American Journal of Anatomy 12 (1911-12) - Revision history

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CONTENTS

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	===No. 1. JULY, 1911===		===No. 1. JULY, 1911===
	{{Ref-~~Pohlman1912~~}}		{{Ref-Pohlman1911}}

	Augustus G. Pohlman. The development of the cloaca in human embryos. Seven figures 1		Augustus G. Pohlman. The development of the cloaca in human embryos. Seven figures 1

	{{Ref-~~Lisser1912~~}}		{{Ref-Lisser1911}}

	H. LissER. Studies on the development of the human larynx. Thirty-nine figures 27		H. LissER. Studies on the development of the human larynx. Thirty-nine figures 27
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	S. Walter Ranson. Non-medullated nerve fibers in the spinal nerves. Seven figures 67		S. Walter Ranson. Non-medullated nerve fibers in the spinal nerves. Seven figures 67

	{{Ref-~~Whitehead1912~~}}		{{Ref-Whitehead1911}}

	{{Ref-~~WhiteheadWaddell1912~~}		{{Ref-WhiteheadWaddell1911}

	R. H. Whitehead and J. A. Waddell. The early development of the mammalian sternum. Eight figures 89		R. H. Whitehead and J. A. Waddell. The early development of the mammalian sternum. Eight figures 89

	===No. 2. SEPTEMBER, 1911===		===No. 2. SEPTEMBER, 1911===
	{{Ref-~~Lowrey1912~~}}		{{Ref-Lowrey1911}}

	Lawson G. Lowrey Prenatal growth of the pig. Five figures 107		Lawson G. Lowrey Prenatal growth of the pig. Five figures 107
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	R. R. Hknsley. Studies on the pancreas of the guinea pig. Fifteen figures . 297		R. R. Hknsley. Studies on the pancreas of the guinea pig. Fifteen figures . 297

	{{Ref-~~Lewis1912~~}}		{{Ref-Lewis1911}}

	Fredeuic T. Lewis. The bi-lobed form of the ventral pancreas in mammals. Twelve figures.		Fredeuic T. Lewis. The bi-lobed form of the ventral pancreas in mammals. Twelve figures.

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	VOLUME 12 1911-1912		VOLUME 12 1911-1912


	THE WISTAR INSTITUTE OF ANATOMY AND BIOLOGY		THE WISTAR INSTITUTE OF ANATOMY AND BIOLOGY

	PHILADELPHIA, PA.		PHILADELPHIA, PA.



	COMPOSED AND PRINTED AT THE WAVERLY PRESS		COMPOSED AND PRINTED AT THE WAVERLY PRESS
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	Baltimore, U. S. A.		Baltimore, U. S. A.



	==CONTENTS==		==CONTENTS==
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	1911-1912		1911-1912

			===No. 1. JULY, 1911===
			{{Ref-Pohlman1912}}

			Augustus G. Pohlman. The development of the cloaca in human embryos. Seven figures 1

	~~No. 1. JULY, 1911~~		{{Ref-Lisser1912}}

	~~Augustus G~~. ~~Pohlman~~. ~~The~~ development of the ~~cloaca in~~ human ~~embryos~~. ~~Seven~~ figures 1		H. LissER. Studies on the development of the human larynx. Thirty-nine figures 27

	H. ~~LissER~~. ~~Studies on~~ the ~~development of the human larynx~~. ~~Thirty-nine~~ figures 27		S. Walter Ranson. Non-medullated nerve fibers in the spinal nerves. Seven figures 67

	~~S. Walter Ranson. Non~~-~~medullated nerve fibers in the spinal nerves.~~		{{Ref-Whitehead1912}}

	~~Seven figures 67~~		{{Ref-WhiteheadWaddell1912}

	R. H. Whitehead and J. A. Waddell. The early development of the mammalian sternum. Eight figures 89		R. H. Whitehead and J. A. Waddell. The early development of the mammalian sternum. Eight figures 89

			===No. 2. SEPTEMBER, 1911===
			{{Ref-Lowrey1912}}
	No. 2. SEPTEMBER, 1911

	Lawson G. Lowrey Prenatal growth of the pig. Five figures 107		Lawson G. Lowrey Prenatal growth of the pig. Five figures 107

	W. H. Long LEY. The maturation of the egg and ovulation in the domestic		W. H. Long LEY. The maturation of the egg and ovulation in the domestic cat. Thirteen figures 139

	~~cat~~. ~~Thirteen~~ figures ~~139~~		Hubert Dana Goodale. The early development of Spelerpes bilineatus (Green). One plate and seventy-seven text figures 173

	~~Hubert Dana Goodale~~. ~~The early development of Spelerpes bilineatus~~		===No. 3. NOVEMBER, 1911===

	(~~Green)~~. ~~One plate and seventy-seven text~~ figures ~~173~~		(/ H. E. Jordan. The histogenesis of the pineal body of the sheep. Eleven figures 249

			J. S. Ferguson. The application of the silver impregnation method of Bielschow.sky to reticular and other connective tissues. I. The mature tissues. thirteen figures 277

	~~No. 3. NOVEMBER, 1911~~

	~~(/ H. E. Jordan. The histogenesis of the pineal body of the sheep. Eleven~~

	~~figures 249~~


	J. S. Ferguson. The application of the .silver impregnation method of Bielschow.sky to reticular and other connective tissues. I. The mature tissues. ~~'IMiirteen~~ figures 277

	R. R. Hknsley. Studies on the pancreas of the guinea pig. Fifteen figures . 297		R. R. Hknsley. Studies on the pancreas of the guinea pig. Fifteen figures . 297

	~~Fredeuic T. Lewis. The bi~~-~~lobed form of the ventral pancreas in mammals.~~		{{Ref-Lewis1912}}

	Twelve figures.		Fredeuic T. Lewis. The bi-lobed form of the ventral pancreas in mammals. Twelve figures.

			===No. 4 JANUARY, 1912===

			John H. Stokes. The acoustic complex and its relations in the brain of the opossum (Didelphys Virginiana). Fourteen figures 401
	~~No. 4 JANUARY, 1912~~

	John H. Stokes. The acoustic complex and its relations in the brain of the

	opossum (Didelphys Virginiana). Fourteen figures 401

	Charles F. Silvester. On the presence of permanent communication between the lymphatic and the venous systems at the level of the renal veins in adult South American monkeys. Twelve figures 447		Charles F. Silvester. On the presence of permanent communication between the lymphatic and the venous systems at the level of the renal veins in adult South American monkeys. Twelve figures 447

	A. M. M'LLEr. The development of the jugular lymph sac in birds. Ten		A. M. M'LLEr. The development of the jugular lymph sac in birds. Ten figures 473

	figures 473

	~~Frank Reagan. The fifth aortic arch of mammalian embryos; the nature of~~

	~~the last pharyngeal evagination. Sixteen figures 493~~

			{{Ref-Reagan1912}}

			Frank Reagan. The fifth aortic arch of mammalian embryos; the nature of the last pharyngeal evagination. Sixteen figures 493

	==~~THE DEVELOPMENT OF THE CLOACA IN HUMAN EMBRYOS~~==		==The Development Of The Cloaca In Human Embryos==

	AUGUSTUS G. POHLMAN Indiana University		AUGUSTUS G. POHLMAN Indiana University

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THE DEVELOPMENT OF THE CLOACA IN HUMAN EMBRYOS

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	STUDIES ON THE DEVELOPMENT OF THE HUMAN		==STUDIES ON THE DEVELOPMENT OF THE HUMAN LARYNX==

	LARYNX

	H. LISSER		H. LISSER
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	The investigation of the embryonic larynx has been by no means neglected. It has received the attention of many investigators resulting in several valuable contributions. Either by reason of limited material or of especial interest in particular features, these researches have usually been directed into the consideration of only parts of this complex field. There is still lacking a well rounded comprehensive review of the whole subject. Nor does this paper propose to accomplish this. The scope has been limited to a study of the cartilages, muscles, and nerves, during that period of embryonic life, when the most active development occurs. This comprises those stages where these respective elements are first definitely recognizable, to where they assume more or less, their adult relationship, namely, from the 10.5 mm. (5 weeks?) human embryo to the 20 mm. (7^ weeks?) human embryo. Investigations on earlier stages although perhaps somewhat problematic in results have been made, and excellent accounts of still later development can be found in the literature.		The investigation of the embryonic larynx has been by no means neglected. It has received the attention of many investigators resulting in several valuable contributions. Either by reason of limited material or of especial interest in particular features, these researches have usually been directed into the consideration of only parts of this complex field. There is still lacking a well rounded comprehensive review of the whole subject. Nor does this paper propose to accomplish this. The scope has been limited to a study of the cartilages, muscles, and nerves, during that period of embryonic life, when the most active development occurs. This comprises those stages where these respective elements are first definitely recognizable, to where they assume more or less, their adult relationship, namely, from the 10.5 mm. (5 weeks?) human embryo to the 20 mm. (7^ weeks?) human embryo. Investigations on earlier stages although perhaps somewhat problematic in results have been made, and excellent accounts of still later development can be found in the literature.

	. METHODS AND ~~MATERL\LS~~		. METHODS AND MATERIALS

	The material at my disposal was Dr. Mall's collection of human embryos, for the use of which I am sincerely grateful. Likewise at this time, I desire to express my appreciation of the interest and advice of Dr. Warren H. Lewis, under whose direction these stud 27



	28



	~~H. LISSER~~


			The material at my disposal was Dr. Mall's collection of human embryos, for the use of which I am sincerely grateful. Likewise at this time, I desire to express my appreciation of the interest and advice of Dr. Warren H. Lewis, under whose direction these studies were undertaken, and to Mr. Max Broedel for valuable suggestions regarding some of the illustrations.
	~~ies~~ were undertaken, and to Mr. Max Broedel for valuable suggestions regarding some of the illustrations.

	I have included a large number of drawings in the hope that they would help to make lucid the descriptions of the various structures involved. Accuracy and fidelity to the original sections has alwaj's been the main object. However, it must be said, that in reproducing embryonic tissue, peculiar difficulties are encountered. Condensed mesenchyma is not as a rule clearly outlined, but shades gradually into the surrounding tissue, so that some exaggeration is permissible, and in fact necessary in giving sharp clear outlines to such structures for proper interpretations and the construction of models.		I have included a large number of drawings in the hope that they would help to make lucid the descriptions of the various structures involved. Accuracy and fidelity to the original sections has alwaj's been the main object. However, it must be said, that in reproducing embryonic tissue, peculiar difficulties are encountered. Condensed mesenchyma is not as a rule clearly outlined, but shades gradually into the surrounding tissue, so that some exaggeration is permissible, and in fact necessary in giving sharp clear outlines to such structures for proper interpretations and the construction of models.
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	1 24th Session. Amer. Assoc, of Anat., Baltimore, Md., December 1908-January 1909. Lisser: Models to show development of human larynx.		1 24th Session. Amer. Assoc, of Anat., Baltimore, Md., December 1908-January 1909. Lisser: Models to show development of human larynx.



	~~DEVELOPMENT OF THE HUMAN LARYNX 29~~

	In considering the musculature of the larynx, it soon became evident, after dissections and studies of the adult larynx, that there was by no means unanimity of opinion among different authorities as to just what constituted the muscles of the larynx. Of course, every one recognized and mentioned the cricothyreoideus, the cricoarytaenoideus posterior and the cricoarytaenoideus lateralis. But some spoke of an inter artyaenoideus muscle, others divided this into an arytaenoideus transversus and artyaenoideus obliquus. Some added an aryepiglotticus and a thyreoepiglotticus ; others merely pictured one of these, and some, neither of them. Others again, divide the thyreo-arytaenoid into an internus and externus, and Sewell Seymour ('05) has dissected a 'small or superficial thyreo-arytaenoid muscle' which, moreover, he differentiates into four tj^pes. It is therefore apparent that there is not yet a clear cut conception of the exact musculature of the normal adult larynx. Certainly, if such difficulties present themselves in the adult fully formed larynx, how much more confusion would there be in attempting to isolate these varieties in a human embryo. So that I arbitrarily selected the following nomenclature for the laryngeal musculature, as being the simplest and most reliable:		In considering the musculature of the larynx, it soon became evident, after dissections and studies of the adult larynx, that there was by no means unanimity of opinion among different authorities as to just what constituted the muscles of the larynx. Of course, every one recognized and mentioned the cricothyreoideus, the cricoarytaenoideus posterior and the cricoarytaenoideus lateralis. But some spoke of an inter artyaenoideus muscle, others divided this into an arytaenoideus transversus and artyaenoideus obliquus. Some added an aryepiglotticus and a thyreoepiglotticus ; others merely pictured one of these, and some, neither of them. Others again, divide the thyreo-arytaenoid into an internus and externus, and Sewell Seymour ('05) has dissected a 'small or superficial thyreo-arytaenoid muscle' which, moreover, he differentiates into four tj^pes. It is therefore apparent that there is not yet a clear cut conception of the exact musculature of the normal adult larynx. Certainly, if such difficulties present themselves in the adult fully formed larynx, how much more confusion would there be in attempting to isolate these varieties in a human embryo. So that I arbitrarily selected the following nomenclature for the laryngeal musculature, as being the simplest and most reliable:
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			THE CARTILAGES
	~~30 Jl. LISSER~~

	~~TPIE~~ CARTILAGES

	The cartilages of the larjaix, or better what become the cartilages of the laryiix, are at this stage but very imperfectly formed, as is to be expected, since they reveal themselves for the first time in a 10.5 mm. human embryo. Consequently their appearance is introduced by condensations of mesenchyma, very well termed "pre cartilage." No true cartilage formation, whatsoever, has occurred at this time in any of the laryngeal cartilages, nor yet in the hyoid bone and styloid process. I have investigated the sections of 8, 8.5, 9, and 10 mm. embryos for earlier traces of these structures, but have been consistently unsuccessful, so that I place the first recognizable stage of the larynx skeleton (if one may be permitted to use such an expression) at 10.5 mm. in man. Of course, it is well known that all the structures of the larynx have their ultimate foundation in the gill arches, and I refer those interested in this very early stage of the subject to Frazaer's interesting studies ; but I would be skeptical of finding, wdth the present means of study at our disposal, actual structural status, as regards separate cartilage and muscle masses, earlier* than 10.5 mm. in man.		The cartilages of the larjaix, or better what become the cartilages of the laryiix, are at this stage but very imperfectly formed, as is to be expected, since they reveal themselves for the first time in a 10.5 mm. human embryo. Consequently their appearance is introduced by condensations of mesenchyma, very well termed "pre cartilage." No true cartilage formation, whatsoever, has occurred at this time in any of the laryngeal cartilages, nor yet in the hyoid bone and styloid process. I have investigated the sections of 8, 8.5, 9, and 10 mm. embryos for earlier traces of these structures, but have been consistently unsuccessful, so that I place the first recognizable stage of the larynx skeleton (if one may be permitted to use such an expression) at 10.5 mm. in man. Of course, it is well known that all the structures of the larynx have their ultimate foundation in the gill arches, and I refer those interested in this very early stage of the subject to Frazaer's interesting studies ; but I would be skeptical of finding, wdth the present means of study at our disposal, actual structural status, as regards separate cartilage and muscle masses, earlier* than 10.5 mm. in man.

	The cricoid-cartilage. {Figs. 1-2.) Contrary to expectations, the cricoid cartilage does not appear to develop from two lateral portions of condensed mesenchyma, separate and independent of each other, which later grow together ventrally and dorsally about the lumen of the larynx; nor as one might be led to anticipate, form a large area posterior or dorsal to the larnyx lumen (this portion later becoming more prominent by far than the anterior arcus). But at this stage, there is a predominance of condensed mesenchyma about the ventral portion or arcus, which fades ofT laterally, and then becomes more emphasized again, by greater compactness and deeper stain, dorsal or posterior to the lumen, but not so extensively nor so well marked as ventrally. This is true provided one considers the deeper staining, more strikingly isolated portions of condensed mesenchyma, as the anlage. So that it seems reasonable to assume that the cricoid cartilage originates from an anlage primarily ventral in the position of what later be		The cricoid-cartilage. {Figs. 1-2.) Contrary to expectations, the cricoid cartilage does not appear to develop from two lateral portions of condensed mesenchyma, separate and independent of each other, which later grow together ventrally and dorsally about the lumen of the larynx; nor as one might be led to anticipate, form a large area posterior or dorsal to the larnyx lumen (this portion later becoming more prominent by far than the anterior arcus). But at this stage, there is a predominance of condensed mesenchyma about the ventral portion or arcus, which fades ofT laterally, and then becomes more emphasized again, by greater compactness and deeper stain, dorsal or posterior to the lumen, but not so extensively nor so well marked as ventrally. This is true provided one considers the deeper staining, more strikingly isolated portions of condensed mesenchyma, as the anlage. So that it seems reasonable to assume that the cricoid cartilage originates from an anlage primarily ventral in the position of what later becomes the anterior arcus, and also, though less prominently from a posterior portion, of itself perhaps, originating from two slightly separated posteriorly lateral portions. The lateral portions then, develop by a welding of the anterior and posterior portions lateralward. At this stage then, the ventral portion of the cricoid cartilage is appreciably advanced in development over the rest of this structure. It does seem difficult to reconcile this with the appearance, for instance, in the 20 mm. stage, when the ventral portion still persists as condensed mesenchyma, while the lateral parts have undergone considerable chondrification.


	~~DEVELOPMENT OF THE HUMAN LARYNX 31~~

	~~comes~~ the anterior arcus, and also, though less prominently from a posterior portion, of itself perhaps, originating from two slightly separated posteriorly lateral portions. The lateral portions then, develop by a welding of the anterior and posterior portions lateralward. At this stage then, the ventral portion of the cricoid cartilage is appreciably advanced in development over the rest of this structure. It does seem difficult to reconcile this with the appearance, for instance, in the 20 mm. stage, when the ventral portion still persists as condensed mesenchyma, while the lateral parts have undergone considerable chondrification.

	The arytaenoid cartilages. It is doubtful whether these structures can be determined at this early period in development; probably not. There are two faint, indefinite masses which suggest beginning condensation, but as there is a possibility of these being a part of the superior portion of the cricoid, no positive assertions can be made as to their independence.		The arytaenoid cartilages. It is doubtful whether these structures can be determined at this early period in development; probably not. There are two faint, indefinite masses which suggest beginning condensation, but as there is a possibility of these being a part of the superior portion of the cricoid, no positive assertions can be made as to their independence.

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	~~THE AMERICAN JOURNAL~~



	OF



	~~ANATOMY~~



	EDITORIAL BOARD Charles R. Bardeen George S. Huntington		EDITORIAL BOARD Charles R. Bardeen George S. Huntington
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	University of Cincinnati		University of Cincinnati



	VOLUME 12 1911-1912		VOLUME 12 1911-1912



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	~~/ :xa ^~~		COMPOSED AND PRINTED AT THE WAVERLY PRESS



	COMPOSED AND PRINTED AT THE

	WAVERLY PRESS

	Bt the Williams & Wilkins Company		Bt the Williams & Wilkins Company
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	CONTENTS		==CONTENTS==

	1911-1912		1911-1912
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	figures 249		figures 249

	~~iii~~



	~~iv CONTENTS~~

	J. S. Ferguson. The application of the .silver impregnation method of Bielschow.sky to reticular and other connective tissues. I. The mature tissues. 'IMiirteen figures 277		J. S. Ferguson. The application of the .silver impregnation method of Bielschow.sky to reticular and other connective tissues. I. The mature tissues. 'IMiirteen figures 277
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	Fredeuic T. Lewis. The bi-lobed form of the ventral pancreas in mammals.		Fredeuic T. Lewis. The bi-lobed form of the ventral pancreas in mammals.

	Twelve figures. ~~'■^^^~~		Twelve figures.


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	THE DEVELOPMENT OF THE CLOACA IN HUMAN		==THE DEVELOPMENT OF THE CLOACA IN HUMAN EMBRYOS==

	EMBRYOS

	AUGUSTUS G. POHLMAN Indiana University		AUGUSTUS G. POHLMAN Indiana University
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	The active work in the embryology of the urogenital system began about 1880 and continued for a period of some fifteen years when Keibel's monograph was published. The writer's investigation of the cloacal region was started in 1903, and was undertaken in part as a control of Keibel's work, and in part with a view toward solving some of the points on which differences in opinion existed. The publication of this report has been delayed in the hope that certain facts in comparative embryology might be established and help to clarify some of the obscure relations found in the human embryo. Inasmuch as the extensive investigations of Fleischmann and his students have come to naught in this respect, the major differences in opinion will be considered,		The active work in the embryology of the urogenital system began about 1880 and continued for a period of some fifteen years when Keibel's monograph was published. The writer's investigation of the cloacal region was started in 1903, and was undertaken in part as a control of Keibel's work, and in part with a view toward solving some of the points on which differences in opinion existed. The publication of this report has been delayed in the hope that certain facts in comparative embryology might be established and help to clarify some of the obscure relations found in the human embryo. Inasmuch as the extensive investigations of Fleischmann and his students have come to naught in this respect, the major differences in opinion will be considered,

	~~THE AMERICAN JOURNAL OF ANATOMY, VOL. 12, NO. 1 JULY, 1911~~

	1



	~~2 AUGUSTUS G. POHLMAN~~

	and the doubtful points answered in so far as it is possible. The short literature review covers the essential facts and effort has been made to reduce the description of the material to a concise tabulation. The writer expresses his indebtedness to Prof. Keibel at whose suggestion the development of the later stages in the embr^^ologj' was undertaken, and to Prof. F. P. Mall for the use of his collection of embrj os and for the many courtesies shown him in the Anatomical Laborator}^ of Johns Hopkins University.		and the doubtful points answered in so far as it is possible. The short literature review covers the essential facts and effort has been made to reduce the description of the material to a concise tabulation. The writer expresses his indebtedness to Prof. Keibel at whose suggestion the development of the later stages in the embr^^ologj' was undertaken, and to Prof. F. P. Mall for the use of his collection of embrj os and for the many courtesies shown him in the Anatomical Laborator}^ of Johns Hopkins University.

	Born's article ('93) reviews the earlier development of the cloacal region in a very complete manner, and the substance is as follows: The entoderm of the enteron comes into direct relation with the surface ectoderm in the pharyngeal and cloacal membranes during the formation of the head and tail folds. Both of these membranes lose their primitive position and become folded into the substance of the embryo through increase in the surrounding mesoderm. The allantois, which is developed dorsally in the mammalian embryo (human and guinea pig excepted), shifts to a ventral position on the gut, and is gradually displaced from its intimate relation to the yolk sac through increase in the amount of mesodermal tissue. The primitive streak is carried to the ventral surface of the body during the formation of the tail fold, and forms the whole or part of the cloacal membrane. Kolliker f'83), Strahl ('83, '84), and Bonnet ('88) believe that the caudal end of the primitive streak is made up of applied layers of ectoand entoderm, and that it enters as such into the formation of the cloacal membrane. Keibel ('88) argues that this primitive relation of the ecto- and entoderm is lost through interposition of mesoderm; the latter disappearing later with restoration of the original two layered condition.		Born's article ('93) reviews the earlier development of the cloacal region in a very complete manner, and the substance is as follows: The entoderm of the enteron comes into direct relation with the surface ectoderm in the pharyngeal and cloacal membranes during the formation of the head and tail folds. Both of these membranes lose their primitive position and become folded into the substance of the embryo through increase in the surrounding mesoderm. The allantois, which is developed dorsally in the mammalian embryo (human and guinea pig excepted), shifts to a ventral position on the gut, and is gradually displaced from its intimate relation to the yolk sac through increase in the amount of mesodermal tissue. The primitive streak is carried to the ventral surface of the body during the formation of the tail fold, and forms the whole or part of the cloacal membrane. Kolliker f'83), Strahl ('83, '84), and Bonnet ('88) believe that the caudal end of the primitive streak is made up of applied layers of ectoand entoderm, and that it enters as such into the formation of the cloacal membrane. Keibel ('88) argues that this primitive relation of the ecto- and entoderm is lost through interposition of mesoderm; the latter disappearing later with restoration of the original two layered condition.

	The model of the 4.2 mm. human embryo presented by Keibel ('88) shows the hind gut and widely lumened allantois opening cephalward into the caudal entodermal sac or cloaca. Ventrally, this cloaca is limited as far as the dermal navel by the epithelial cloacal membrane. Caudalward, the limit of the cloacal membrane comes about by a mesodermic separation of the epithelial layers. The gut segment distal to the lower limit of the cloacal		The model of the 4.2 mm. human embryo presented by Keibel ('88) shows the hind gut and widely lumened allantois opening cephalward into the caudal entodermal sac or cloaca. Ventrally, this cloaca is limited as far as the dermal navel by the epithelial cloacal membrane. Caudalward, the limit of the cloacal membrane comes about by a mesodermic separation of the epithelial layers. The gut segment distal to the lower limit of the cloacal membrane may be termed the tail gut and terminates in an undifferentiated cell mass formed by itself, the chorda and neural tube. Born emphasizes the length of the cloacal membrane as follows: I call particular attention to the original extent of the cloacal membrane. It reaches cephalward to the point where the allantois leaves the body at right angles; i.e., as far as the caudal border of the dermal navel."



	~~THE CLOACA IN HUMAN EMBRYOS 3~~

	membrane may be termed the tail gut and terminates in an undifferentiated cell mass formed by itself, the chorda and neural tube. Born emphasizes the length of the cloacal membrane as follows: I call particular attention to the original extent of the cloacal membrane. It reaches cephalward to the point where the allantois leaves the body at right angles; i.e., as far as the caudal border of the dermal navel."

	The cloacal membrane does not extend as far as the allantois in the model of an 8.0 mm. human embryo presented by Keibel ('91) . Mesodermic tissue has apparently wandered in from above and separated the layers of epithelium. The cloacal membrane is therefore shorter than in the 4.2 mm. stage. The tail gut shows degeneration. The precloacal mesodermic tissue has increased in amount but instead of displacing the cloacal membrane caudalward, has folded it into the lower surface of the genital tubercle as was first described by Tourneux ('89), and verified by Retterer ('90) and Reichel ('93). No intermediate stage in the development has been described up to this time ('93). With the increase in size of the genital tubercle, the entodermal cloaca becomes more deeply placed as was demonstrated by Reichel ('93). Born states that the epithelial plate contained within the genital tubercle is ectodermic and that it is continuous with the superficial ectoderm. The epithelial plate occupies the caudal surface of the eminence and is bordered laterally by folds of mesoderm (repli ano-genitaux of Retterer) , while the cloacal membrane itself terminates at the postanal fold (replis postanal). , The depression arises (as in the mouth region) through increase in the height of the limiting borders. The depression is always closed in by epithelium, and the base of the depression is never separated from the entodermal cloaca by mesodermic tissue." (Born.)		The cloacal membrane does not extend as far as the allantois in the model of an 8.0 mm. human embryo presented by Keibel ('91) . Mesodermic tissue has apparently wandered in from above and separated the layers of epithelium. The cloacal membrane is therefore shorter than in the 4.2 mm. stage. The tail gut shows degeneration. The precloacal mesodermic tissue has increased in amount but instead of displacing the cloacal membrane caudalward, has folded it into the lower surface of the genital tubercle as was first described by Tourneux ('89), and verified by Retterer ('90) and Reichel ('93). No intermediate stage in the development has been described up to this time ('93). With the increase in size of the genital tubercle, the entodermal cloaca becomes more deeply placed as was demonstrated by Reichel ('93). Born states that the epithelial plate contained within the genital tubercle is ectodermic and that it is continuous with the superficial ectoderm. The epithelial plate occupies the caudal surface of the eminence and is bordered laterally by folds of mesoderm (repli ano-genitaux of Retterer) , while the cloacal membrane itself terminates at the postanal fold (replis postanal). , The depression arises (as in the mouth region) through increase in the height of the limiting borders. The depression is always closed in by epithelium, and the base of the depression is never separated from the entodermal cloaca by mesodermic tissue." (Born.)

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