THE FERTILIZATION OF THE OVUM. 



do not go back into the nucleus after every division, as 

 Brauer 1 asserts is the case in Ascaris. It therefore seems 

 highly probable that the entire aster is a cytoplasmic struc- 

 ture, temporarily modified or differentiated for the purpose 

 of contraction and expansion, whose chief function in the 

 reproductive cells is to bring the pro-nuclei together, accom- 

 plish nuclear division, and move the nuclei from place to place 

 in the resulting cells. 



To return to the process of fertilization : the spermatozoon 

 usually, though not invariably, enters the ovum near the vege- 

 tal pole. I have not observed it in the actual process of enter- 

 ing, though I have seen it immediately afterward. In such 

 cases it consists of a small conical or sometimes fusiform 

 nucleus, surrounded by a clear non-granular area, Fig. 3. At 

 this stage the sperm nucleus 

 consists entirely of chromatin 

 closely packed together; there 

 is no nuclear sap and no aster 

 is visible, though, perhaps, the 

 clear area surrounding the 

 nucleus may be taken as an 

 indication of the presence of 

 such a body. In Fig. 3 the 

 egg nucleus is shown in the 

 process of division preparatory 

 to the formation of the first FlG ^_ c , plana . formation of the 



polar body. There are two first polar body ; at the left the sperma- 



centrosomes at the upper pole tozo5n has J ust entered the ovum, 

 of the spindle, though but one is to be seen at the lower pole. 

 The centrosome at the upper pole has divided thus early, 

 preparatory to the division of the chromatin of the first 

 polar body. This division occurs soon after the polar body 

 is formed. The surface of the egg is indented over the 

 upper pole of the spindle, thus indicating, as it seems to me, 

 that the astral fibres are here attached to the surface and are 

 drawing it in by their contraction. 



1 Brauer, Zur Kenntniss der llerkunft des Centrosomes. Biol. Ccntralblatt, 

 13d. 13, Nos. 9 and TO. 



