THE MAMMALIAN EGG 



It cannot be doubted that the choice of experimental animal 

 played a most important part in the advancement of knowledge of 

 early mammalian development. Despite the fact that Harvey was 

 a painstaking and experienced investigator, he quite failed to draw 

 the proper conclusions from his studies in the deer; ruminant 

 blastocysts rapidly attain a highly extended state, and Harvey 

 interpreted this structure as a mass of mucous strands, among which 

 the embryo was to arise. De Graaf and Cruickshank were fortunate 

 to select the rabbit as experimental animal, because in this species 

 ovulation is induced by coitus, the tubal egg is easily visible to the 

 naked eye owing to its possession of a wide mucin layer, and the 

 blastocyst is a very distinctive object. Von Baer's discovery, which 

 was made with the dog egg, must have been facilitated by the fact 

 that the follicle in the dog ovary is large and comparatively clear, 

 and the egg stands out in transmitted light owing to its almost 

 opaque cytoplasm. 



Following von Baer's historic announcement, events moved more 

 rapidly. Studies on the structure of follicles, eggs and developing 

 embryos were made by Coste (1834) and Barry (1838, 1839) in the 



JJO Tenth Stage 



Fig. 4 

 Rabbit eggs as described by Barry (1839). 



