1907.] 



NATURAL SCIENCES OF PHILADEM'HIA. 



543 



to a much later period. In or near the centre of many of these yolk 

 spheres a diffusely staining substance can be found when the egg is well 

 stained and cleared. This is not a nucleus, for it has no regular 

 boundaries anil often several such bodies are seen in a single yolk 

 spherule. They do not stain deeply, but appear as a sort of cloudy 

 material. These botlies may be merely the small portions of cyto- 

 plasm remaining in these large cells or, what seems more likely to me, 

 they may be of the nature of nuclear sap. If they were of a cytoplasmic 

 nature there is no reason why they should not show in earlier stages. 

 Instead they became evident only after the yolk begins to break up 

 into spherules. In many respects they resemble the archiplasmic 

 material often found around a nviclcus just after the nuclear mem- 



end 



Fig. 4. — Transverse section of an embryo, showing the ciUated lumen of the 

 alimentary canal. The endoderm cells are spreading over the yolk spheres. 

 end., endoderm; mes., mesoderm; ph., pharynx; y., yolk. 



brane has broken. Further, the large nuclei, which are at first spherical, 

 in the late stages become irregular in shape or even very much shrunken 

 (fig. 35). That these yolk granules should be broken up through the 

 action of enzymes coming from the nucleus is not at variance with the 

 modern views of nuclear activity. 



In such a case it seems probable that this material would become 

 aggregated at those places where the most rapid dissolving action is 

 going on. This offers an explanation for the relatively enormous size 

 of the nuclei of these three cells. These nuclei do not show evidences 

 of degeneration; instead the chromatin granules can be seen scattered 

 through the nucleus, often along distinct linin threads. 



After the ectoderm has covered the lower hemisphere of the egg it 



