5i8 MONTGOMERY. [Vol. XV. 



In the ova of Montagna and Rodalia they are never in contact 

 with the nucleolus. In none of these three cases observed by 

 me does there seem to be any genetic connection between the 

 paranucleoli and the nucleoli proper. And in other cases, 

 where the two are separated (this separation is the most usual 

 state), no genetic connection between the two has been de- 

 scribed ; and even in that smaller number of cases where 

 they are in contact with each other at some period of their 

 development, no positive proof of their genetic relation has 

 been offered. Therefore we might conclude, though with re- 

 serve, that in the greater number, if not all, cases the para- 

 nucleoli are not derivatives of the nucleolus, but are products 

 sui generis. It is the rule that the nucleolus proper appears 

 in the nucleus before the paranucleoli arise, the latter usually 

 arising first towards the close of the growth stages. Accord- 

 ingly, though I cannot corroborate Hacker's ('95) conclusions 

 as to the origin of the nucleolar substance, I am inclined to 

 agree with him that portions of nucleolar substance are succes- 

 sively deposited in the nucleus, and that those portions which 

 are deposited last, after the nucleus has undergone important 

 physiological and chemical changes, would differ from the 

 portion first produced (that of the nucleolus proper), and so 

 would represent the paranucleolus. And there are certain facts 

 from my own observations which would support this view. In 

 the earlier stages of the maturation of the ovum of Tetrasiemma 

 and Zygonemertes there are a large number of nucleoli produced 

 successively at the periphery of the nucleus ; these then wander 

 successively to the center of the nucleus, and then from that 

 point again to the periphery. Now in this last stage, when the 

 nuclear filaments are commencing to arise, we find, usually in 

 contact with the latter, much smaller, more deeply stained 

 nucleoli, and these I have termed "nucleoli of the second 

 generation." We have found, accordingly, that after the nu- 

 cleus has passed through very marked physiological changes 

 (increase in size, redistribution of chromatin), another kind of 

 nucleoli appears, which may or may not be morphologically com- 

 pared to the paranucleoli of other ova. These nucleoli of the 

 second generation have neither a genetic nor a physiological 



