and the embryos themselves. The existence of the umbilical 

 cord, amnion, chorion, allantois and placenta in mammals had 

 been known for a long time. In the human embryo, according 

 to Baer, until recently the importance of the small vesicle 

 between the chorion and amnion and connected with the umbilicus 

 by a long stalk could not be interpreted. 



17 



Figure 30. Baer's illustration of the development of turtles. 



17 - embroyonic corselet "Emys europea," view 

 from above ; a- entrance in the spinal canal ; 

 18- the same also, in transverse section (enlarged 

 10 times); ab- spinal plates; be- abdominal plates; 

 d- boundary vessel. 



In the first and second decades of the nineteenth century, 

 however, it was learned that in the mammalian embryo the yolk 

 sac is actually present. Thus the umbilical vesicle of man 

 and other mammals and similar to the yolk sac of birds, is 

 connected with the intestine. In a special report, Baer 

 carried out on different mammals studies that showed that the 

 formation of allantois and chorion changes with changes in 



A 



uterine structure.' 1 In the initial period of development of 



4. K. E. v. Baer, "Untersuchungen iiber die Gef assverbind.ung 

 zwischen Mutter und Frucht in den Saugethieren, " EIN 

 CLUCKWUNS'CH ZUR JUHELFEIER S. T. SOMNERINC, 

 Leipzig, 1828, pp. 30 ff. 



407 



