364 
MESSES. E. M. BALEOUR AND W. N. PARKER ON THE 
the region of the body. In the head (Plate 22, fig. 18, m.c.), it is very prominent, and 
forming, as it does, the major part of the axial tissue of the body, impresses its own 
shape on the other parts of the head and gives rise to a marked ridge on the surface 
of the head directed towards the yolk. In the trunk (Plate 22, figs. 19, 20) the keel 
is much less prominent, but still projects sufficiently to give a convex form to the 
surface of the body turned towards the yolk. 
In the head, and also near the hind end of the trunk, the nervous layer of the epi- 
blast continuous with the keel on each side is considerably thicker than the lateral 
parts of the layer. The thickening of the nervous layer in the head gives rise to 
what has been called by Gotte* 4 4 the special sense plate,” owing to its being sub¬ 
sequently concerned in the formation of parts of the organs of special sense. We 
cannot agree with Gotte in regarding it as part of the brain. 
In the keel itself two parts may be distinguished, viz. : a superficial part, best 
marked in the region of the brain, formed of more or less irregularly arranged 
polygonal cells, and a deeper part of horizontally placed flatter cells. The upper part 
is mainly concerned in the formation of the cranial nerves, and of the dorsal roots of 
the spinal nerves. 
The mesoblast (ms.) in the trunk consists of a pair of independent plates which are 
continued forwards into the head, and in the prechordal region of the latter, unite 
below the medullary keel. 
The mesoblastic plates of the trunk are imperfectly divided into vertebral and 
lateral regions. Neither longitudinal sections nor surface views show at this stage 
any trace of a division of the mesoblast into somites. The mesoblast cells are poly¬ 
gonal, and no indication is as yet present of a division into splanchnic and somatic 
layers. 
The notochord (nc.) is well established, so that its origin could not be made out. 
It is, however, much more sharply separated from the mesoblastic plates than from 
the hypoblast, though the ventral and inner corners of the mesoblastic plates which 
run in underneath it on either side, are often imperfectly separated from it. It is 
formed of polygonal cells, of which between 40 and 50 may as a rule be seen in a 
single section. No sheath is present around it. It has the usual extension in front. 
The hypoblast ( hy .) has the form of a membrane, composed of a single row of oval 
cells, bounding the embryo on the side adjoining the yolk. 
In the region of the caudal swelling the relations of the germinal layers undergo 
some changes. This region may, from the analogy of other Vertebrates, be assumed 
to constitute the lip of the blastopore. We find accordingly that the layers become 
more or less fused. In the anterior part of the tail swelling, the boundary between 
the notochord and hypoblast becomes indistinct. A short way behind this point 
(Plate 22, fig. 21), the notochord unites with the medullary keel, and a neurenteric 
* “Tib. d. Entwick. d. Central-Neryen Systems d. Teleostier,” Archiv fur mikr. Anat., vol. xv., 1878. 
