DEVELOPMENT AND LIFE-HISTORIES OF TELEOSTEAN FISHES. 725 



as a single layer, except at the margin where they are heaped together (PI. II. fig. 10), # 

 are very much flattened towards the animal pole, and merge with the cells from other 

 parts of the rim. The effect of this union (especially where the cells from the rest 

 of the rim meet the cells of the scutum as it proceeds towards the same pole, as well 

 as laterally) is, that the original very definite outline of the shield becomes irregular, 

 and finally almost wholly disappears. The rim, however, does not vanish with the 

 appearance of the carina, as Kupffer and Van Bambeke hold, nor are the two structures 

 really so intimately connected as is often supposed. The rim continues even after the 

 alar expansion of the scutum, for the reason just stated, is no longer visible. The shield, 

 in fact, exists before invagination of the hypoblast, if by the shield be really meant the 

 embryonic thickening, and not merely a visible scutiform appearance; but it passes 

 insensibly away on all sides, save posteriorly. The invagination-cells do not so much 

 produce the shield or carina as make both optically visible.t 



The ectodermal and periblastic cells, which are inflected, result in the establish- 

 ment of a single layer of flattened cells — a sheet, in fact, of continuous hypoblast, 

 which, as Haeckel held (No. 63, p. 91), limits ventrally the embryonic lamella. It 

 separates the carina from the yolk, save in the caudal region, where sections even more 

 than the study of the living ovum indicate the special activity which centres there. 

 It is noteworthy that the rim does not contain any mesoblastic cells, as in Rana, the 

 Teleostean resembling the Cyclostome (Petromyzon) in this feature. In the region of 

 the scutum the hypoblast, of course, includes in its fold lower layer cells, but their 

 significance at this time is indifferent. This view, we think, explains satisfactorily the 

 origin of the primary rim, the thickening of the blastoderm, the extension of both, the 

 definition of the embryonic scutum, and its subsequent gradual disappearance. At any 

 rate, it is difficult to explain these phenomena by any process of delamination such as 

 that of Oellacher, Kyder, and others : differentiation in situ of the lowest stratum of 

 the primary entoderm would hardly produce the definitely-bounded thickening, and the 

 centripetal progress of the same. The whole appearance and behaviour of the cells of 

 the rim in the very transparent blastoderms here considered, strongly suggests invagina- 

 tion rather than delamination. Oellacher's figures (No. 114, Taf. i. and Taf. ii.), it is 

 true, as strongly indicate delamination, though figs. 2 and 3, Taf. i. might represent an 

 inflection of the lowest layer. At a later stage, when Oellacher recognises a definite 

 " unteres Keimblatt," the cells are rounder and larger than the superjacent cells, a 

 condition quite the reverse of that which obtains in the Gadoids. It would appear as 

 if the character of the constituent cells of the hypoblast in these groups were not only 

 thus unlike, but that in its mode of origin very marked differences also existed. Mr 



* This centripetal passage of cells, there can be little doubt, is of profound ancestral significance; it can be no less 

 than " a real survival of the hypoblast cells to grow inwards during the process of involution " (Balfour, he. cit., p. 530). 



t The curious notions of Oellacher (vide No. 113, pp. 21, 40) respecting the various shapes assumed by the scutum 

 at different stages, do not seem to be borne out by study of Gadoid and other forms ; and the opinion formerly expressed 

 by one of us (No. 123), that the shield shows differences in outline, characteristic of different species, also needs 

 modification. 



