sac of hooved animals are situated far apart, so the two 

 inserted sacs are in each other, the internal one deprived 

 of vessels and assumed to be the urinary outlet. Even 

 before Baer, it was called specifically the allantois. The 

 allantoic sac of rodents does not extend through the amnion, 

 situated against it on the ventral side of the embryo. 

 Finally, the allantoic sac of human beings is very small 

 (Figure 29, 23) . 



Baer called the outer vascular layer of the mammalian 

 embryonic vesicle the chorion. It is formed originally from 

 that external embryonic layer deprived of vessels. The 

 placenta, according to Baer, was considered growths of the 

 chorion, which served to create conditions for the maternal 

 blood to influence the embryonic blood. On the basis of 

 observations Baer suggested broadening understanding of 

 the placenta to include all the blood -containing villi 

 present on the surface of the ovum. When the flow of blood 

 to the uterus is strengthened, a structureless material is 

 distinguished on its internal surface; later vessels 

 penetrate it and there arises the caducous membrane, which 

 Baer favored calling the uterus cover or uterine placenta. 



Concerning embryonic respiration and nutrition, Baer 

 noticed that the maternal blood vessels nowhere communicate 

 with the fetal vessels. In the blood of both mother and 

 fetus, which pass in close proximity, "changes must take 

 place, and these changes must be called respiration" 

 (II 9x, p. 275 (205)). The feeding of the ovum at the 

 beginning, when there are still no vessels, takes place 

 by absorption at its surface. Later, with the appearance 

 of blood vessels, this absorption occurs through them. 



Turning to the details of mammalian development, Baer 

 first established the great similarity to avian development 

 in organ formation. For example, in the skeletal system, 

 the spinal column and tail form as in birds. At early 

 stages, the limbs are similar to those of birds, and at 

 first a long fold forms, from which the basic segments of 

 limbs, articulated joints, and digits are formed. The last 

 is at first undifferentiated, so the four hooves of sheep 

 are basically indistinguishable from the digits of dogs and 

 the hooves of swine. Jaw development takes place from the 



411 



