upon the Eggs of Echinus. 5 
Now, the permeability of the egg is changed when a sperma- 
tozoon enters, and presumably the change is constant in degree 
for each species. When, however, the sperm of a foreign species 
is made to enter an egg, is it not possible that the change in 
permeability is not that which would have been caused by a sperm 
of the species to which the egg belongs? If this is so, and the 
degree of change in permeability of an egg when fertilised is there- 
fore a function of the sperm, then the cytological behaviour of 
reciprocal crosses is explicable. 
Let the change in permeability for 
FE. acutus eggs (fertilised by) H. acutus sperm be P, 
E. esculentus m ce . esculentus ae ao 
Then EL. esculentus A 9 Bin CHEUTEUS ss JE 
and E. acutus FP » . esculentus eprel? 
i 
Let the difference between P and P, be about equal to the 
change of permeability in normally fertilised eggs of H. acutus 
which is brought about by the action of hypertonic solutions of 
appropriate strength. 
Now the chromatin of E. esculentus can withstand a change of 
permeability in the surrounding protoplasm equal to P—P,, 
without becoming abnormal*, as is shewn by its reaction to a 
hypertonic solution capable of producing such a change. Hence 
when the egg of H. esculentus is fertilised by the sperm of F. 
acutus, and so attains the permeability peculiar to fertilised eggs 
of /. acutus, no abnormality occurs. 
On the other hand, when the egg of H. acutus is fertilised by 
the sperm of ZF. esculentus, the EL. acutus element becomes patho- 
logical, as it cannot endure a permeability in its surrounding proto- 
plasm equal to that characteristic of an egg of H. esculentus. 
This suggestion is, however, in entire opposition to most of 
the conclusions of other workers in hybridisation. The work of 
Kupelwieser, Baltzer and Tennent all tends to shew that it is the 
male chromatin which becomes abnormal in such cases. Tennent, 
however, has shewn that in the cross Arbacia x Towxopneustes not 
only are all the chromosomes derived from the male eliminated, but 
also some of those from the female parent. Again, G. Hertwig 
has shewn that fusion with an abnormal sperm can cause the 
female chromatin of a normal egg to be affected. 
If the abnormalities are to be entirely confined to the male 
element in the egg, there is apparently no explanation for the 
eytology of such reciprocal crosses as Sphaerechinus x Strongylo- 
centrotus, or Hchinus acutus x H. esculentus; for if, as Baltzer 
* Presumably the change P—P, is a little less than that caused in eggs of 
E. esculentus by 50 ¢.c. sea-water + 6c.c. 24 M. NaCi, as in the latter case one vesicle 
is produced, while in the hybrid H. acutus x E. esculentus no such abnormality 
occurs. 
