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. I. 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 GOrrE 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 Hennecuy, Acasstz, 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 Kiyestry 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 Hennecuy’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 OrLLacnERr’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 Pl. IL. 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. 
Hennecuy, 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 Kinas_ey and Conn, while holding that 
the epiblast is really inflected as stated above, also regard the intermediary layer as 
adding cells to the invaginated hypoblast (No. 78, p. 209). The inflected cells creep up 
