BANCROFT: OVOGENESIS IN DISTAPLIA OCCIDENTALIS. 97 



the nucleolus (Fig. 1, ov^go.). At this period all of the granules, 

 including the prospective nucleolus, are differentiated similarly — taking 

 a bluish tint — with the methyl green and acid fuchsin double stain. 

 Soon achromatic fibres make their appearance. At first mainly periph- 

 eral, and inserted on the chromatic granules and the membrane, 

 they soon come to traverse the centre of the vesicle. It is rarely, 

 however, that chromatin is found upon them (Plate l, Fig. 3, ov'go.). 

 At the same time that the achromatic network becomes differentiated, 

 the largest chromatin granule increases in size, and becomes more 

 nearly spherical and not so closely approximated to the membrane. 

 Although it still takes the same stain as the other granules, the 

 disparity in size and its regular shape justify us in calling it the 

 nucleolus. Its later development will be discussed in a separate 

 section. 



The further changes that occur in the growth of the rest of the 

 germinative vesicle consist entirely in the elaboration of the achromatic 

 network and the chromatin granules upon it. The achromatic fibres 

 are not always seen, especially in the later stages, but staining in iron 

 haematoxylin brings out the chromatic elements very distinctly (Plate 

 3, Fig. 13; Plate 4, Fig. 27; Plate 6, Fig. 48). The latter forma 

 reticulum extending through all parts of the germinative vesicle, the 

 complexity of the structure increasing with the size of the vesicle, and 

 culminating in the condition shown in Figure 48 (Plate 6). 



In addition to the reticulum, many isolated granules are found 

 throughout the vesicle and especially on its membrane. While all 

 these granules when stained with iron haematoxylin are very deeply 

 colored, and resemble closely the chromatic structures described for 

 other eggs, still, strictly speaking, we are not justified in calling them 

 chromatin ; for chromatin is that substance which takes the chromatic 

 or basic aniline stain when treated with a combination such as methyl 

 green and acid fuchsin. Malfatti ('91), in testing the electivity of 

 this stain for known chemical substances, found that free nucleic acid 

 was stained pure green; nucleins, which contain less phosphorus, were 

 colored blue, while substances with still less phosphorus took the red 

 stain. Thus, as nucleic acid and the nucleins are derived from the 

 nucleus, it becomes highly probable that chromatin is made up of 

 these substances. 



Now, when the germinative vesicle of this stage is treated with the 

 stain mentioned, the network that is so distinct in haematoxylin prep- 

 arations is colored red, and the only substance staining green is con- 



