598 GRIFFIIV. [Vol. XV. 



spherical sperm-nucleus so as to lie closely applied to the amphi- 

 aster, with an aster at each pole (PL XXXII, Fig. 32). The 

 application of the fused or more often incompletely fused egg 

 vesicles to the base of the sperm-nucleus and their subsequent 

 fusion give rise to a segmentation-nucleus with the asters at 

 opposite poles (PI. XXXII, Figs. 33-37). 



When, as often happens, the sperm enters from the side, the 

 axis of the amphiaster is more or less nearly parallel to the egg- 

 axis, as marked by the polar bodies. With the progress of the 

 sperm-nucleus along its curved copulation path, the amphiaster 

 is gradually rotated so as to bring its axis (the axis of the future 

 cleavage-spindle) perpendicular to that of the egg. The relative 

 position of the germ-nuclei at contact depends upon the extent 

 to which the rotation has already progressed. A tardy rotation 

 may cause the egg-vesicles to be received first upon one of the 

 asters (PI. XXXII, Fig. 32. Cf. Brauer, '92, for same phe- 

 nomenon in Bi-anchipus). Eventually, however, the rotation is 

 effected so as to bring the egg-nucleus between the asters in 

 the normal copulation position. 



In some forms {e.g., Ascaris, Mouse, Unio, ProstJieceraeus, 

 Pleuroyphyllidea, and others) the astral systems completely fail 

 to appear, or disappear entirely about the time of copulation of 

 the germ-nuclei ; and in others (notably the sea-urchins) the 

 rays become far less distinct, and a resting-stage results. On 

 account of the extreme difificulty of detecting the centrosomes 

 in these stages, such facts as the above have been considered by 

 some observers as strong evidence against the persistence of 

 the centrosome. In Thalassema, the " pause "is of short dura- 

 tion, and while the asters are a trifle less distinct, they never- 

 theless show clearly throughout, and the persistence of their 

 focal centrosome is easily demonstrated. With the approach 

 of the germ-nuclei the egg-vesicles seem to exert a disturb- 

 ing influence upon the amphiaster, or render it more easily 

 affected by reagents ; for the astral centers are often somewhat 

 distorted, the centrosomes pushed aside (carrying the asters 

 with them) against the membrane of one of the nuclei or fur- 

 ther out into the cytoplasm. In most instances the presence of 

 the centrosomes can be made out with comparatively little dififi- 



