upon the Hggs of Echinus. 3 
cause the formation of one vesicle in each nucleus exactly similar 
in appearance and behaviour to those seen in eggs of the hybrid 
E. acutus 3 x E. esculentus f'. There was, however, this important 
difference; whereas in the hybrid eggs the vesicles are formed 
directly from individual chromosomes and are never present inside 
the nuclear membrane, those in the hypertonic eggs of H. esculentus 
are always formed inside the nuclear membrane of the female pro- 
nucleus. It was found impossible to determine whether the number 
of chromosomes formed from such a nucleus was one less than 
the full somatic number for the species, but what evidence there 
is, is certainly not unfavourable to the view that this vesicle 
represents morphologically either the whole or part of one chro- 
mosome. It resembles the vesicles of the hybrid eggs in being 
either omitted from or included in the daughter nuclei of the first 
division. 
The cytology of eggs of H. acutus which were treated in 
exactly the same way as is above described for EH. esculentus, 
_shews a marked difference between the two species. Within the 
nuclear membrane of the zygote nuclei, it is only very exceptional 
to find any trace of vesicles. As soon as the nuclear membrane 
disappears, however, numerous vesicles are found among the 
chromosomes. Their number varies from three to five, and there 
is absolutely no doubt that they are formed directly from indi- 
vidual chromosomes just as in the hybrid #. acutus 2 x E. escu- 
lentus §, for not only is the number of normal chromosomes 
correspondingly reduced, but every stage between typical chromo- 
somes and fully formed vesicles has been found. If it could be 
shewn that the vesicles are always derived from the same indi- 
vidual chromosomes in the complex, there would be an interesting 
proof of the physiological individuality of these bodies. Unfortu- 
nately so far this has been found to be impracticable. 
The ultimate effect of these hypertonic solutions on the eggs 
of both species is to cause irregularity in segmentation of the 
cytoplasm. In no case did an egg develop beyond the blastula 
stage. 
i hiese experiments shew, therefore, that in the formation of 
vesicles and the consequent elimination of chromosomes, which 
normally takes place in the nuclei of the hybrid HL. acutus 2 x H. 
esculentus ff, it is essentially the female chromatin that becomes 
pathological. It is extremely unfortunate that it is impossible to 
test this hypothesis by a study of the hybrid eggs themselves; yet 
the effects of hypertonic solutions on the eggs of the two species 
gives strong support for such an assumption. 
It will be remembered, however, that the mitoses of the hybrid 
E. esculentus 2 x E. acutus f are quite normal, and therefore we 
cannot explain the pathological condition of the reverse cross by 
1—2 
