302 NATURAL, SCIENCE. APRIL, 
stages, which are even smaller than one-eighth the normal size. 
These probably result either from fission of the half or quarter 
blastomeres, or by some fragmentation due to the shaking-up. 
‘‘ These minute embryos prove that a mass one-eighth the size of. 
the normal ovum, or less, is capable of producing a gastrula,” yet 
the one-eighth blastomeres did not. 
It seems to me, however, that this is not quite conclusive ; 
for although I am not one of those who exalt the nucleus at the 
expense of che cell-substance, it seems that we cannot exclude the 
supposition of a quantitative limit until we know more about the size 
and state of the nucleus (a) in the fraction or fragment of the two- or 
four-celled stage which did form a gastrula, and (b) in the one-eighth 
blastomere which did not form a gastrula. Yet, on the whole, I 
should agree with Mr. Wilson that the limitation is most likely to 
be qualitative, for by the eight-celled stage the difference between 
micromeres and macromeres has become pronounced. In short, the 
embryonic cells are beginning to be specialised. 
It is very important to notice that the segmentation of the 
isolated blastomere of the two- or four-celled stage agrees exactly with 
that of the entire ovum. ‘ The isolated blastomere develops as a unit, 
not asa half-unit, and the two cells to which it gives rise cannot be 
individually identified with those of a normal embryo-half. The 
development is transformed from the beginning; but in sea-urchin 
(Driesch) and frog (Roux) the development is at first that of an 
embryo-half, which subsequently, much earlier in Echinus than in 
Rana, gives rise to a perfect embryo by regeneration.” 
X. It is well-known that the entrance of a spermatozoon into an 
Echinoderm ovum is at once followed by the appearance of a thin 
enveloping membrane, which rises from the surface of the ovum and 
prevents the entrance of other spermatozoa. The spermatozoon 
which has entered influences the ovum so that others are excluded. 
It is not, however, certain that this membrane is really necessary for 
the prevention of polyspermy. 
Now, the Hertwigs showed in 1887 that if unfertilised ova were 
placed in sea-water shaken up with chloroform, a protective membrane 
was formed. Herr Herbst has recently repeated the experimert, 
shaking up 1c.cm. of chloroform with 50 c.cm. of sea-water. All 
the unfertilised ova placed in this mixture formed the protecting 
pellicle. He also succeeded when he used, instead of chloroform, 
benzol, toluol, xylol, creosote, or clove-oil, but the two last produced 
a pathological appearance in the cell-substance of the ova. 
The protective, membrane is really referable to the hyaline 
marginal layer of the ovum, which seems to have a greater consistence 
than the internal cell-substance; it has its analogue around the larval 
stages; in short, it is not in itself in any way remarkable. What is 
remarkable is the manner in which this marginal layer is deliminated 
and hardened after the entrance ofaspermatozoon. The hardening must 
