STUDIES OF FERTILIZATION 385 
part that contains the sperm nucleus undergoes cleavage and 
develops farther; in the other part the female sex-nucleus under- 
goes remarkable transformations, dissolving and reappearing, a 
process which is repeated several times.’’ In spite, therefore, 
of the presence of the sperm-nucleus in a constricted part of 
the same egg, the part containing the egg nucleus was not 
fully fertilized. It made abortive attempts at division, but the 
karyokinetic figure was too feeble to carry the process through. 
Wilson observed (’03) that if the fertilized eggs of Cerebrat- 
ulus be cut in two shortly after the penetration of the sperm- 
atozoon “‘only a single fragment develops even though the 
fragments be refertilized immediately after the operation. In 
such cases it is almost invariably the nucleated fragments that 
develop, but in a very few cases I have observed that the enu- 
cleated fragment develops,’ while the nucleated one forms the 
polar bodies, but proceeds no further.”’ By the nucleated frag- 
ment in this case Wilson means the fragment containing the 
maturation spindle. Farther on he adds ‘‘The few cases in 
which the enucleated fragment of the bisected fertilized egg 
develops are doubtless those in which the plane of section sep- 
arates the sperm-nucleus from the egg-nucleus.”’ This is indeed 
the only possible explanation. In such cases the fragment con- 
taining the egg-nucleus is only partially fertilized. 
Boveri has also observed that if freshly fertilized sea-urchin 
eggs be broken into fragments by shaking, certain of the frag- 
ments contain the egg nucleus alone. If such fragments are not 
subsequently entered by a spermatozoon, the nucleus enlarges, 
dissolves and reappears again; but they do not segment (96). 
Later he observed that such pieces may divide at least twice (’02). 
These experiments demonstrate. that fertilization is still 
partial even some time after entrance of the spermatozoon into 
the egg; but they do not show at what stage it is complete. 
Boveri’s very interesting observations on ‘partial fertilization’ 
in the sea-urchin egg (’88 and ’9G) carry the solution of the prob- 
lem a step farther (cf. also Teichmann ’02). In the experiments 
which furnished the material for his observations both eggs and 
sperm were weakened, the former by standing for fourteen hours 
