THE CENTROSOME . 259 



5. TJie Nucleus in Maturation 



Scarcely less convincing, finally, is the contrast between nucleus 

 and cytoplasm in the maturation of the germ-cells. It is scarcely 

 an exaggeration to say that the whole process of maturation, in its 

 broadest sense, renders the cytoplasm of \he germ-cells as unlike, 

 the nuclei as like, as possible. The latter undergo a series of com- 

 plicated changes which are expressly designed to establish a perfect 

 equivalence between them at the time of their union, and, more re- 

 motely, a perfect equality of distribution to the embryonic cells. 

 The cytoplasm, on the other hand, undergoes a special and per- 

 sistent differentiation in each to effect a secondary division of labour 

 between the germ-cells. When this is correlated with the fact that 

 the germ-cells, on the whole, have an equal effect on the specific 

 character of the embryo, we are again forced to the conclusion that 

 this effect must primarily be sought in the nucleus, and that the 

 cytoplasm is in a sense only its agent. 



C. THE CENTROSOME 



Nearly all investigators have now accepted Van Beneden's and 

 Boveri's conclusion that the centrosome is an organ for cell-division, 

 and that in this sense it represents the dynamic centre of the cell (cf. 

 p. 56). This is most clearly shown in the ordinary fertilization of the 

 ovum, in which process, as Boveri has insisted, it is the centrosome 

 that is the fertilizing element par excellence, since its introduction 

 into the egg confers upon the latter the power of division, and hence 

 of development. Boveri's interesting observations on " partial fertil- 

 ization" in the sea-urchin referred to at p. 140 afford a beautiful illus- 

 tration of this point. In certain exceptional cases the egg may divide 

 before conjugation of the germ-nuclei has occurred, the sperm-nucleus 

 lying passive in the cytoplasm until after the first cleavage and then 

 conjugating with one of the nuclei of the two-celled stage. The egg 

 is here fertilized i.e. rendered capable of division by the centro- 

 some, which separates from the sperm-nucleus, approaches the egg- 

 nucleus, and gives rise to the cleavage-amphiaster as usual. 



Again, Boveri has observed that the segmenting ovum of Ascaris 

 sometimes contains a supernumerary centrosome that does not enter 

 into connection with the chromosomes, but lies alone in the cytoplasm 

 (Fig. 117). Such a centrosome forms an independent centre of divi- 

 sion, the cell dividing into three parts, two of which are normal 

 blastomeres, while the third contains only the centrosome and attrac- 



