62 EOBERT SOHAEPF. 



the egg grows older. Whether the variously shaped particles 

 of the remaining nucleoli, which we have seen in figure 7, 

 subsequently congregate in the centre, in order to become 

 round again, I have not been able to observe. Such a trans- 

 formation, however, according to Iwakawa, 1 seems to obtain in 

 the young ova of Triton. At any rate the nucleus becomes 

 smaller and smaller, to the benefit of the yolk. In an almost 

 ripe ovum very little of it is left (fig. 12). It then lies excen- 

 trically, and consists of clear contents, with the exception of a 

 few round nucleoli. More of the latter are seen just outside 

 the nucleus in the surrounding yolk. Large vacuoles (va.) 

 have also made their appearance in the immediate neigh- 

 bourhood of the nucleus, but their presence may be due to 

 the fixing reagent. It is important to note that no trace of a 

 membrane can be made out at this stage of the germinal 

 vesicle. 



The gradual decomposition of the nucleus in the growing 

 egg has been seen by many authors, and we have descriptions 

 not only from vertebrate, but also from invertebrate ova. 

 Thus Hertwig, 3 in his researches on the formation of the egg 

 in Toxopneustes lividus, says, "When the ovum becomes 

 ripe, the nucleus undergoes regressive metamorphosis, and is 

 driven by means of protoplasmic contractions to the surface of 

 the yolk. Its membrane dissolves, and its contents break up 

 and are reabsorbed by the yolk. The nucleolus, however, seems 

 to remain unchanged, and appears to travel into the yolk mass, 

 becoming the permanent nucleus of the ripe ovum/' 



With regard to the question as to the presence or absence of 

 a membrane in the nucleus, I could see a double contour dis- 

 tinctly in the section represented by figure 7. I believe it is 

 now generally recognised that the nucleus is surrounded by a 

 membrane, and most of the references I can find support this 

 fact. Thus Hertwig, in the paper quoted above, speaks of the 



1 Iwakawa, "The Genesis of the Egg in Triton," 'Quart. Journ. Micr. 

 Sc.,' vol. xxii, 1882. 



2 Hertwig, " Bildimg d. thierischen Eies," • Morphol. Jahrbuch,' vol. i, 

 1875. 



