94 DIFFERENTIATION III 



selves made up of unlike elements, the elements which conju- 

 gate in the synapsis prior to the maturation divisions. For 

 normal development of the whole and of each part a complete 

 set must be present in every cell. In sexual reproduction two 

 such sets are present, but one suffices. 



To the evidence thus brought forward by Boveri must be 

 added certain cytological observations on the different size 

 and form of one or more of the chromosomes. Thus Sutton 

 found that in the Insect Brachystola the chromosomes of the 

 spermogonia could be arranged in n pairs according to their 

 sizes, and that in the maturation divisions the members of the 

 several pairs conjugated and then separated from one another. 

 Baltzer again and Tennent have found straight, hook-shaped, 

 and horse-shoe shaped chromosomes in Echinids, while Wilson 

 and others have demonstrated the heterochromosomes in many 

 Insects and possibly some other animals. 



In what way these various chromatic elements call forth 

 the development of those characters which they transmit is 

 unknown, but it seems certain that the differential activity of 

 nuclei which are alike must depend on differences in the 

 environment in which they are placed, that is, on differences 

 in the cytoplasm. Such we know to exist, and we know also 

 that they can provoke dissimilar behaviour in similar nuclei. 

 We are therefore brought back to the structure of the cyto- 

 plasm as a necessary condition of the transmission of characters 

 by the nuclei. 



It remains for us to consider exactly what kind of characters 

 are handed on by the plasma directly and by the nuclei. 

 Experiments on heterogeneous hybridization enable us to give 

 at least a provisional answer to this question. 



The possibility of fertilizing an ovum with the spermatozoon 

 of an animal of quite a different kind was the discovery of 

 Loeb, who found that by the addition of a small quantity 

 of calcium chloride and sodium hydrate to sea-water, the 

 eggs of the Sea-urchin Strongylocentrotus purpuratus would 

 permit the entrance of spermatozoa of the Starfish Asterias 

 ochracea. From these eggs pluteus larvae with the charac- 



