EARLY STAGES OF SEG-MENTATION OF ECHINUS HYBEIDS. 507 
chromosomes Ijiug in it. If, then, each chromosome can be 
regarded as a closed semipermeable membrane containing 
substances at a higlier concentration than the surrounding 
cytoplasm now possesses, the chromosomes will absorb water, 
and in some cases swell so much as to produce vesicles. 
In the cross acutus ? x esculent us cj vesicle for- 
mation could be explained on the same hypothesis, if the 
esculent us spermatozoon takes up more fluid from the 
cytoplasm and especially from the egg-nucleus in forming the 
male nucleus than does the acutus sperm. It will act like 
the hypertonic solution in withdrawing water from the rest 
of the egg. The egg-cytoplasm will replace this water from 
the surrounding sea-water, but the chromosomes which 
have formed meanwhile in the egg-nucleus will have a 
higher concentration than the normal, and, as in the case of 
the eggs treated with Iwpertonic solutions, will show a 
tendency to swell and form vesicles. That the chromo- 
somes may be regarded as being each enclosed in a semi- 
permeable membrane is suggested by their normal behaviour 
in the late anaphase, when each chromosome swells up to 
form a vesicle in appearance like a small nucleus. Evidence 
in the same direction has been recently brought forward by 
A. A. Lawsoifl, though we doubt whether his conclusions are 
entirely applicable to our cases. The suggestion that in 
the cross acutus ? x esculentus (J, the esculentus 
sperm acts like a hypertonic solution in withdrawing water 
from the egg-nucleus is supported by the fact, i-eferred to 
in oui' account of the conjugation of the nuclei, that in this 
cross the sperm-nucleus conjugates with the egg-nucleus 
while it is still quite small and contains a compact mass of 
chromatin, and while the chromosomes of the egg-nucleus 
are not yet definitely visible. In the converse cross, on the 
other hand, in which no vesicles are formed, the sperm- 
nucleus becomes nearly as large as the egg-nucleus before 
the two come into contact, so that conjugation does not 
‘ Lawson, “Nuclear Oniosis as a Factor in Mitosis,” ‘Trans. Roy. 
Soc. Edin.,’ vol xlviii, pi. i, 1912, p. 1.37. 
VOL. 58, PAKT 3. NEW SERIES. 
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