510 
SUMMARY OF CURRENT RESEARCHES RELATING TO 
terms ectoderm and endoderm, and the very use of these led gradually 
to emphasis on position. The author goes on to trace the development 
of the planula-theory (Ray Lankester) and the gastraea-theory (Haeckel), 
the slow evolution of the biogenetisches Grundgesetz which he traces 
back to Aristotle, and the growth of scepticism as to the homology of 
the germinal layers, which had been assumed, where only analogy was 
evident. From the development of Bryozoa, &c., he shows how difficult 
it is to homologise the analogous layers throughout. He maintains 
that the homology of Keimschichten has got hopelessly mixed up with 
the analogy of Keimblntler , and that embryologists have a bad habit of 
shifting at random from morphological to physiological conceptions. 
Relative Proportions of Yolk, Shell, and White of Egg.* — Dr. R. W. 
Bauer, continuing his quantitative estimates, finds that a double-yolked 
duck’s egg has 42*554 g. of yolk, 59*06 g. of albumen, and 11*453 g. 
of shell. In a thrush’s egg there was 2 * 31 g. of yolk, 4*642 of albumen, 
and * 999 of shell. 
j8. Histology* 
Particular Type of Nerve-cell in the Middle Ganglionic Layer of 
the Bird’s Retina.| — Prof. A. S. Dogiel finds, besides the large nerve- 
cells and amacrine cells of the middle ganglionic layer, other cells of 
much interest. He describes two types:— (1) Nerve-cells whose very 
long axis-cylinder process enters into and branches within the optic 
centres, in the layer of optic fibres, and (2) nerve-cells whose relatively 
short axis-cylinder process does not go beyond the retina, but branches 
on the external surface of the inner reticular layer. In many, parti- 
cularly in large cells of this second type, there are two axis-cylinder 
processes, either both from the body of the cell, or one from a proto- 
plasmic process, and the other from the body of the cell. The two kinds 
of centrifugal fibres which end in the retina come into close relations 
with the cells of the middle ganglionic layer ; the terminal branches of 
the first kind of fibre surround the amacrine cells (which have no axis- 
cylinder process), those of the second kind of fibres form a feltwork 
around the protoplasmic processes of the second type of nerve-cells 
above described. 
Retinal Cells in Eyes of Fishes.J — The late Prof. J. A. Ryder had 
a note on an arrangement of the retinal cells in the eyes of Fishes which 
partially simulate compound eyes. He recently found that in the eye of 
a larval salmon there is an arrangement of the sensory cells of the retinal 
epithelium that is so regularly and definitely repeated throughout the 
whole extent of the retina as to admit of no question. The first thing 
that strikes the observer in examining these sections is the extraordinary 
regularity with which the rods and cones form a pattern. The author 
recognises that a comparison of these rods and cones in the retina of the 
Salmon cannot be exactly instituted with the cell groups in the Arthropod 
eye which are known as ommatidia. The regularity and repetition of 
the arrangement of cellular elements in the retina of a fish suggests that 
* Biol. Centralbl., xv. (1895) p. 448. 
f Anat. Anzeig., x. (1895) pp. 750-60 (2 figs.). 
X Proc. Acad. Nat. Sci. Philad., 1895, pp. 161-6 (2 figs.). 
