reaching four to five inches, whereas the embryonic vesicles, 

 while also not reaching the normal size, nonetheless grow to 

 eight inches (20 cm) and more (H 2 ) • In addition, during 

 normal development in swine the embryonic vesicles in different 

 stages may be of different sizes. From these data it follows 

 that when determining the age of embryos, one must draw 

 attention to the formation of the embryo itself, rather than 

 to the size of the embryonic vesicle. 



In concluding notes, Baer considered the structure and 

 development of embryonic membranes. Undoubtedly, his histor- 

 ical interest is represented by the discussions of the most 

 important question at that time, the relation of the pear- 

 like vesicle, given by Pokel the name Erythrois, to the 

 intestine and to the formation of the chorion. Baer con- 

 sidered that the vesicle, which in the human embryo corre- 

 sponds to the Erythrois of other mammals, is nothing other 

 than the urinary bladder, or since the intestine arises 

 from it, it is considered to be the cloaca. 



The formation of the chorion in mammals Baer described 

 as a result of concrescence of the urinary bladder with the 

 outer embryonic membranes and receiving vessels growing near 

 the allantois. Baer was positive that the chorion in man is 

 formed in this manner also. Here he gave a very interesting 

 footnote: "I consider that it is an honor for me," Baer 

 wrote, 



that this opinion was repeated by Weber, Frorin 

 and Bischoff . Nearly in the same words they 

 called it a' serious and brilliant hypothesis. 

 In my work, "On the Vascular Connection between 

 Mother and Fetus in Mammals," 4 I suppose, I have 

 shown in detail in different animal forms the 

 gradual formation of the chorion by the union of 

 the urinary sac with the outer membranes of the 

 embryonic ovum. Without drawings, doubt arises 

 because of difficulty in understanding these 



4. Baer, "Untersuchungen iiber die Ge fas s verb indgung 

 Zwischen Mutter und Frucht in Saugethieren , " 1828, 

 305 ff (113). 



450 



