252 



IV. i 



and not only for its metabolism (Figs. 152, 153) but for the 

 production of form (Fig. 154). 



2. It is obvious that the germ-cells are the vehicles whereby 

 the inheritable characters of the species are transmitted from 

 one generation to the next ; they are the material basis of in- 

 heritance. The germ-cells of the two sexes are, however, as 

 unlike as possible in every character except their nuclei, in 

 which, as the study of maturation has abundantly shown, they 

 are exactly alike. In fertilization, the 

 union of the two germ-cells, two distinct 

 processes are involved. The first is the 

 mutual stimulation whereby the lost 

 power of cell-division is restored. This 

 is a process independent of the nuclei (in 

 Metazoa). The other is, however, the union 

 of the nuclei (or their chromosomes) ; and 

 since the offspring are held to inherit 

 equally from the two parents, it has been 

 supposed that with the union of the nuclei, 



the only parts of the cells that are 

 FIG. 152. Egg-cell of J .* 



Dytiscus marginalia in its exactly alike, the paternal and maternal 



follicle with two nurse contributions are intermingled, and that 

 cells. From the nurse , ., ,, , ,- ,, 



cells nutritive material therefore it is in the chromatm of the 



passes into the egg-cell, nuclei that the vehicle of transmission 

 towards which the nucleus PI j-, v/- . r i -\ j c 



sends out pointed pseudo- of hereditary qualities must be looked for. 



podia. (After Korschelt, 3. Thirdly, it has been pointed out 



Heider) K rSChelt *** ( ori g inall y b y Roux ) that karyokinesis 

 appears to be a mechanism expressly 



adapted to the simultaneous division of a large number of qualita- 

 tively different units. From the existence of karyokinesis it is 

 argued that the nucleus is not homogeneous, and that in de- 

 velopment the various units of which it is composed are directly 

 concerned in the process of differentiation. 



The doctrine of the individuality of the chromosomes has been 

 brought forward in support of this belief, as well as the 

 constancy of their number. 



4. The diminution of the chromosomes. Boveri has observed 

 in Ascvris, in the nuclei of purely somatic cells, a peculiar process 



