_J 



Figure 349. Photographs ilkistrating the development of the mammahan egg. A-F, stages in 

 the development of the rabbit's egg. A, two-cell stage, 24 hours after fertilization; the surrounding 

 membrane is the zona pellucida in which are embedded several sperms; the polar body lies near 

 the upper end of the cleavage furrow. B, four-cell stage (29 hours). C, eight-cell stage (32 

 hours). D, morula stage (55 hours). E, trophoblast stage (71 Vz hours), showing the differentia- 

 tion of cells into the outer trophoblast layer and the inner cell mass from which the embryo 

 arises. F, blastocyst stage (90 hours), showing the segmentation cavity, trophoblast, embryonic 

 disk (above), and zona pellucida with part of the albuminous coat around it. G-H, human 

 embryos. G, one month old (6.7 mm.), showing arm and leg buds, growing at caudal end, 

 umbilical cord, heart, branchial bars, olfactory pit, and eye. H, six weeks old (19 mm.), showing 

 developing hands and feet, elbow and knee, nose, eye, and ear. The cranial vault appears too 

 large for the body, due to the fact that it is a hollow vesicle. (Photos A-F by courtesy of P.W. 

 Gregory; G-U by courtesy of Carnegie Institute of Washington, Department of Embryology.) 



477 



