OVARY AND OVARIAN OVA IN CERTAIN MARINE FISHES. 137 



nucleus does not pass through a genuine resting pliase. Thus 

 Val. Hacker believed tliat in the ova of certain fresh-water 

 Copepods throughout the growth-period a (loul)lc chromatic 

 thread could be traced in more or less evident form, and that 

 it was directly derived from the dyaster of the preceding 

 division, so that the division of the chromosomes (tetrads) in 

 the first reduction division was prepared in the previous 

 mitosis. E-uckert has reached similar conclusions in the 

 study of the ova of Selachians. No further information being 

 apparently available concerning the history of the centrosome 

 in the spermatocyte in the period just before the reduction 

 divisions, we may proceed to consider more precisely the 

 history of the germinal vesicle in the ovum, and may con- 

 veniently refer to Ruckert's observations on the Selachian 

 ovum as affording the most definite and comprehensive view of 

 the subject, and then inquire how far they harmonise with 

 what I have been able to see in the Teleostean egg. 



Many authors have maintained that the nuclear network 

 temporarily vanishes in the germinal vesicle. Riickert re- 

 marks that the reticulum which is considered to be the vehicle 

 of heredity, and has been studied to the farthest possible 

 detail in the final stage of its development, was not known 

 with certainty, when he began his investigation, even to exist 

 in the young stages of the egg. In 1890 Boveri, considering 

 each tetrad as a single chromosome, proved that the reduc- 

 tion to half the normal number of chromosomes had already 

 taken place when the first directive spindle appeared, and 

 maintained that the reduction must take place in the ger- 

 minal vesicle, if not at some earlier stage. But he had not 

 minutely traced the fate of the chromatic substance in the 

 germinal vesicle. 



O. Hertwig, on the other hand, considered that the number 

 of chromosomes was doubled, which means, of course, that 

 each member of the tetrad represents a chromosome. Then 

 the reduction follows from the two divisions without the 

 interposition of a resting phase or further splitting of the 

 chromosomes. 



