EARLY STAGES OE SEGMENTATION OF ECHINUS HYBRIDS. 505 
led chiefly to study the vesicle formation in this cross, on 
which a few words will be added below. 
In 1912 we have obtained the remaining crosses with 
miliar is. Of these, we find that esculentus ? x 
miliaris c^' behaves normally, with no elimination; miliaris 
? X either esculentus or acutus shows only a small 
percentage of developing eggs, of which a considerable pro- 
poi'tion at least show elimination of one, or at most a few 
chromosomes. We do not feel able to say with confidence 
wliether these are paternal or maternal ; in the few examples 
which are at the right stage of division, the length of the 
eliminated chromosomes suggests that they are paternal. 
We cannot, however, correlate this result with any confidence 
with the observation that the advanced plutei show maternal 
characters, for as Shearer, De Morgan and Fuchs have 
recorded, in 1912 the crosses with miliaris ? have shown a 
different behaviour from that of previous years, and have 
been as a rule of the paternal rather than of the maternal 
type. It is not certain, thei’efore, that our results obtained 
in 1912 are similar to what w'ould have been obtained in 1911 
or previous years. It is worthy of note, however, that in the 
cross esculentus ? x miliaris (J, which in 1912, as in 
other years, has given purely maternal plutei, no chi'omo- 
some elimination of any kind has been found. 
With regard to vesicle-formation little need be said here, 
for the matter is thoroughly discussed in a paper by one of 
us published concurrently with this. We assumed at first 
that the chromosomes Avhich develop vesicles are paternal, 
and that vesicle-formation is to some extent comparable Avith 
the elimination of paternal chromosomes described byBaltzer. 
Experiment Avith hypertonic solutions on eggs of the pure 
species, hoAvever, has made it very probable that the vesicles 
in the eggs of the cross acutus ? x esculentus c? are 
derived from the acutus (maternal) chromosomes, due pro- 
bably to an alteration of the permeability or osmotic condition 
of the egg consecjuent upon the development within it of a 
foreign spermatozoon. Baltzer gives evidence on several 
