354 bulletin: museum of compaeative zoology. 



tion in the egg. At the time of meeting the egg nucleus is almost in- 

 variably in the granular condition just described ; the sperm nucleus, 

 however, shows great variation in this respect. In the majority of 

 cases it has perhaps three times the diameter of the mature sperm head 

 before penetration, is of distinctly granular composition, has an oval 

 outline, and is surrounded by a clear area (Figs. 143 and 144). At this 

 stage the asters may or may not be visible, though, from conditions ob- 

 taining in earlier and later phases, we must suppose that they are nor- 

 mally present. When seen they lie as a rule on opposite sides of the 

 nuclei, sometimes in or near their plane of contact (Fig. 143), but more 

 often at nearly opposite sides of the egg nucleus (Fig. 144), showing now, 

 as in later stages (Figs. 147 and 148), great individual variation in this 

 respect. Since all trace of the achromatic figure of the second matura- 

 tion spindle disappears previous to the re-formation of the egg nucleus, 

 there is no doubt that these asters are the two sperm asters which have 

 accompanied the sperm nucleus in its migration through the egg. 



The sperm nucleus increases in bulk (Fig. 144), becomes more and 

 more flattened against the membrane of the egg nucleus, and gradually 

 changes its shape until it resembles a cap-like appendage to the latter 

 (Fig. 145). It is still surrounded by a clear area, and is granular in ap- 

 pearance. The portion of the nuclear membranes separating the two 

 nuclei now disappear, and the spongy substance of the sperm nucleus 

 breaks up into a number of small chromatin bodies (Fig. 146), which 

 collectively form a roughly spherical mass. Although accurate count- 

 ing is impossible, there are certainly many more than twenty-four of 

 these bodies. Meanwhile the chromatin of the egg nucleus, previously 

 in a diffused condition, collects into an irregular reticulum composed of 

 separate segments and thickenings connected by linin threads (Fig. 146). 

 The two nuclei are now enclosed by a single continuous membrane. 

 The fusion nucleus as a whole is now pear-shaped, the portion derived 

 from the sperm forming the smaller end. The larger egg-nucleus por- 

 tion frequently shows various constrictions, remotely suggesting the 

 amoeboid form assumed by the egg nucleus in many other Metazoa, for 

 example, in Cerebratulus (Coe, '99). 



In the present example (Fig. 146) the two asters are clearly visible, 

 one lying nearly in the plane of conjugation, the other near the opposite 

 pole of the egg nucleus. The chromatin derived from the sperm nucleus 

 now assumes more and more the appearance of an irregular network, 

 until finally the paternal and maternal constituents are no longer sharply 

 distinguishable from each other-, although the fusion nucleus still pre- 



