MUSEUM OF COMPARATIVE ZOOLOGY. ' 545 



can be satisfactorily answered only by further and extensive comparative 

 studies. There are already many important observations which make 

 such a direct division (without fibrous differentiation) probable ; but 

 even in such cases an interchange of substance may not be completely 

 excluded, and the certainty that there is an increase in the total amount 

 of nuclear substance with successive generations makes the acquisition 

 of new material on the part of the nucleus unquestionable. That this 

 acquisition is facilitated by the division may at least be claimed as 

 probable. 



It is extremely doubtful whether new nuclei arise in animal cells 

 without the least visible connection with the nuclear substance of pre- 

 existing nuclei. Even in plant cells this process may be less certain 

 than has been claimed. I have endeavored to show how the case of 

 Isoetes might possibly be less indicative of this mode of formation than 

 Strasburger teaches ; but the evidence from the conifers and from Pha- 

 seolus seems at present capable of no other interpretation than that 

 there is often a complete dissolution — a morphological obliteration — 

 of the old nucleus. It perhaps is not entirely unreasonable to indulge 

 the hope that even in these cases new methods of investigation may 

 ultimately prove that there is not an entire dissipation of the substance 

 of the old nucleus. In either event, however, the interpretation which 

 Strasburger has given is that which most fully explains the extreme 

 cases. 



Germinative Vesicle. — It has been conclusively shown both by 

 Fol and by 0. Hertwig, that the first maturation spindle is formed at 

 the expense of constituents of the germinative vesicle. The latter is 

 neither totally dissolved in the yolk nor totally eliminated from the egg. 

 I have not proved the same to be the case in Limax ; but there is no 

 occasion to doubt that the first archiamphiaster is there produced in the 

 same manner as in the starfish. So much being granted, there is every 

 reason to agree with Hertwig that a genetic connection exists between 

 the germinative vesicle and subsequent generations of nuclei. I have 

 never found an egg which did not, under proper treatment, exhibit some 

 definite morphological evidence of the existence of a nuclear structure, 

 and am certain that in no case is the formation of the polar globules 

 accompanied by an elimination of the whole of the spindle. The female 

 pronucleus is formed primarily from the inner half of the nuclear plate 

 of the second maturation spindle, and its substance enters into the com- 

 position of the first cleavage spindle. The evidence of the continuity 

 VOL. VI. — NO. 12. 35 



