408 THE HEREDITARY FACTORS AND DIFFERENTIATION 



§4 



Another line of work concerns the time at which the hereditary 

 factors in the chromosomes begin to exert their action in differ- 

 entiation, Cidaris and Lytechinus are two species of sea-urchins 

 which differ considerably in the times at which corresponding pro- 

 cesses take place during their developments. The larva of Cidaris 

 gastrulates about 20 hours after fertilisation, and later, mesen- 

 chyme is formed from the inner end of the archenteron at about 23 

 hours. The larva of Lytechiiius gastrulates after about 9 hours, but 

 mesenchyme has already been formed at 8 hours after fertilisation ; 

 this mesenchyme therefore cannot be produced from the formed 

 archenteron but is derived from the outer surface of the larva before 

 gastrulation has begun, at the place where the archenteron will later 

 begin to invaginate. 



The hybrid obtained by fertilising eggs of Cidaris with sperm 

 of Lytechinus begins by developing as a larva of typical maternal 

 (i.e. Cidaris) character, up to the end of the blastula stage. This 

 indicates that the paternal factors have not yet exerted any effect 

 up to this stage. But the mesenchyme is produced just as the 

 archenteron begins to invaginate, not from its inner end (as in 

 Cidaris) but from the sides of its base, near the outer surface of the 

 larva, thus resembling the conditions in Lytechinus. In this respect 

 the hybrid is intermediate between the two parent-species, and it 

 is clear that the paternal factors begin to make their effects observable 

 just at the beginning of the gastrula stage. ^ 



It is probable, therefore, that it is in the immediately preceding 

 stage, that of the late blastula, that the action of the hereditary 

 factors in the nuclei commences. In this connexion it is most in- 

 teresting to note that the late blastula is precisely the stage at which 

 the ratio of cytoplasm to nucleus in the blastomeres reverts to the 

 value at which it stood in the oocyte, before maturation of the egg 

 took place (see Chap, v, p. 132). It may therefore perhaps be 

 suggested that the time of onset of the action of the hereditary 

 factors of the nuclei depends upon the reversion of the cytoplasmic- 

 nuclear ratio to its initial value. ^ 



^ Tennent, 1914, 1922. 

 2 Boveri, 1905. 



