THE FERTILIZATION OF THE OVUM. 29 



distinguished even after the first cleavage spindle begins to 

 form, and their chromatin masses remain distinct, Fig. 10, 

 until the equatorial plate stage. 



After the chromatin of the two pronuclei has assumed the 

 shape of distinct rods, the chromosomes, and before these rods 

 have been pressed together into the equatorial plate, it is 

 possible to determine that each pronucleus contains twelve of 

 these elements. Since these elements are doubled at every 

 division of the .nucleus, it follows that each of the first two 

 nuclei contains twenty-four chromosomes, twelve derived from 

 the sperm nucleus and twelve from the egg nucleus, and this 

 number is probably constant for all the nuclei of this species. 



It is difficult to define exactly the time at which the two 

 pronuclei have sufficiently united to be considered a single 

 element, but perhaps this can be most conveniently located at 

 the time when the boundary wall between the two vesicular 

 nuclei disappears, although, as just said, the two masses of 

 chromatin remain distinct until a somewhat later period. Cer- 

 tain it is that a fusion of the pronuclei to form a resting seg- 

 mentation nucleus, as described by Hertwig, does not take 

 place. 



With the union of the pronuclei, the entire process of 

 fecundation may be considered as ended. So far from being 

 a purely nuclear phenomenon, it must be evident to any one 

 who follows the history of the asters that they take an 

 extremely important part in this process. If we are amazed 

 at the precision with which the chromatic elements of the 

 nucleus are divided and distributed, we can be no less astonished 

 at the wonderful directive influence exercised by the asters 

 upon the nuclei ; and if it is justifiable to conclude that the 

 chromosomes have an extremely important function in the 

 building of the new organism because of the way in which 

 they are distributed, how is it possible to avoid the same con- 

 clusion with regard to the asters, which, as we have seen, are 

 equally divided in a manner not clearly understood into half- 

 asters, which then fuse in such a way that one-half of each 

 aster of the new organism comes from the paternal and one- 

 half from the maternal organism ? 



