MUSEUM OF COMPAKATIVE ZOOLOGY. 225 



surfixce of one or both the pronuclei. Most observers locate the two 

 points very definitely : both stars are made to lie in the plane along 

 which the two flattened pronuclei are at first in contact and then con- 

 fluent, and they are represented as occupying two diametrically oppo- 

 site points in the circumference of that plane. From this it follows that 

 the new centres are made to lie in the surface of the new nucleus (or 

 nucleus of the first cleavage sphere). However it may be in other cases, 

 it no longer holds good as a distinctive position for Limax. Unexcep- 

 tionally, there is a more or less marked antipodal relation expressed in 

 the position of the two new centres ; but they are not uniformly in con- 

 tact ivith, nor even in close proximity to^ either of the pronuclei (Fig. 85), 

 and when such approximation does exist (Figs. 52, 74), it may be that 

 one of the new centres is in relation with only one of the still separate pro- 

 nuclei. So great an ontogenetic concentration as the contemporaneous 

 existence of the arc/«"aster and one or both of these new asters has never 

 been observed. The question may arise whether the single centres of 

 radiation above mentioned (Figs. 52, 73, 79, 80) may not have been the 

 last remnants of archiasters, since the latter persist for a long time. I 

 think the answer may be most positive that they are not ; for the archi- 

 aster fades gradually in all parts, and if the central portion remains visi- 

 ble a trifle longer than the rest, it is only as a very indistinct structure. 

 But the asters which concern us now are vigorous, though not yet of 

 extensive influence ; they are sharply marked by rays of comparatively 

 limited extent. Furthermore, their positions are not favorable to such 

 an interpretation. They are uniformly nearer the middle of the vitellus 

 than are the centres of the pronuclei. There is, however, some varia- 

 bility in the closeness of the latter to the surface of the yolk, which is 

 manifest in both fresh and hardened specimens. The two light spots 

 seen in the living egg, just prior to and during the first segmentation, 

 appear from their positions to coincide with the asters. 



The condition of the pronuclei at the time of the origin of these new 

 centres and during their increasing ascendency is of special interest. 

 As has been already indicated, it is very difficult to prove in eggs 

 treated with acetic acid that there is a direct union of the pronuclei. 

 Even in cases where the amphiaster of the first cleavage sphere has 

 acquired a considerable extent (Fig. 85), it is clear that the two pro- 

 nuclei have not become fully amalgamated into a single structure, and 

 it may possibly be questioned if any portions of their substance have 

 become confluent. The nucleoli, though more faintly outlined than in 

 an earlier stage, are still easily recognizable. 



VOL. VI. — NO. 12. 15 



