BANCROFT: OVOGENESIS IN DISTAPLIA OCCIDENTALS. 105 



it has been subjected; and believe that the refractive bodies are really 

 composed of nuclein or nucleic acid. 



Of course the most interesting problem connected with the nucleolus 

 is whether the refractive bodies contain any of the chromatin that goes 

 to form the chromosomes of the maturation nucleus, but on account of 

 the small number of ova that I have found in the oviduct, and the 

 difficulty experienced in making perfect series of sections, I am unable 

 to offer any direct evidence on the subject. One series from an ovum 

 within the oviduct shoAved three of these bodies situated within the 

 remains of the disintegrated germinative vesicle, but no other distinct 

 structural elements were detected ; and when the ovum has reached the 

 pouch, the tetrads (Plate 6, Figs. 60, 61) are already formed, and no 

 trace of the nucleolus remains. On the whole, it seems rather improb- 

 able that the refractive bodies should form the chromosomes, and in 

 the latest ovarian stages there is other chromatic material in the ger- 

 minative vesicle. Occasionally the whole vesicle will be suffused with 

 a faint green coloration, and sometimes very minute green microsomes 

 are seen on the projections of the shrivelled vesicle. Accordingly, at 

 this stage, there is not the entire abs3nce of other chromatic material 

 that is encountered earlier. 



4. Maturation. 



The condition obtaining among the ova of Class 4, where the yolk 

 bodies are completely formed and the germinative vesicle much shriv- 

 elled, stellate, and about 20 /jl in diameter, is the last stage that I 

 have observed in the ovary. The next changes take place in the pas- 

 sage through the oviduct, during which the tetrads are formed. 



A few observations have been made on these and subsequent stages, 

 but, owing to the difficulty of obtaining perfect series, they are so 

 scattered that they cannot be easily interpreted, and a discussion of 

 them would be unprofitable. Enough, however, has been seen to show 

 that the number of tetrads in the maturation nucleus is probably 

 twelve, and that two polar bodies are formed. No centrosome or 

 achromatic fibres of any kind have been detected either during matura- 

 tion or the first cleavage of the ovum, although I have several good 

 preparations of these stages. But, as before mentioned, the number of 

 figures is so small, and the variability in their appearance so great, that 

 as yet no connected history of the processes can be made out. 



Cambridge, May, 1898. 

 VOL. XXXV. — NO. 4. 4 



