STRUCTURE AND DEVELOPMENT OE LEPIDOSTEUS. 
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of (1) an outer area (p.z.) with some resemblance to the area pellucida of the Avian 
embryo, forming the parietal part of the body; and (2) a central portion consisting 
of the vertebral and medullary plates and the axial portions of the embryo. In 
hardened specimens the peripheral part forms a shallow depression surrounding the 
central part of the embryo. 
The central part constitutes a somewhat prominent ridge, the axial part of it being 
the medullary plate. Along the anterior half of this part a dark line could be 
observed in all our specimens, which we at first imagined to be caused by a shallow 
groove. We have, however, failed to find in our sections a groove in this situation 
except in a single instance (Plate 22, fig. 20, x), and are inclined to attribute the 
appearance above-mentioned to the presence of somewhat irregular ridges of the 
outer layer of the epiblast, which have probably been artificially produced in the 
process of hardening. 
The anterior end of the central part is slightly dilated to form the brain (b.); and 
there is present a pair of lateral swellings near the anterior end of the brain which we 
believe to be the commencing optic vesicles. We could not trace any other clear 
indications of the differentiation of the brain into distinct lobes. 
At the hinder end of the central part of the embryo a very distinct dilatation may 
also be observed, which is probably homologous with the tail swelling of Teleostei. 
Its structure is more particularly dealt with in the description of our sections of this 
stage. 
After the removal of the egg-membranes described above we find that there remains 
a delicate membrane closely attached to the epiblast. This membrane can be isolated 
in distinct portions, and appears to be too definite to be regarded as an artificial product. 
We have been able to prepare several more or less complete series of sections of 
embryos of this stage (Plate 22, figs. 18-22). These sections present as a whole a most 
striking resemblance to those of Teleostean embryos at a corresponding stage of 
development. 
Three germinal layers are already fully established. The epiblast (ep .) is formed of 
the same parts as in Teleostei, viz. :—of an outer epidermic and an inner nervous or 
mucous stratum. In the parietal region of the embryo these strata are each formed 
of a single row of cells only. The cells of both strata are somewhat flattened, but 
those of the epidermic stratum are decidedly the more flattened of the two. 
Along the axial line there is placed, as we have stated above, the medullary 
plate. The epidermic stratum passes over this plate without undergoing any 
change of character, and the plate is entirely constituted of the nervous stratum of 
the epidermis. 
The medullary plate has, roughly speaking, the form of a solid keel, projecting 
inwards towards the yolk. There is no trace, at this stage at any rate, of a medullary 
groove ; and as we shall afterwards show, the central canal of the cerebro-spinal cord 
is formed in the middle of the solid keel. The shape of this keel varies according to 
