ZOOLOGY AND BOTANY, MICROSCOPY, ETC. 
(301 
The proximal retinulae are pigment-cells which closely invest the 
rhabdome. Accessory pigment-cells occupy the open space at the base 
of the ommafcidia, and their function seems to be that of filling what 
would otherwise be an unoccupied space, as though they gave solidity 
to the tissue in the base of the retina ; similar cells have been seen by 
Carriere in Astacus , and by Patten in Penseus. As the fibres of the 
optic nerve pass into the proximal retinulae, it is most likely that the 
rhabdome and not the cone is the perceptive body ; this conclusion 
is further supported by the ultimate distribution of the optic fibrillae. 
The author shows that the cone- cells form a transparent axis which 
leads directly to the rhabdome, and through which light could readily 
have reached that structure, and it is, no doubt, in it that the light is 
transformed into that kind of energy which is transmitted by nerve- 
fibres. 
In treating of the development of the eye of the Lobster, the author 
points out that four different types of structure have been indicated as 
possible by the work of various investigators. He is himself led to reject 
that of Patten as unsupported by embryology, while Reichenbach and 
Kingsley are said to have misinterpreted structures. On the whole, the 
balance of evidence seems at present to be in favour of the view that the 
retina originates as a thickened layer of hypodermis, and is not modified 
by any form of involution. When there is an involution it is connected 
only with the formation of the optic ganglion ; and in the production 
of this ganglion, the involution can be replaced by a proliferation of 
cells. In Crustaceans the nerve-fibres are always attached to the 
proximal ends of the retinulae, and we may, therefore, suppose that tho 
retina has never been inverted, but retains its original position ; any 
explanation which involves the inversion of the retina is, in all proba- 
bility, wrong. 
It is difficult to draw any general conclusion as to the number 
of retinulae in the ommatidia of the - higher Crustacea, but Herrick’s 
statement that there are seven in AlpJieus coincides fairly with the 
results obtained from the lobster. The author concludes with some 
discussion as to the types of ommatidia, in which he attempts to bring 
the ommatidia of all Crustaceans into relation by suggesting a process 
of cell-division, but the question of what constitutes the simplest form 
of ommatidium is one which still requires study. 
Blastoderm of Isopoda.* — M. L. Roule describes the formation of 
the blastoderm in Porcellio scaber as a twofold process : — (1) The 
peripheral differentiation of the deutoplasm, under the influence of the 
nucleated “ islet,” into a formative layer ; and (2) the successive nuclear 
divisions which establish the blastoderm. The deutoplasm is in no 
sense nucleated ; all the nuclei of the blastoderm are derived from the 
segmentation-nucleus ; the nuclei of the “ vitelline cells ” of other inves- 
tigators are, like those of the embryonic body, derived from the nuclei 
of the blastoderm accounted for above. 
The Oxycephalids.f — Under this title Mr. C. Bovallius has 
published a memoir on these Ampliipoda. He discusses the principles 
* Comptes Rendus, cx. (1890) pp. 1373-4. 
t Nova Acta Reg. Soc. Sc. Upsal., iii. (1890) 141 pp., 7 pis., and 87 figs, in text. 
