STRUCTURE AND DEVELOPMENT OF LEPIDOSTEUS. 
365 
cord, homologous with the neurenteric canal of other Ichthyopsida, is thus established. 
In the same region the boundary between the lateral plates of mesoblast and the 
notochord, and further back (Plate 22, fig. 22), that between the mesoblast and the 
medullary keel, becomes obliterated. 
Fifth day after impregnation .—Between the stage last described and the next stage 
of which we have specimens, a considerable progress has been made. The embryo 
(Plate 21, figs. 6 and 7) has grown markedly in length and embraces more than 
half the circumference of the ovum. Its general appearance is, however, much the 
same as in the earlier stage, but in the cephalic region the medullary plate is 
divided by constrictions into three distinct lobes, constituting the regions of the 
fore-brain, the mid-brain, and the hind-brain. The fore-brain (Plate 21, fig. 6, fb.) is 
considerably the largest of the three lobes, and a pair of lateral projections forming 
the optic vesicles are decidedly more conspicuous than in the previous stage. The 
mid-brain ( m.b .) is the smallest of the three lobes, while the hind-brain (h.b.) is 
decidedly longer, and passes insensibly into the spinal cord behind. 
The medullary keel, though retaining to a great extent the shape it had in the last 
stage, is no longer completely solid. Throughout the whole region, of the brain and 
in the anterior part of the trunk (Plate 22, figs. 23, 24, 25) a slit-like lumen has become 
formed. We are inclined to hold that this is due to the appearance of a space between 
the cells, and not, as supposed by Oellacher for Teleostei, to an actual absorption of 
cells, though we must admit that our sections are hardly sufficiently well preserved to 
be conclusive in settling this point. Various stages in its growth may be observed in 
different regions of the cerebro-spinal cord. When first formed, it is a very imperfectly 
defined cavity, and a few cells may be seen passing right across from one side of it to 
the other. It gradually becomes more definite, and its wall then acquires a regular 
outline. 
The optic vesicles are now to be seen in section (Plate 22, fig. 23, op.) as flattish 
outgrowths of the wall of the fore-brain, into which the lumen of the third ventricle 
is prolonged for a short distance. 
The brain has become to some extent separate from the superjacent epiblast, but 
the exact mode in which this is effected is not clear to us. In some sections it appears 
that the separation takes place in such a way that the nervous keel is only covered 
above by the epidermic layer of the epiblast, and that the nervous layer, subsequently 
interposed between the two, grows in from the two sides. Such a section is repre¬ 
sented in Plate 22, fig. 24. Other sections again favour the view’ that in the isolation 
of the nervous keel, a superficial layer of it remains attached to the nervous layer of the 
epidermis at the two sides, and so, from the first, forms a continuous layer between the 
nervous keel and the epidermic layer of the epiblast (Plate 22, fig. 25). In the absence 
of a better series of sections we do not feel able to determine this point. The posterior 
part of the nervous keel retains the characters of the previous stage. 
At the sides of the hind-brain very distinct commencements of the auditory vesicles are 
