ZOOLOGY AND BOTANY, MICROSCOPY, ETC. 
761 
preceding this there is the layered, yolk-free, lecithoderm. From the 
merocytes in the floor of the subgerminal cavity there is during the 
diblastic stage of the germ an abundant giving off of cells on the lower 
germinal layer over its whole extent ; at the beginning of gastrulation 
there is an abundant proliferation at the marginal pad of the lecitho- 
derm, especially on its distal zone ; but whether the merocytes continue 
to form cells in later stages is undecided. So is the fate of the mero- 
cytes. The volk-cells first appear superficially, finally in the centre. 
They probably arise from yolk-free cells, which in the middle stages 
of development occur abundantly in the perilecithal cleft and in the 
superficial layers of the internal yolk-mass. The yolk-cells may dis- 
appear without becoming yolk-sac epithelium; but this is uncertain. 
In the final state there are two forms of cells : yolk-cells and ripe yolk- 
sac epithelial cells ; and there are five preliminary forms — merocytes, 
lecithoderm cells (early epithelium), yolk-free cells of the lecithoderm 
margin, very minute yolk-free cells and other yolk-free cells round and 
flat. 
The yolk-organ of reptiles is in its parietal layer like that of birds, 
in its internal cell-mass like that of amphibians. The development of 
the wall shows four stages, as in birds. The yolk-cells do not develope 
by yolk-segmentation as in amphibians, but arise from the formation 
of yolk-free cells from the parietal layer. It is possible, however, 
that the merocytes produce a first generation of non-persistent yolk- 
cells. The yolk-cells and the yolk-sac epithelial cells are originally 
homologous ; the latter are morphologically derivable from the former. 
In birds the palingenetic formation of yolk-cells appears to have been 
entirely lost. 
Vertebrae and Protovertebrae.* — Prof. V. v. Ebner notes that the 
cartilaginous vertebrae of snakes (and perhaps of all Amniota) develope 
earlier than the cartilaginous arches ; that the so-called “ primitive 
vertebral arches ” of Amniota are embryonic structures which cannot be 
brought into direct relation with any definite skeletal parts ; they are 
metameric vertebral streaks. Coming’s protovertebral cleft is the same 
as his intervertebral cleft, and both correspond to von Ebner’s inter- 
vertebral cleft, which however is not identical with the articular cavity. 
The primitive constrictions of the notochord, as described by Corning 
in blind-worms, have no persistent importance ; the permanent con- 
strictions develope after the beginning of the vertebral ossification in 
association with the formation of the articular surface and cavity. The 
latter appears long after the intervertebral cleft has disappeared from 
the intervertebral cartilage. The intervertebral cleft disappears en- 
tirely outside the vertebral body ; the intervertebral foramen does not 
represent it. The cleft does not lie in a myoseptum, for the latter 
represents the boundary of two protovertebrae, whereas the former 
corresponds to the cranio-caudal middle region of a proto vertebra. 
Parietal Eye and Nerve.-f— Prof. E. Beraneck, having studied the 
parietal nerve and the pineal body, especially in Anguis fragilis , comes 
to the following conclusions. The parietal eye is not a simple diver- 
* SB. K. Akad. Wiss. Wien, ci. (1892) pp. 235-60 (1 pi.), 
t Anat. Anzeig., vii. (1892) pp. 671-89 (6 figs.). 
3 F 
1892. 
