MUSEUM OF COMPARATIVE ZOOLOGY. 511 



melting together which is so generally observed to ensue. Hertwig says 

 the nucleoli suffer disintegration into clusters of small granules, which 

 collect on both sides of the conjugation surface ; then two dull stellar 

 systems make their appearance at two diametrically opposite points in 

 this plane of contact ; next, the contour of the two vacuoles suddenly 

 disappears, the nuclear fluid apparently mingling with the surrounding 

 protoplasm. 



This agrees with what I have shown to take place in Limax, so far as 

 concerns the origin of the amphiaster of the first cleavage sphere before 

 the actual fusion of the pronuclei. Hertwig does not seem to have 

 observed any want of synchronism in the appearance of the two asters, 

 and moreover clearly places them on the boundary of the nucleus and 

 the protoplasm, just as Fol and many others have done. Without giv- 

 ing particular prominence to the fact, Hertwig makes the disintegra- 

 tion of the pronuncleoli precede the appearance of the asters. That I 

 consider to be a point of cardinal importance, and one which, in view 

 of the contrary results I have obtained, and in view of the fact that 

 Hertwig may not have observed the very first indications of an exist- 

 ing aster, should hereafter demand especial attention. Where do the 

 first signs of the approaching segmentation make their appearance, — 

 in the nucleus, in the yolk, or at the boundary of the two] The 

 definite answer to this question will, I believe, advance us a step in the 

 appreciation of the mutual relations of nucleus and protoplasm. 



According to Hertwig, these changes occur in both Phyllirrhoe and 

 Pterotrachea in the same manner as in the pteropods, save that in the 

 latter genus the pronuclei secrete each only a single nucleolus. When 

 their contours disappear, the pronuclei contain, in his opinion, only 

 nuclear fluid (Kernsaft). 



Blanchard ('78, pp. 754 - 758) summarizes the results of the recent 

 studies on fecundation which he has reviewed, and expresses his belief 

 that the homogeneous clear mass, at the expense of which the male pro- 

 nucleus grows, is not vitelline protoplasm, but rather that part of the 

 germinative vesicle which is dissolved in the vitellus at the moment 

 when the directive (first maturation) spindle is formed. He admits that 

 there is no direct evidence to prove this hypothesis : it simply seems to 

 him more rational than the other supposition. 



