382 OUTLINES OF CHORDATE DEVELOPMENT 



develops only as far as a cleavage stage that season; its develop- 

 ment then continues the following spring. 



There is considerable variation in the details of cleavage in 

 the Mammalia, and since we are not describing any single form, 



FIG. 147. Cleavage of the ovum of the rabbit. After Assheton. A. Two- 

 cell stage, twenty-four hours after coitus, showing the two polar bodies separated. 

 B. Four-cell stage, twenty-five and one-half hours after coitus. C. Eight-cell 

 stage, a, Albumenous layer derived from the wall of the oviduct; z, zona radiata. 



we must be limited to very general terms; many of the details 

 may be learned from the accompanying figures (Figs. 147, 148). 

 Cleavage is total and at first adequal, very early becoming quite 



FIG. 148. Morula and early blastodermic vesicles of the rabbit. After 

 Assheton. The zona radiata and albumenous 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 blas- 

 todermic vesicle; i, inner cell mass; w, wall of blastodermic vesicle (subzonal 

 layer, trophoblast). 



unequal, and in many cases very irregular, so that stages of 

 three-, four-, five-cells, etc., may be found. The first cleavage 

 plane, and usually the second, pass nearly through the chief 



