ZOOLOGY AND BOTANY, MICROSCOPY, ETC. 
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every starfisli cannot open every bivalve, and that the size and strength 
of the two must be in suitable proportion. 
Continuity of Mesenchyme-Cells in Echinoid Larvae.* — Mr. E. W. 
M‘Bride reminds the reader of the view of Mr. Adam Sedgwick that 
free stellate mesoblast-cells, such as are often figured, are not to be 
found in the elasmobrancli embryo, but that the appearances thus inter- 
preted were really only the thickened nodes of a protoplasmic network. 
The examination of numerous larvae of Echinus esculentus enables 
Mr. M'Bride to support the observations of Sedgwick in a very striking 
manner. The larvae referred to were in the gastrula stage, and pos- 
sessed a comparatively narrow alimentary tract, outside which was a 
wide primary body-cavity. In this latter there were numerous mesen- 
chyme-cells. These cells were of two main kinds ; some were aggre- 
gated masses of rounded cells, which formed the matrix of the future 
larval skeleton, while others were wandering cells or amoebocytes. Of 
these latter there were two varieties, of which the first were obviously 
stellate in form, and connected with each other and with the walls of the 
cavity by long processes. The second kind of wandering cell is rounded 
in form, and has always been described and figured as if it were per- 
fectly free. If, however, one examines a slightly compressed living 
gastrula of Echinus esculentus , one observes that these rounded cells, 
which at first sight look as if they were completely free, are in every 
case connected, either with neighbouring cells, or with the walls of the 
cavity by excessively fine threads, along which they appear to travel. 
Without wishing to go as far as Mr. Sedgwick in denying reality to 
the conception of the cell, Mr. M c Bride is inclined to hold that the cell- 
structure of the Metazoa is largely due to secondary differentiation, and 
that a multinucleate Protozoon like Actinosph&rium is to be compared to 
the Metazoan body, and not to a single unit of the same. 
Spermatozoa of Echinoderms.f — Prof. G. W. Field began at Naples, 
and has carried on elsewhere, an investigation of the morphology and 
physiology of the Echinoderm spermatozoa. Many of the facts found 
range themselves with a large mass of details which are being accumu- 
lated from all branches, not only of the animal, but also of the vegetable 
kingdom, and tending still more to strengthen the theory first advanced 
by Mark, and lately confirmed by 0. Hertwig and others, that the polar 
bodies are aborted eggs ; that the egg, the spermatozoa, and the polar 
bodies are strictly homologous, and that any difference apparent is to be 
regarded as a specialisation for specific purposes. The accumulation of 
all the food-yolk in one of the ova results in the uselessness of the other 
three (polar bodies). The modification of the four spermatids derived 
from the spermatogone by differentiation of the cytoplasm into a vibratile 
tail, the separation and subsequent extrusion of the no longer useful 
material of the nuclear spindle, in the form of the mitosome, are modi- 
fications merely of parts of the cell. 
The author calls attention to the following points : — The size and 
shape of the spermatozoa differ in the various classes; in Holothurians, 
Ophiurids, and Asterids the head is spherical; in Crinoids andEchinids 
* Proc. Camb. Phil. Soc., ix. (1896) pp. 153-4. 
f Journ. Morphol., xi. (1895) pp. 235-70 (2 pis.). 
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