INTRODUCTION 



and nucleus, show that the nucleus must be deciding what kind of 

 hat is grown. Tlie nucleus seems to bear the genotype (Fig. i). 



Sexual reproduction is just such a transplantation as we see in the 

 alga. A sperm nucleus or generative nucleus of a pollen grain enters 

 an egg cell hundreds or thousands of times larger, and fuses with 

 the female nucleus. Thus the nucleus of the fertilized egg or zygote 

 is of mixed origin; but its cytoplasm is almost exclusively from the 

 mother. The new organism develops from this zygote and shows, 



S S?xEcJ [S]xEc^ E 



Fig. 2. — Larvae of sea urchins: S, Sphaerechinus granuhtus; E, Echinus microtuber- 

 culatus; S $ x E (^ , the normal cross; [S] X E (^ , the haploid larva produced by the 

 same cross when the nucleus of the egg has been removed. The larva is then scarcely 

 anything other than a dwarf edition of the sperm parent which is the source of its 

 nuclei (after Boveri i{ 



where they differ sufficiently, the characters of father and mother 

 in equal measure. Going a step further, Boveri was able to fertilize 

 a piece of egg, lacking a nucleus, of one species of sea urchin with 

 the sperm of another. The resulting larva (termed a merogon) closely 

 resembled a dwarf of its male parent from which it had derived its 

 nuclei and showed no obvious influence of its female parent from 

 which it had derived its cytoplasm (Fig. 2). 



We thus see the overriding action of the nucleus. The study of its 

 structure and movements may therefore be expected to tell us why 

 individuals are so constant within themselves in their inborn character 

 and also why they differ from one another. 



18 



