724 PROFESSOR W. C. M'INTOSH AND MR E. E. PRINCE ON 



in situ, and partly by invagination ; in Cyclostomes and Amphibians it is in all likelihood 

 invagination purely, and the prevailing view, that Teleosteans illustrate this latter process 

 also, is probably true. In a section of an early blastoderm (PI. II. fig. 15, a) the infold- 

 ing has apparently begun at one point, but the cells of the single stratum — becoming 

 crowded together — lie over each other so as to produce a multi-layered appearance (hyp). 

 The layer inflected is, however, the outer or corneous layer, as Gotte holds, and this 

 point is of some importance, for many authorities who favour the invagination-theory, 

 differ as to the layer that undergoes inflection. Thus Henneguy, Agassiz, Whitman, 

 and others, though holding strongly to invagination, declare that the outer layer is not 

 concerned in the process — a linear fissure, it is maintained, wholly separating the lower 

 or sensory epiblast from the outermost layer, the latter indeed ceasing at a certain 

 distance from the margin. That the outer or corneous layer alone is inflected is the view 

 of Kingsley and Conn (No. 78, p. 201) and others. Teleostean blastoderms are 

 particularly unfavourable for deciding critical points such as this, the cells of the various 

 layers being almost destitute of those peculiar distinctive features shown in many other 

 groups, and an element of uncertainty must necessarily be connected with such a point 

 as this. So far as Henneguy's view (No. 64, pp. 402-3) depends upon observations 

 on the living ovum, it cannot be relied on, for this point must be determined by sections. 

 If Oellacher's well-known figures be referred to, we find in very early blastoderms that 

 not only is the epiblast shown extending quite up to the periphery, but the flattened 

 cells pass beyond the limits on to the surface of the yolk (No. 114, cf. figs. 4, 5, 6, 

 Taf. i.); but such an extension beyond the margin of the blastoderm does not take place 

 in the ova dealt with here, though the limits of the germ in section are difficult to 

 distinguish, save in such a section as PI. II. fig. 15, a. 



In the living egg a fissure certainly can be distinctly made out, but it apparently 

 ceases before the margin is reached. Optical considerations, again, would favour this. 

 Henneguy, however, also urges that even in sections this point may be wrongly inter- 

 preted, as chromic acid preparations show the same appearance as that we have just re- 

 ferred to, and the obliteration of the fissure he attributes to the reagent. The view 

 has been suggested (No. 122, p. 449), that while the process is one of invagination, it is 

 more than that, since it embraces also a species of budding, such as Lereboullet alludes 

 to (No. 94, p. 253), cells segmented from the periblast being added to the blastodermic 

 margin, and folded in along with ectodermal cells. This vegetation of periblastic cells 

 will probably be most active along the posterior edge of the scutum, but no evidence of 

 this is indicated until a later stage. The entire rim is thus a region where peculiarly 

 complex processes are going on, for not only is the outer edge continuously progressing 

 towards the vegetal pole, but the inner edge is also advancing towards the opposite 

 pole, and this is rendered possible by the combined inflection of epiblast-cells, and the 

 inclusion of periblast-elements. It appears that Kingsley and Conn, while holding that 

 the epiblast is really inflected as stated above, also regard the intermediary layer as 

 adding cells to the invaginatecl hypoblast (No. 78, p. 209). The inflected cells creep up 



