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 

 "Wenckebach 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, PI. III. and figs. 4, 5a, and 56, PI. 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 

 Gotte 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 in 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 Balfour and Deighton. In their paper (No. 19) part of 

 the mesoblast is determined to be from the indifferent cells of the primitive streak, prim- 

 arily epiblastic {Ibid., 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, I, p. 150), and really "lower layer" cells, Balfour con- 

 siders "have also a share in forming the future mesoblast" (p. 154). Kingsley 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). 



Hypoblast. — 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, 11, 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 fine 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 (PI. 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. 56 and 6, PI. IV.). We see here the very phenomenon which 

 Kingsley and Conn* and Cunningham have suggested, viz., that the invaginated hypo- 

 blast is really " dorsal hypoblast, roofing over a primitive enteric cavity, whose floor is 



* Op. rib, p. 201. 



