in the Animal Kingdom. 123 



situation occupied by the Germinal Vesicle, 'before the bursting 

 of the latter ; the part exuberant, projecting, but not being dis- 

 tinguishable from the rest, by a well-defined border. The pro- 

 jecting portion becomes more and more distinct, until its grow- 

 ing independence is manifested, in a tendency to withdraw itself 

 from the remainder. (See Plate I. fig. 5.) 



This separation of the central part of the Germinal Mem- 

 brane from its periphery and from the yolk, gives rise eventual- 

 ly to the appended Umbilical Vesicle in Man and other Mam- 

 mals. In Birds, the corresponding part is taken into the abdo- 

 men. In Frogs, the embryo occupies, from the first, so large a 

 portion of the Germinal Membrane, and the latter so nearly sur- 

 rounds the yolk, that the yolk becomes contained in the em- 

 bryo, before the independence of the latter has time to manifest 

 itself by a tendency to withdrawal. 



Explanation of Plate I.* 



Fig. 3. Transverse section of the Germinal Membrane and incipient Em- 

 bryo of the Common FowL 

 Fig. 4- Ditto, more advanced. 



Fig. 5. Transverse section of an Embryo of the Common Fowl. 



* Primary membrane enclosing the Germ and Yolk. 



1 * Serous, or Animal layer. 



'* Mucous, or Vegetative layer. 



B Sinus or Vein, bounding the Vascular Area. 



a Chorda vertebralis. 



b Outer margin of the Lamina dorsalis. 



c Upper margin of the same ; afterwards the Mesial Line of the back. 

 be Lamina dorsalis. 



d Outer (and afterwards under) margin of the Lamina ventralis. 

 bd Lamina ventralis. 



e Flexure of the serous lamina. 

 de Membranous portion of the abdominal paries. 



/ Margin of the lateral envelope. 



ff Lateral part of the fold of the Amnion ; afterwards, the closing-point of 



the Amnion. 

 dea Amnion. 



h Upper angle of the mesenteric lamina. 



i Under angle of the mesenteric lamina ; afterwards the suture of the 



Mesentery. 

 hi The mesenteric lamina. 



k The vascular lamina on the Intestine. 



/ Mucous lamina of the Intestine. 

 m Corpora Wolffiana. 



n Mesenteric space. 



o Aorta. 



Fig. 6. Ideal transverse section of the Embryo of a Vertebrated Animal* 

 a Stem of the vertebral column. 



b Laminae dorsales their union forms the upper or dorsal Tube. 

 o Lamina; ventrales their union forms the under or ventral Tube. 

 d Central portion of the Nervous System or nervous Tube. 

 e Vascular Tube. 

 / Mucous Tube. 

 g Corpora Wolffiana. 



Containing transverse sections only : selected from Ton Bar. 



