10 ART. 4. N. YATSU I 



nucleus occupying a central position : a single and well marked 

 vesicular principal nucleolus (plasmoseme) is present, the accessory 

 nucleolus which had been found in the ovarian ovum having 

 disappeared in the course of growth. 1 The principal one too 

 degenerates in situ shortly after the accessory one has disappeared, 

 and at the time when no changes as yet have taken place in the 

 nucleus. The nucleus is vesicular and stains in any basic dyes 

 far less intensely than the cytoplasm, a small amount of chromatic 

 substance being scattered on the nuclear reticulum — showing that 

 the ovum is very poor in nucleïnic acid at this stage. Surrounding 

 the nucleus is a thick layer of cytoplasmic net-work, in whose 

 meshes fine yolk-granules are packed, which, of course, take 

 readily erythrosin or any other plasm stains. On examining 

 iron-hœmatoxylin preparations we can distinguish two kinds of ova 

 according to the appearances presented by the yolk : the one with 

 yolk-granules of an almost uniform size (PI. IL, Fig. 14.), and 

 the other with those of different sizes (PI. II., Fig. 15.). This 

 distinction may be due not to natural differences, but to the state 

 of precipitation brought about by the action of fixing reagents, 

 and to the degree of extraction in the iron-hiematoxylin method. 

 Exterior to this yolk layer is a layer of vacuoles, 2-4 vacuoles 

 in thickness. They are probably formed by the dissolving away 

 of yolk-granules by the action of certain enzymes ; for the vacuoles 

 increase in number with the age of the egg. In some cases external 

 to the vacuolar layer and just beneath the vitelline membrane is 

 found a layer of yolk-granules varying somewhat in thickness, 

 while in others no such layer is found. There are some cases 

 in which no vacuolar layer is present at all. All these variations 



1. Cf. N. Yatsu : — Notes on the Histology of Linyula anulina Brug. This Volume 

 Art. 5. 



