730 PROFESSOR W. C. M‘INTOSH AND MR E. E. PRINCE ON 
and gills, besides the eyes and mouth themselves, as developed from wandering mesoblastic 
cells as well as unsegmented mesoblast (No. 150, p. 336), and these wandering cells 
Wencxesacu has recently affirmed to be active in Teleosteans in building up the heart 
and its connected trunks, and other parts of the embryo (No. 158). It cannot be 
denied that in fig. 2, Pl. IL and figs. 4, 5a, and 5b, Pl. IV., the mesoblast has more 
intimate relation to the hypoblast than to the epiblast, and the condition presented by 
these early sections corroborates the view that the mesoblast is of hypoblastic origin, as 
Gorre strongly holds (No. 58). That the mesoblast in the Teleostei has in fact a three- 
fold origin is consonant with the figures given in various plates,—part being formed 
directly by conversion of lower layer cells im situ, while part is proliferated from the 
invaginated hypoblast beneath, and lastly to make up for the forward growth of these 
cells into the cephalic region, other mesoblastic cells are derived from the indifferent mass 
constituting the caudal region. It is singular that this account of the multiplex growth 
of the mesoblast should coincide, even down to many details, with the derivation of this 
layer in the chick, according to BaLrour and Dreicuton. In their paper (No. 19) part of 
the mesoblast is determined to be from the indifferent cells of the primitive streak, prim- 
arily epiblastic (Jbid., p. 182); some mesoblastic cells, which are stellate, are differentiated 
from the hypoblast (pp. 184-5); while certain others lying below the epiblast in the early 
blastoderm (see No. 11, fig. 91,7, p. 150), and really “lower layer” cells, BaLrour con- 
siders “have also a share in forming the future mesoblast” (p. 154). KainesLey and 
Conn, though they furnish no account of the process, come to a similar conclusion, and 
hold that this middle layer is derived partly from hypoblast and partly from lower layer 
cells (No. 78, p. 200). 
Hyrostast.—The hypoblast, hy, which there can be ‘little doubt is pushed in 
from the periphery as an inflected layer of ectodermal, for the most part “ corneous 
layer” cells, ep, with some cells derived from the periblast, per, insinuates itself 
between the under surface of the germ, //, and the cortex of the yolk, y, forming the 
limiting layer on the ventral aspect of the embryo. It separates the neurochord (ne, 
Pl. IV. fig. 5a) in the middle line and the lateral cells, mes, destined to form, in part, 
the mesoblast, from the yolk, y. It remains for some time as a single layer of flattened 
cells, hy, in the anterior and mid portions of the embryo; but at the posterior termina- 
tion (Pl. IV. figs. 5d and 6) its character alters, for it is there less definite, merging, in 
fact, with the heaped-up periblast, per, like the thickened layer of dubious cells, which in 
the chick continue into the “germinal wall” behind (No. 19, p. 179). This tract of 
mingled hypoblast and periblast is the site of much developmental activity, and about 
the time that the blastopore closes it becomes defined as a bridge of swollen columnar 
cells, hy, in the median line, arching over a fissure below, and pressing against the 
neurochord, ne, above (figs. 5b and 6, Pl. IV.). We see here the very phenomenon which 
Kinestry and Conn * and CunniycHam have suggested, viz., that the invaginated hypo- 
blast is really “dorsal hypoblast, roofing over a primitive enteric cavity, whose floor is 
* Op. cit., p. 201. 
