30 EDWAKD MCCRADY, JR. 



the other, and that these cells are descended from the larger 

 of the first two blastomeres. 



The second specimen showing the first cleavage in process 

 is a telophase (no. 16256). The equatorial constriction of the 

 cytosome is not yet complete, but the wells of the nuclei have 

 formed. The only indication of polarity recognizable at this 

 stage is a difference in size of the two prospective daughter 

 cells. With reference to the polarity indicated in these two 

 specimens the plane of the first cleavage is equatorial. 



The extrusion of yolk. A very extraordinary change has 

 taken place in the cytoplasm while the nuclear events just 

 described were in progress. It was observed in the last 

 chapter that during the passage of the ovum down the tubes 

 the yolk granules and fat droplets collect into fewer units 

 of larger size. The next change which can be recognized takes 

 place when the pronuclei are approaching each other in the 

 center of the egg and before the dissolution of their mem- 

 branes. At this time a few large fat droplets can be seen pro- 

 truding from the surface of the egg, as if they were being 

 slowly extruded. That they meet considerable resistance at 

 this surface is witnessed by the degree of distortion they 

 undergo. The globules below the surface are spherical, but 

 those at the surface are invariably much flattened more, 

 probably, than the laws of capillarity w r ould account for. 

 The resistance responsible for the compression is in part due 

 to a true cell membrane, but the zona pellucida is still in con- 

 tact with the surface of the ovum and may participate in this 

 resistance. The globules apparently burst through the sur- 

 face of the ovum, and their contents are dissipated into a space 

 which now arises for the first time between the zona and 

 the egg proper. This perivitelline space first appears only in 

 the equatorial region when the ovum elongates during the 

 first cleavage. 



But not only fat globules are extruded. A little later, as the 

 first cleavage furrow sinks into the ovum, large balls of cyto- 

 plasm including numerous granules of yolk from the sub- 

 peripheral zone are forced through the surface and deposited 



