404 THE HEREDITARY FACTORS AND DIFFERENTIATION 



complete set of all the chromosomes. As has already been seen, the 

 blastomeres of the sea-urchin can be separated, and the hypothesis 

 can be tested by seeing how many of such blastomeres of dispermic 

 eggs are capable of development. As a matter of fact, the observa- 

 tions are in accordance with the calculated probabilities. Further, 

 in some dispermic eggs, there is formed not a quadripolar but a 

 tripolar spindle, and the egg cleaves into three. Here, the pro- 

 babilities of any blastomere receiving a complete set of chromo- 

 somes are different, but again, observation accords with calculation. 

 Thus the chromosomes of any haploid set (genome) are functionally 

 different, and the presence of all of them is essential.^ 



The problem has also been attacked from another angle by means 

 of experiments on frogs' eggs which have been subjected to X-rays 

 or mechanical injuries to the nucleus, and which are fertilised by 

 sperms subjected likewise to X-rays, ultra-violet rays, or trypa- 

 flavine. The effect of such treatment on the sperm is to incapacitate 

 the nucleus from playing any further part in development, without 

 destroying the activating power of the sperm. In no case can 

 normal development ensue if both the egg and the sperm nuclei 

 have been affected, but it has been possible to determine the stages 

 at which the normality of the developmental processes breaks down. 

 In the first place, it has been found that the presence of a certain 

 amount of chromatic material on the spindle is necessary if cleavage 

 is to take place at all.^ Next, it appears that as a result of slight 

 irradiation of the egg (the sperm having been treated with trypa- 

 flavine), a normal though retarded cleavage may take place, but 

 gastrulation is seriously affected. Either the blastopore closes very 

 slow^ly and nothing more happens, or the blastopore lip is merely 

 ephemeral, or it does not even appear at all. In all these cases it is 

 clear that the damaged nuclear apparatus is responsible for the 

 failure to develop.^ 



Further evidence is supplied by experiments with larval hybrids, 



i.e. larvae resulting from the. fertilisation of eggs of one species 



by sperm of another. This is well shown in some sea-urchins, where 



the larval skeleton may show considerable specific differences. 



The pluteus of Echinus microtuherculatus is of an elongated 



^ Boveri, 1904, 1907. - Dalcq and Simon, 1932. 



^ Dalcq and Simon, 1931. 



