MUSEUM OF COMPARATIVE ZOOLOGY. 565 



been eliminated as two polar globules, there are left in the yolk these two nu- 

 clei. They may, I think, unhesitatingly be considered the " pronuclei." He 

 seems never to have observed them at any great distance apart, although " some- 

 times one sees them separated a little from each other, sometimes closely ap- 

 proximated." Each contains a single nucleolus. The nuclei grow. Their 

 nucleoli undergo a series of irregular segmentations. These divisions do not 

 seem to have been directly followed, but are inferred from the frequency with 

 which one finds biscuit-shaped or constricted corpuscles. Perez is unable to 

 say what becom'es of the two nuclei. He concludes there is no other alter- 

 native : either one of the nuclei disappears, the other alone being called upon 

 to inherit the individuality of the ovule, and to assume the dignity of vitelline 

 nucleus, or the two nuclei fu'se. He has not seen the nuclei mutually approach, 

 but has always found them at some distance from each other, never in imme- 

 diate contact. For this reason, and on account of a theoretical consideration, — 

 namely, that the amphiaster immediately preceding the first cleavage is the 

 same as the first amphiaster, and must, like it and all later amphiasters, arise 

 from a single nucleus, — Perez is of the opinion that only one survives and fur- 

 nishes a lineage, while the other perishes. 



He has arrived at the conclusion that there is a genetic connection between 

 the germinative dot and all subsequent generations of vitelline nuclei, but he 

 has done so by the introduction of two fundamental errors ; for the corpuscles 

 at the centres of the asters are not derived, as he claims, from the germinative 

 dot, nor do they, on the other hand, constitute the nuclei of succeeding gen- 

 erations. 



The mistake concerning the origin of the two nuclear structures (pronuclei) 

 to be found after the formation of the polar globules, deprives of its impor- 

 tance his negative statements relative to the penetration of spermatozoa. It 

 may be, or it may not, that " fecundation is simply the result of the dissolu- 

 tion of the spermatozoon at the surface of the vitellus and of the absorption of 

 its substance by the ovule." Reasoning from the analogy of cases easier of 

 control and observation, it is highly probable that in Helix a penetration of the 

 spermatazoon takes place. Nothing which Perez has presented in the way of 

 observation diminishes the probability that such is the case, nor that one of 

 the nuclei he has seen is the equivalent of the male pronucleus. 



Concerning the existence of a vitelline membrane, I think it is quite hazard- 

 ous to speak dogmatically. It is rarely that I have seen in Limax evidence 

 (Fig. 57, compare also Figs. 80'', 80") of the existence of anything resembling 

 such a membrane. That, however, does not prevent its constant occurrence 

 in the case of Helix. I am led to suspect, however, by the statement that it 

 is easily ruptured, that the author may have assumed that it previously existed 

 in some eases where he had no direct evidence of its presence. At least, his 

 Fig. 19 shows no trace of such a membrane. 



Perez has contributed very interesting observations on the nature of the egg 

 just before the formation of the polar globules. No one, I believe, has seen 

 such strongly marked and numerous pseudopodal protrusions of the vitellus of 



