Cnapter 1 



THE EARLY EMBRYOLOGY OF THE MOUSE 



By George D. Snell, Roscoe B. Jackson Memorial Laboratory. 



Fertilization, 2. Cleavage, 4. The blastula, 5. Implantation and early growth, 5 

 The formation of the entoderm, 7. Embryonic and extra-embryonic ectoderm, 8 

 The ectoplacental cone, 10. The inversion of the germ layers, 10. The primitive 

 streak and mesoderm formation, 15. The orientation of the embryo in the uterus, 15 

 Amnion, chorion and exocoelom, 16. The head process, 20. The neural groove, 23 

 The notochord, 24. The archenteron, 25. The allantois, 25. Fore-gut and hind-gut 

 26. The head fold, 28. The somites, 28. The primitive streak as a growth center 

 31. The coelom, ^2. Reichert's membrane, ;^,i. The amnion, 36. The yolk-sac, 36 

 The blood islands, 37. Changes in the uterus. 37. The nourishment of the embryo 

 3q. The giant cells, 40. The seven somite embryo, 41. The tail fold, 42. The 

 turning of the embryo, 44. The mid-gut, 44. The heart, 45. Blood vessels, 50 

 Change in shape of the yolk-sac, 51. Bibliography, 51. 



The early embryology of the mouse and rat has been the subject of 

 numerous studies during the past 50 or 60 years. Because the results of 

 these studies are published in several languages and in many different 

 journals, some of them not accessible in most libraries, because errors were 

 inevitably present in the earlier articles, and because many of the published 

 figures are not adequate for conveying a quick and clear understanding of the 

 subject, the author has undertaken, and here presents the results of, a com- 

 plete reinvestigation of nearly the whole field. The material used in the 

 study consists of sections of embryos spaced at six hour intervals from 4 

 days to 9 days. In some cases ten or more embryos of a single stage have 

 been sectioned. The sections were prepared by Olive Bartholomew, 

 Elizabeth Fekete and the author. The technique used has been described 

 elsewhere (14). To this description need only be added that, because in 

 most cases the females used as mothers were hybrids between two strains, 

 and because the fathers were from a third strain, thus giving both embryos 

 and mothers a maximum of hybrid vigor, the stages as here described are 

 usually earlier, often by as much as a day or more, than comparable stages 

 described by other authors. While this procedure gave embryos which 

 developed rapidly and were normal in a high proportion of all cases, it did 

 not eliminate variability. No attempt has been made to describe the varia- 



