86 PRIMITIVE FORMS OF LIFE 



separate from one another forming as many distinct bodies, 

 the chromosomes. The number of chromosomes is generally 

 even. Throughout the entire animal and vegetable king- 

 doms the cells that are later to give rise to the ova and 

 spermatozoa, as the case may be, divide twice in succession, 

 in such a fashion that down to the last division in the case of 

 animals, and to the penultimate division in the case of plants, 

 the chromosomes, instead of splitting before the cell divides 

 and then distributing themselves equally between the two 

 resulting new cells, form two equal groups without preliminary 

 fission. The number of the chromosomes in the germ cells 

 is consequently less by half than that in the somatic cells. 

 The penetration of the spermatozoon within the ovum re- 

 establishes the proper number of chromosomes and makes the 

 fecundated ovum the first complete, normal body-unit, which 

 will produce others by bipartition. 



From the above it appears that in order to make use of the 

 reserve materials it has accumulated, which distinguish it 

 from the male element, the ovum must possess a predetermined 

 quantity of chromatin. Thus, chromatin must be the active 

 substance which controls the digestion of the reserves in the 

 structural cells. The experiments which Balbiani performed 

 long ago on Infusoria, and which many investigators have 

 since repeated, seem to confirm this view completely. The 

 chromatin can only perform this digestive function with the 

 aid of ferments no doubt associated with other secretions, 

 affecting by their quantity and quality the nature of the 

 elements formed later, and determining the separation 

 previously described. 



A proof of this influence exerted by chromatin is furnished 

 by the observation of what occurs in the case of certain 

 insects possessing spermatozoa of two kinds, where either half 

 the spermatozoa contain one chromosome more than the others l 

 or where one half contains a chromosome much larger than the 

 others. 2 In both cases ova fecundated by the spermatozoa 

 richest in chromatin produce females, the others males. This 

 fact acquires even greater significance if it be recalled that the 



1 Moths ; Cockroaches, Hemiptera of the genera Pyrrhocoris, Protenor, 

 A nasa, A lydus, etc. 



2 Hemiptera of the genera Lygceus, Caenus, Euschistus ; coleoptera of the 

 genus Tenebrio ; the domestic fly. 



