MAMMALIAN EMBRYOLOGY 



621 



into the uterus, passes into the oviduct in an opposite direction from 

 that which the egg takes. This causes a meeting of egg and sperm. The 

 length of time it takes the egg to reach the uterus, after ovulation, varies 

 in different species of animals. It may vary from a few hours to sev- 

 eral weeks. It is therefore practically impossible to state the exact 

 time when fertilization actually takes place. This is especially true in 

 the human being; but, as soon as the sperm does meet the egg, and fer- 

 tilization does take place, the embryo begins developing. Consequently 

 by the time the fertilized egg reaches the uterus, it has already passed 

 through, or is just passing through, a stage that is even a little advanced 

 beyond the gastrula stage. There are, in fact, several germ-layers already 

 present at this time. 



THE BLASTODERM 



As soon as fertilization takes place, the egg divides equally into 

 two cells, these two into four, and so on in the usual way. However, 

 very early, some of the cells divide more rapidly than others, so that 

 there is an overgrowth of those which grow most rapidly. This gives 

 rise to several terms. The more rapidly growing, or outer layer, is called 

 the sub-zonal layer, while the central mass is called the inner cell mass. 



The sub-zonal layer is only one cell in thickness, so it is easily dis- 

 tinguished from the inner cell mass. Then, too, a cavity forms between 

 the two layers. 



The entire structure is now called a morula (Fig. 356). As soon as 



Fig. 356. 



Morula and early blastodermic vesicles of the rabbit. Th.2 zona radiata and 

 ftlbuminous layer are not shown. A. Section through morula stage, forty-seven 

 hours after coitus. B. Section through very young vesicle, eighty hours after 

 coitus. Taken from uterus ; ordinarily the ova have not reached the uterus at 

 this age. C. Section through more advanced vesicle, eighty-three hours after 

 coitus. Taken from uterus, c, Cavity of blastodermic vesicle ; i, inner cell mass ; 

 w, wall of blastodermic vesicle (subzonal layer, trophoblast ) . (From Assheton.) 



D. Section through the fully formed blastodermic vesicle of the rabbit, 

 /cm, Granular cells of the inner cell mass ; troph, trophoblast cells ; zp, zona 

 pellucida. (From Quain's Anatomy, after Van jJeneden.) 



the cavity has definitely formed between the inner cell mass and the sub- 

 zonal layer, the morula is known as a blastodermic vesicle. This cavity 

 contains a fluid which is supposed to represent the yolk-mass of the 

 blastula and gastrula in the lower forms. 



