340 CHARLES PACKARD 



The entrance of the sperm is normal in every case, and poly- 

 spermy is as rare as in the controls. As the sperm penetrates the 

 peripheral protoplasm of the egg it revolves, and an aster de- 

 velops in front of it (fig. 16). The further course of the sperm 

 is not marked by any abnormalities (fig. 17). In about twenty 

 minutes the sperm head becomes closely applied to the egg 

 nucleus where it remains as a distinct cap for some time before 

 it completely fuses with the egg nucleus. In the meantime the 

 sperm aster divides and the daughter asters migrate to opposite 

 sides of the cleavage nucleus (fig. 18). 



Up to this time the egg nucleus is entirely normal. Before 

 insemination it is filled with a tangle of chromatin threads 

 suspended in a delicate linin network, a condition which persists 

 until the sperm nucleus begins to fuse with it. The first sign 

 of abnormal development appears at this time. Some of the 

 chromatin of the egg nucleus condenses into deeply staining 

 spherical bodies which are scattered throughout the nucleus. 

 As a rule they are comparatively small (fig. 19) but may be 

 very large (fig. 20). In this condensed condition they remain 

 throughout subsequent development and may be seen in the 

 anaphase of the first cleavage lagging behind the chromosomes 

 (figs. 22 and 23). 



The remaining chromatin at first spins out into very delicate 

 threads, during which time the mingling of the parental chro- 

 matin takes place (fig. 19). This stage is followed by a gradual 

 shortening and thickening of the threads (figs. 20 and 21). In 

 the meantime the astral rays (not shown in the figure) grow in 

 and become attached to the rod-like chromosomes which have 

 resulted from the shortening of the chromatic threads. This 

 whole process is normal in every respect. 



The chromosomes can be counted during the anaphase. Many 

 counts show that development is, without exception, diploid. 

 The normal diploid number is 34. The appearance of the mi- 

 totic figure resembles in many ways the figures of Hertwig ('12) 

 In his experiments on Sphaerechinus he radiated the sperm only, 

 and found that the sperm chromatin breaks up into numerous 

 masses of irregular shape which in many instances, are involved 



