176 bulletin: museum of co^niparative zoology. 



is much more conspicuous in one of them (Fig. 21a) than in the other. 

 A single well marked aster, without visible centrosome, sustains an 

 interesting relation to the vesicles of the compound nucleus. It lies 

 somewhat nearer the surface of the egg than the three more or less 

 confluent vesicles, and the more pointed ends of these appear as though 

 drawn out in the direction of the centre of the aster. This aster is on 

 the side of the vesicles which faces the other nucleus, in connection 

 with which there is no aster or other cytoplasmic differentiation. In 

 the absence of an entrance cone or of a cytoplasmic "track," which, 

 •as we have seen, are characteristic of some sperm nuclei, it is difficult 

 to decide which is the sperm nucleus. In another egg the aster was 

 found to accom])any the sperm nucleus (Fig. 16a), but, on the other 

 hand, the mvdti-vesiculate condition is more characteristic of the egg 

 nucleus. The compound nucleus, the one accompanied by an aster 

 (Fig. 21b), has a shortest diameter of about 20 jw, whereas the other 

 nucleus is only 9 fx in diameter; this seems to give evidence that the 

 compound one is the egg nucleus. But even if this is so, the aster 

 cannot be considered as belonging to the maturation spindle, since it 

 is too prominent and not in the proper position, but must represent a 

 new aster, which may perhaps persist to help form the cleavage spindle. 



The most extreme condition of distinct vesicles representing the 

 germ nuclei, though not the only such case found, is shown in Figure 22. 

 This Figure is compiled from two sections by a careful superposing 

 of camera drawings, and shows all the nuclear bodies found in the egg. 

 Three of the vesicles (a) lay wholly in one section, and within 10 fi of 

 the surface of the egg ; the other vesicles (group h) were limited to the 

 preceding section, and all were less than 20 fx from the surface. All 

 the vesicles were in a zone of more deeply staining cytoplasm, which 

 extended to the surface of the egg, and no astral radiations were present 

 in connection Avith any of the vesicles. Spermatozoa were present on 

 the surface of the egg, and one had probably penetrated, since no 

 attraction cone was present, and the surface of the egg showed the 

 irregularities which we have seen to be characteristic of the time 

 immediately following the penetration of a spermatozoon. As neither 

 sperm head nor other nuclear bodies were found in the egg, the vesicles 

 figured must represent both sperm and egg nuclei. However, it is not 

 possible to distinguish between the two. 



Polyspermy occurs frequently in Pennaria, and in such cases each 

 sperm head is accompanied by a distinct aster, though the aster is 

 usually lost when one of the sperms begins to metamorphose into a 

 nucleus. Perhaps the presence of the aster in polyspermic eggs gives 



