728 PROFESSOR W. C. M‘INTOSH AND MR E. E. PRINCE ON 
of the embryonic thickening, the proliferated epiblast, ne (Pl. II. fig. 1), and the lower 
layer cells, of course, lie above the invaginated hypoblast, hyp. These lower layer cells 
probably become largely converted into mesoblast, though it is certain that the hypoblast 
also buds off some mesoblastic cells). W. Woxrr has recently expressed a view similar to 
this, though he denies that the mesoblast (Mittelkeim) arises in any way from the endo- 
derm. The cells which build up the mesoblast represent, he holds, the surplus of those 
blastomeres which are not used in forming the gastrula (No. 160, pp. 425-448). Accord- 
ing to Kurrrer, His, and Kier, the mesoblast results solely from the differentiation of 
the deeper germ-layer, while the hypoblast is stated to originate in the periblast (KLEtN’s 
“parablast”’). GOrre speaks of it as formed from the invaginated layer, which gives 
origin in addition to the hypoblast. The fact would seem to be that much mesoblast is 
formed from the lower layer cells, // (Pl. IL. fig. 15), these cells being a continuous 
sheet, viz., the primary entoderm of the early two-layered blastoderm, and they become 
severed into two longitudinal masses, mes (Pl. II. figs. 2 and 11; also Pl. IV. figs. 5 and 
10), by the proliferation of epiblast, ep, which produces the medullary plate, or 
neurochord, ve. The sub-ectodermic mass, // (Pl. I. fig. 15), cannot be regarded as 
mesoblast until it is severed mesially—the mesoblast, when recognisable as such, is defined 
as two lateral plates, just as in Petromyzon (Calberla), Triton (Scott and Osborn), 
Elasmobranchs, and other forms. KinGsLey and Conn speak of this continuous sheet, at 
an early stage, but their figures are not decisive. Thus their fig. 25, to which they 
specially refer, as also figs. 26 and 27, show a massive dorsal plate, which must be the 
thickened epiblast, ¢.e., the neurochordal proliferation, and against it the notochord abuts 
below. The mesoblast must, in part, constitute the lateral plates, though the authors 
themselves do not so interpret their figures. This interpretation appears, im fact, 
irresistible, though it is not in agreement with the view stated in the text (No. 78, 
p- 200). Ryprer* records a peculiar condition in EHlecate, viz., a precocious metameric 
segmentation in the two parts of the rim which diverge from the posterior end of the 
trunk. This is very remarkable, for no such feature has been seen in any other form, 
while in those referred to in this paper, the posterior portion of the trunk, after the 
mesoblastic plates are defined anteriorly, shows no such differentiation, the three layers of 
the mid-region merging, in fact, in a mass of indifferent cells at the posterior termination 
(vide—prs, PI. IIL. fig. 12, and Pl. IV. figs. 5d and 5e). These two mesoblastic plates, as 
seen in section mes (Pl. LI. fig. 11), have above a thin covering of epiblast, ep, and inferiorly 
an insinuating layer of hypoblast, /yp, which separates the embryo from the yolk below. 
Anteriorly the mesoblast thins away, and in the otocystic region is reduced to a single 
layer of somewhat depressed cells, mes, between the hypoblast, hy, and the greatly 
enlarged neurochord, mo (Pl. IV. fig. 4). In OxrtiacueEr’s figures of the trout at this 
stage, the mesoblast is not so much reduced ; but its larger bulk is probably connected 
* Ryper’s view of the origin of the mesoblast is not clear; he apparently favours delamination with OELLACHER 
(op. cit., pp. 494-95), and hypoblastic proliferation (on p. 570); while on p. 501 he seems to suggest a sundering of the 
“lower layer” mass, such as is insisted upon above. 
