208 LILIAN SHELDON. 



passage into the ovum with the segmentation nucleus, as I 

 hope to be able later to make some further investigations on 

 that subject. It will be enough to state here that in several 

 cases there were ova in the uterus which possessed no nucleus 

 whatever; there was a small amount of protoplasm present as a 

 very loose and not always easily-recognisable reticulum lying 

 among the yolk-spheres. This protoplasmic reticulum was 

 sometimes scattered throughout the egg, but was more often 

 only present at the periphery ; while in some cases it was 

 aggregated at one point only. In one ovum there was a very 

 large compact mass of protoplasm at one point near the 

 periphery, but no trace of a nucleus could be discerned. 



Segmentation. 



Immediately before the segmentation begins the ovum con- 

 sists of a great mass of yolk-spheres, and contains a single 

 nucleus. The position of the nucleus in the ovum varies 

 somewhat in different cases; in fig. 1 it is seen to be situated 

 at some distance from the periphery of the ovum ; in this case 

 it is round in form, and contains a deeply staining wall and 

 also a single mass of chromatin. The protoplasm in which it 

 is embedded is compact and dense, and contains at its periphery 

 several chromatin particles. In the ovum from which fig. 2 is 

 taken the nucleus and its surrounding protoplasm had a some- 

 what different position and form. The nucleus is situated 

 near the periphery of the ovum, being separated from the 

 vitelline membrane by only a thin layer of yolk. The nucleus 

 has a peculiar lobed form, and consists of three masses of 

 deeply staining material, between which is a portion of nuclear 

 substance which stains less deeply. It is surrounded by a very 

 small amount of protoplasm, which forms a loose reticulum, 

 the strands of which pass in and are lost among the yolk- 

 spheres. 



The next stage is that in which two nuclei are present in 

 the egg ; two sections from such an egg are figured in figs. 

 3 a and 3 b. One nucleus is situated at the periphery of the 

 ovum and the other somewhat deeper, but both lie in the centre 



