506 
L. DONCASTKK AND J. GRAY. 
distinct lines that the chromosomes eliminated in his experi- 
ments were all paternal, so that if he is correct, it is probable 
that the vesicle-formation in our case is quite a different 
phenomenon. Tenuent,^ on the other hand, finds that both 
paternal and maternal chromosomes are eliminated in the 
cross Arbacia ? by Tox opn e u stes c? j so that it is po.ssible 
that elimination is not due simply to an incompatibility 
between the chromosomes of one species and the egg-cyto- 
plasm of the other, but that, as suggested above, the physical 
condition of the cytoplasm is altered by the development 
within it of a foreign sperm-nucleus, causing the non-division 
or other abnormal behaviour of certain chromosomes, of either 
one or both species in different cases. 
In any case a point of importance is that all these expeii- 
ments give evidence of a physiological differentiation among 
the chromosomes. Some behave normally, others form 
vesicles, or fail to divide, and from the comparative con- 
stancy of the numbers it may be inferred that it is the same 
chromosomes in each case which are affected. We do not 
feel confident in identifying the chromosomes which form 
vesicles in the acutus $ x esculent us (J cross, but our 
general impression is that some at least of the affected 
chromosomes are the same in each case, suggesting that they 
differ in physiological characters as they do in shape and 
size. The physical cause of the vesicle-formation has been 
discussed by one of ns elsewhere, but we put forward one 
additional suggestion which may possibly be worthy of con- 
sideration. A^esicles are produced in acutus chromosomes 
when the eggs have been treated with hypertonic solutions 
and have been returned to normal sea-water. It seems 
possible that the hypertonic solution withdraws water from 
the egg, causing it to have a higher concentration, both in the 
cytoplasm and nucleus, than the normal. At about the time 
the chromosomes are forming the egg is returned to water of 
lower concentration; the cytoplasm will then absorb water, 
and its concentration will thus be lower than that of the 
' ‘ Journ. Exp. Zool.,’ xii, 1912, p. 397. 
