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, 1 

 should agree with Mr. Wilson that the limitation is most likely ta 

 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. " Tlie isolated blastomere develops as a unit, 

 not as a 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 

 Raiia, 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 experiment, 

 shaking up i c.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 of a spermatozoon . The hardening must 



