TJ/E CEXTROSOME 250 



5. The Nncieus in Maturation 



Scarcely less convincing, iinally, 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 the 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. Thp: Centrosome 



Nearly all investigators have now accepted Van Beneden's and 

 Boyeris conclusion that ///.- ccntrosomcis an onran 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 

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

 that IS the fertilizing element par excellence, since its introduction 

 into the ^g^g 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 ^gg may divide 

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

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

 conjugatmg with one of the nuclei of the two-celled stage The e-o- 

 is \i^x^fertilized~i.e. rendered capable of division — by the centro- 

 some, which separates from the sperm-nucleus, approaches the eo-g. 

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



Again, Boveri has observed that the segmenting ovum of Ascari, 

 sometimes contains a supernumerary centrosome that does not enter 

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

 (l^ig. 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- 



