ACCESSORY EMBRYONIC ORGANS. 



641 



FIG. 194. 



amnion. -. yiteiiu*. 



6. External blasto- 



dermic layer. c.Body 

 of th * f^ff ' V-' 



Am oiotic folds, e. Vi- 



teiiine membrane. 



FIG. 195. 

 I 



by opening a passage for it through the external blastodermic layer. 

 This is done by the formation of the amnion. 



Soon after the body of the embryo has been sketched out, mainly 

 by the thickening of a portion of the external blastodermic layer, a 

 secondary fold of this layer rises up on all sides 

 about the edges of the newly-formed embryo; so 

 that its body appears as if sunk in a kind of de- 

 pression, surrounded by a membranous ridge, as in 

 Fig. 194. The embryo (c)*is here seen in profile, 

 with the external folds, above mentioned, rising up 

 in advance of the head, and behind the posterior 

 extremity. The same thing takes place on the two 



sides, bv the formation of lateral folds simulta- Dia g ram of the FECUN- 



DATED EGG; .^howiu^ 

 neously with the appearance of those in front and the formation of the 



behind. As these folds are destined to form the 

 amnion, they are called the "amniotic folds." 



The amniotic folds continue to grow, covering the 

 embryo and approaching each other over its dorsal 

 region (Fig. 195). Their edges afterward come in 

 contact and unite with each other at this point (Fig. 195, 6), so as to 

 shut in a space between them and the body of the embryo. This 

 space, which contains a layer of clear fluid, is the 

 amniotic cavity. 



There now appears a prolongation or diverticulum 

 (Fig. 195, c), growing from the posterior portion of 

 the intestinal canal, and following the course of the 

 amniotic fold which has preceded it ; occupying, as it- 

 protrudes, the space thus left vacant. This diver- 

 ticulum is the commencement of the allantois. It is 

 an elongated membranous sac, continuous with the 

 posterior portion of the intestine, and containing Diagram of the FE- 

 blood-vessels derived from those of the intestinal 

 circulation. Its cavity communicates with that of 

 the intestine. 



After the amniotic folds have approached and 

 touched each other over the back of the embryo, their adjacent surfaces 

 fuse together at the point of contact ; so that the cavities of the two 

 folds, coming respectively from front and rear, are separated only by a 

 single membranous partition (Fig. 196, c) connecting the inner and 

 outer laminaB of the amniotic folds. This partition is afterward 

 atrophied and disappears; and the inner and outer Iamina3 become 

 separated from each other. The inner lamina (Fig. 196, a) remains 

 continuous with the foetal integument, thus enclosing the embryo in 

 a distinct cavity. It is called the amnion, and its cavity is known as 

 the amniotic cavity. The outer lamina, on the other hand (Fig. 196, 

 6), comes in contact with the original vitelline membrane and fuses 

 with its substance, so that the two form but one. This membranous 



2Q 



Umbilical 



