V. H. Blackman. 
96 
been if produced by a similar fertilisation of normal eggs. It 
certainly seems probable that these two facts are to be causally 
related. 
The later work of Boveri (1907) again speaks strongly in favour 
of the importance of the nucleus in hereditary processes, and as 
the results are of particular interest they may be dealt with in 
some little detail. Boveri caused the eggs of Echinoderms to be 
fertilised by more than one spermatozoon. As a result of this the 
egg-nucleus does not divide regularly into two and then into four, 
but it proceeds directly to a multipolar, simultaneous division into 
three or four parts which are the nuclei of the first blastomeres- 
If now the first four blastomeres of such an egg are shaken apart, 
each will start developing independently into an embryo, but after 
a time these usually become abnormal. It is very interesting to 
note that the four embryos produced from the four blastomeres of 
a single egg often behave quite differently ; some will carry their 
development much further than others and display quite different 
abnormalities. Complete eggs fertilised by two spermatozoa and 
allowed to develop show a normal development up to the gastrula 
stage, but in the later stages defects in the skeleton or some other 
part appear. 1 The striking point about these defects is that they 
are usually confined to one part of the embryo, that arising from 
one of the first three or four blastomeres. 
The irregularity of distribu¬ 
tion of the malformations Boveri 
explains by the irregularity of 
distribution of the chromosomes 
during the first multipolar division. 
If we take the case of Strongy- 
locentrotus lividus there are 
eighteen chromosomes in the 
sexual cells; if such an egg is 
fertilised by two sperms it will 
have fifty-four chromosomes. 
When the egg divides, a spindle 
with four poles will be formed 
and the chromosomes may be 
distributed in the spindles as in 
Fig. A, where the dots represent 
1 The defects do not appear earlier because up to the gastrula 
stage the development of the egg is controlled by the egg- 
cytoplasm as shown in the experiments of Godlewski 
mentioned above. 
6 
26 12 
10 
Fig. A. 
