM. TADPOLE, SHORTLY AFTER HATCHING ; SHOWING THE BRANCHIAL BLOOD VESSELS FROM THE VENTRAL SURFACE, THE HEART HAS BEEN REMOVED, B.—SAME EMBRYO, FROM THE RIGHT SIDE, THE HEART IS REP- RESENTED IN SITU, BUT THE EXTERNAL GILLS OF THE FIRST AND SECOND BRANCHIAL ARCHES HAVE BEEN CUT OFF SHORT AT THEIR BASES. (After Marshall.) A, dorsal aosta. A B, basilar artery. AC, carotid artery. A #1, A F.2, A F.3, afferent branchial vessels of the first, second, and third branchial ‘arches. AP, ‘pulmonary artery. 4 R, anterior cerebral artery. A 7, anterior palatine artery. CA, anterior commissural vessel. CP, posterior commissural vessel. EFA, Ef 2, £F3, EFA, efferent branchial vessels of the first, second, third, and fourth branchial arches. £ H, efferent vessel ot proud arch. EM, efferent vessel of mandibular arch. GA, external gills. glomerulus. KA, seg- mental duct. AP, head- kidney or pronephros. CLF AS.3, first and ‘third nephrostomes of head-kidney. LV.4, efferent lacunar vessel of fourth branchial arch. RA, auricle. RV, ventricle. X 7, truncus arteriosus, / D, Cuvierian vein, VA, hepatic veins. VA, vein of sucker, VM, mandibular vein, VY, hyoidean vein. 65 FIG. 24.—A 12-MM. TADPOLE DISSECTED FROM THE VENTRAL SURFACE TO SHOW THE HEART, THE INTERNAL GILLS, THE BRAN- CHIAL VESSELS, AND THE HEAD-KIDNEYS AND THEIR DUCTS. THE TAIL, WHICH IS ABOUT DOUBLE THE LENGTH OF THE HEAD AND BODY, HAS BEEN REMOVED. XX 22. (After Marshall.) A, dorsal aorta. A F.1, A F.3, afferent branchial vessels of first and th’ri branchial arches. 4 Z, lingual artery. CG, carotid gland. A, junction be. tween afferent and efferent branchial vessels of first branchial arch. & /.1, & 4.3, efferent branchial vessels of first and third branchial arches. GM, glomerulus. & A, archinephric or segmental duct. AM, Wolffian tubules. AP, prone- phros or head-kidney. A’ S.1, A’S.3, first and third nephrostomes of head-kidney- LJ, upper lip. Z/, lower hp. LZ, hind limb. OA, aperture of opercular cavity. OP, opercular cavity. AS, sinus venosus. 7, truncus arteriosus. RV, ventricle. TC, cloaca.” YO, cesophagus, cut short. 7 R, rectal spout. ° 66 The Development of the Frog 67 from the bulbus arteriosus directly around the pharynx, through the four aortic arches, into the dorsal aorta. Previous to this time a branch has grown out from the fourth aortic arch to the lung, the pulmonary artery (Fig, 26, AP), and as the gills diminish more and more in size, this vessel becomes larger and larger until it carries all of the blood that formerly went to the gills for purification. From the lungs the blood is brought back di- rectly to the heart by the pulmonary veins. The first aortic arch, on the completion of metamorphosis, becomes the carotzd arch of the frog, which carries blood to the head (Fig. 27,1). That portion of the dorsal aorta be- tween the openings of the first and second aortic arches may remain open, but more com- monly becomes entirely obliterated. The second aortic arch of the tadpole be- comes the systemzc arch of the adult frog (Figs 27, 2), The third aortic arch gradually diminishes in size, and eventually entirely disappears. The fourth aortic arch of the tadpole be- comes the pulmo-cutancous arch of the frog, carrying blood to the lungs and skin, as the name would indicate (Figs. 26 and 27, 3). 68 Vertebrate Embryology It retains its connection with the dorsal aorta for a considerable time, but eventually be- comes separated from it, so that all of the blood that now passes from the bulbus arte- riosus directly to the aorta must pass through the third or systemic arch (Fig. 27, 2). Fic, 25. DIAGRAMS TO ILLUSTRATE THE MODE OF DEVELOP- MENT OF THE HEART, En, entoderm, £c, ectoderm, £, endothelial lining of the heart. MM, muscular wall of heart. AZes, mesoderm. Pc, pericardium. PA, pharynx. (Somewhat altered from Morgan.) The caroted gland, a characteristic structure in the anatomy of the frog, is formed as an elaboration of the direct communication be- tween the afferent and efferent vessels of the first branchial arch. The development of the other blood vessels will be described in connection with the chick, The Development of the Frog 69 where they approach more nearly the condition in man and other mammals. The spleen, since it is so intimately associated with the blood, may be mentioned at this time. ‘The spleen arises as a spherical bud on the mesenteric artery : it consists of cells similar to those of the lymphatic tissue, and, like these, is said to be derived originally from the ento- blast cells of the mesenteron.”? DEVELOPMENT OF THE C@&LOM AND THE Mus- CULAR SYSTEM The formation of the mesoblast as a layer of cells between the ectoblast and entoblast, and the splitting of this mesoblast into two layers, the somatopleure and splanchnopleure, with the body-cavity or cceelom between them, has already been mentioned (Fig. 15). The sheet of mesoblast on each side of the body rapidly grows ventralward until it meets and fuses with its fellow of the opposite side, so that there is very early a complete layer of mesoblast over the ventral side of the embryo (Figs. 13, JZ, and 15). Along the mid-dorsal line, however, the two sheets of mesoblast remain distinct, being separated by 1 Marshall. 70 Vertebrate Embryology the notochord (Fig. 13, WV). In the anterior end of the embryo the mesoblast is much thinner, and does not there split into the two layers. On each side of the notochord the meso- blast becomes thickened to form the segmental plate, which does not, at first, show any sepa- ration into two layers; later, however, the body-cavity does extend into the segmental plate, for a time, but eventually disappears from that region. At about the time when the medullary folds are coming together to form the neural canal, the segmental plate on each side of the noto- chord begins to be broken up into blocks by a series of vertical connective tissue septa at right angles to the notochord. These blocks or segments are the sesoblastzc somites or myotomes. The mesoblastic somites are, at first, not separated from the lateral sheets of mesoblast, but very soon after their formation they become separated from the lateral meso- blast, and, by the thickening of their walls, especially the inner, their cavities are ob- literated. At a somewhat later stage the mesoblastic somites are largely converted into muscles, whose >-shaped arrangement may be The Development of the Frog 71 easily seen in the transparent tail of the young tadpole (Fig. 1, /—O). The lateral plates of the A GM AB AU EFs Efe CP EF oq | Co Ti aD AP EFa vP vi VH vo Are RB RA RV Afe BT Fic, 26.—A DIAGRAMMATIC FIGURE OF THE HEAD AND NECK OF A 12-MM. TADPOLE, FROM THE RIGHT SIDE, TO SHOW THE HEART AND BRANCHIAL VESSELS. THE GILLS AND THE GILL CAPILLARIES ARE NOT REPRESENTED. X 35. (After Marshall.) A, dorsalaorta.