spoke about the movement of the ovum in the tubes and the 

 formation of the corpus luteum (11, 9 g-k (176-183)). He 

 supposed that "in the ovum of mammals following entry into 

 the uterus or shortly after that, the embryo is isolated" 

 (II, 91; p. 244 (184)). The ovum, which is composed of 

 vitelline globules soon becomes diluted, quickly grows at the 

 expense of the liquid composing parts of the surrounding 

 albumen, and hence becomes covered by a membrane closely 

 adjacent to the ovum reservoir. This membrane, which Baer 

 studied in swine and sheep, was called the outer ovum 

 membrane. Under it was present that membrane directly 

 covering the yolk (vitelline membrane), which later dis- 

 solved. 



The primary form of the embryo Baer described as follows: 

 When the yolk becomes liquid and transparent, it is clear, 

 that the sac-like rudiment is composed of two unequal parts: 

 the smaller is the embryo, in the middle; the larger part 

 surrounding it is the embryonic membrane. The part that 

 is the origin of the embryo first has a rounded form, then 

 it changes to the form of a shield, becoming transparent 

 and deprived of all organization" (II 9p, p. 252 (189-190)). 

 Later the shield elongates, and along the edge appears a 

 strip similar to the primary strips in the bird's egg. 



Just after embryonic formation begins, it is untwisted 

 from the rest of the blastoderm by formation of the umbilicus, 

 As a result of this, the embryo itself and the yolk sac are 

 formed. During the study of development of dogs, rabbits, 

 swines and sheep, Baer noticed that the yolk duct connects 

 the yolk sac with the intestine. In mammals, as in birds, 

 the yolk sac and intestine represent two parts of the 

 vegetative part of embryo, which are only untwisted from each 

 other. Their connection by means of the yolk duct in dif- 

 ferent families of mammals is kept for different lengths of 

 time: the longer the time, the greater the size of the yolk 

 sac. Not in any mammal does the yolk sac enter the body of 

 the embryo. It is torn away with the ovum membranes or it 

 disappears earlier. The yolk sac of mammals and of birds 

 is composed on the outside of a vascular layer and inside of 

 a mucous layer, which are never completely separated. The 

 form and size of yolk sacs of different mammals are very 

 different. For example, in carnivora it is large, the 

 form changes from spherical to ellipsoidal and then to 



409 



