of the Fishery Board, for Scotland. 
307 
out. This widened portion (h) is the rudiment of foundation of the 
future brain- region of the skate. The embryo has grown somewhat in 
length. The medullary groove is deeper, and in the following phase (fig. 
10) the medullary folds (m.f.) are commencing to grow upwards and in- 
wards as a preliminary to the closing in of the central nervous system. 
This upgrowth reaches its maximum in fig. 11, where towards the middle 
region of the embryo the folds meet. The steps of the closure would 
require several figures for their illustration. The first fusions of the folds 
are usually at two points ; the one lying about the middle of the embryo, 
the other a little further back. 
The stage of almost complete closure of the folds is depicted in fig. 12 ; 
the only points still left open lie at the extreme anterior and posterior 
ends of the neural axis. The anterior opening also soon closes. A fictitious 
importance, both physiological and morphological, has been attached to it 
by Van Wyhe and one or two others ; but it is beyond the limits of my 
subject to enter into any discussion of that matter in this paper. 
In figures of the embryo, as an opaque object, nothing of the internal 
structure is revealed, but it may be stated that at this period considerable 
differentiation has taken place within the embryo, especially in the meso- 
blast. In the stage figured in fig. 11, some 25 or 26 mesoblastic somites 
had been formed. In fig. 12 a few more were present. In fig. 11 there 
was hardly any trace of a tail swelling, but this structure, which forms 
the future tail of the skate, is well marked in fig. 12 (t.s.). 
It becomes raised above the level of the blastoderm, and begins to grow 
backwards, and soon forms a long process. In the stage of fig. 12 the 
first gill cleft is also in course of formation. 
Some of the following stages are best studied in preparations of embryos 
stained and mounted whole as transparent objects. Figs 13, 14, and 15 
are from such preparations. In these figures much of the internal 
structure of the embryo is represented. 
The embryo of fig. 13 measured about 5*8 mm. It was well raised 
and folded off from the gigantic yolk-sac, from which it draws the supplies 
of nutrition for its subsequent growth through the medium of its blood, 
which has already commenced to be formed. In that yolk-sac lies its 
nourishment for the next six months at least (Raja batis). 
By the rapid growth of the brain (b) the cranial flexure, so character- 
istic of elasmobranch and some other vertebrate embryos, has commenced. 
The mouth has not yet broken through, but the point at which it will be 
formed can be seen as a depression at m. At this point also the hypo- 
physis cerebri, or rather its oral portion, will also arise. Whatever the 
meaning of this structure may be, its close connection with the formation 
of the mouth is a point which should not be lost sight of. The anus is 
also not yet developed — it arises late in all elasmobranch fishes. 
The neural canal (n.c.) is wide, and communicates posteriorly with the 
cavity of the alimentary canal (ax.) by means of the 'neurenteric canal' 
(n.c). The notochord (n.o.) is now entirely split off from the endoderm or 
hypoblast along nearly its entire length, posteriorly and in front of the 
neurenteric canal it is fused with or passes into the floor of the medullary 
tube above, and the endoderm or hypoblast below. 
In the anterior end of the embryo, the nose (not seen in the figure) is 
represented by a pair of epiblastic thickenings. The primary optic 
vesicles (o.) which will form the sensory portion of the future eye, are 
present as a pair of outgrowths from the anterior cerebral vesicle. The 
pineal body (p.b.) can be seen as a minute structure lying on the roof of 
the hinder portion of the anterior cerebral vesicle. The auditory organ 
(au.) is represented by a vesicle. 
