3 

 and at the sides. By this means develops the circular fold, 



whose top fuses to form the amnion which thus grows from the 



animal layer. 



When the opening of the amnion closes, the lower layer 

 of its forming fold contributes to the construction of the 

 amnion, while the upper is not connected with amnion formation 

 For this part, Baer suggested keeping Pander's name serous 

 vesicle or serous membrane. The serous vesicle includes the 

 amnion with the embryonic and yolk sac. Between the serous 

 membrane, amnion and the yolk sac there remains an interspace 

 connected with a gap between the dermal and intestinal 

 umbilicus. This ring-shaped gap leads into the embryonic 

 abdominal cavity and could be named the abdominal umbilicus. 

 Therefore, the cavity of the serous vesicle is an extra- 

 embryonic part of the abdominal cavity, as the yolk sac is 

 the continuation of the digestive canal situated outside the 

 body. 



Turning to the development of the allantois, Baer des- 

 cribed how on the third day from the most posterior end of 

 the digestive canal, i.e. from the future cloaca, a small 

 rounded sac protrudes. Upon elongating, it passes through 

 the abdominal umbilicus and appears in the space between the 

 amnion, yolk sac and serous membrane. Later it spreads, 

 covers the amnion, penetrating between it and the serous 

 membrane, and then also grows over the yolk sac. Baer called 

 this formation the urinary sac, not only because it develops 

 from the cloaca, into which the urinary tracts flows, but 

 also because urine collects in it the second half of incuba- 

 tion. (100) The stem of the urinary sac is called the 

 urinary duct (urachus) . Because the intestine consists of 

 two layers (internal -mucous and external -vascular) , these 

 same layers should be present also in the urinary sac, and 

 they actually can be distinguished there very early. In the 

 external layer of the urinary sac a network of vessels develop 

 to receive the blood from the two branches of the aorta, the 

 umbilical arteries. Through the single umbilical vein, the 

 blood goes from the urinary sac backwards into the body of 



3 . Its cavity represents the extra-embryonic part of the 

 complete circular cavity. 



379 



