MUSEUM OF COMPARATIVE ZOOLOGY. 415 



same reagent. I will therefore state somewhat more explicitly my con- 

 ception of how this condition may have been brought about. The sadden 

 loss of a quantity of fluid would not be covered by a gradual and uniform 

 shrinkage of the whole nucleus, but would be followed by a giving way 

 of the wall at its weakest point. There is certainly considerable evidence 

 tending to show that that portion of either pronucleus which is directed 

 toward its mate is the one which first shows signs of losing its integrity 

 (compare Limax, Fig. 70), — is therefore, we may assume, least capable 

 of withstanding external pressure. It would not be surprising, then, to 

 find either of the nuclei yielding first at this point. There are manifest 

 reasons (their close approximation) why the apposed faces would not 

 yield by a movement in opposite directions ; the one which, from any 

 cause, exhibited the earlier or stronger tendency to such a change, would 

 entail in its action the corresponding face of its mate. The latter would 

 thus fill the depression caused in the surface of the former. Where the 

 depression in the latter nucleus to balance this out-pushing should occur, 

 would depend, aside from the point of least resistance, upon the direction 

 already given to its substance by the process just described. Thus the 

 pole opposite the eminence already formed would be the point to yield. 

 Although described as successive, these events may nevertheless be con- 

 ceived as simultaneous in their occurrence. Such a conception would, it 

 seems to me, be quite feasible in explaining the shapes presented by the 

 pronuclei in the case of Limax, and at the same time offer a possible ex- 

 planation of the apparent absence of nucleoli. In the case of the rabbit, 

 as described above, however, it is only the central pronucleus which thus 

 suffers an involution. This appears at first to offer an objection to the 

 above explanation, but when one reflects that the central pronucleus is 

 described as being much larger and less conspicuous than its mate, it is 

 possible to believe that this alone is enough to indicate that the central 

 pronucleus may lose much more fluid than does the peripheral. A more 

 serious obstacle appears to lie in the fact that here the nucleoli probably 

 remain visible notwithstanding this condensation. Moreover, these are 

 not occasional but constant conditions in the approximated pronuclei of 

 rabbits' eggs, so far at least as can be inferred from the description. If 

 I had been able to reproduce these conditions, even with other reagents 

 than osmic acid, I should be less confident that they represented relations 

 not normal for living pronuclei. 



Schulze's ('75'', p. 267) excellent paper on the development of Sycan- 

 dra, unfortunately, does not afford much insight into the changes which 

 overtake the germinative vesicle. He believes it disappears, but has not 

 seen any polar globules. 



