DEVELOPMENT AND LIFE-HISTORIES OF TELEOSTEAN FISHES. 715 
So long as the yolk-ball can be distinguished even in advanced embryonic stages 
(see Pl. VII. figs. 1, 9, &e., ep), it is provided with an envelope of unsegmented proto- 
plasm especially noticeable round the margin of the dise (per, PI. II. fig. 12), and forming 
in the early stages of cleavage a thickened peripheral belt. This envelope is the “ feuillet 
végétatif ou muqueux” of LEREBOULLET (No. 93, p. 771); the “trophic or glandular layer” 
of Remak (No. 135, p. 342); the “parablast” of Kier (No. 79, p. 116) and His (No. 
67); the ‘‘ Korner-zone” of Kuprrer (No. 88, p. 217, fig. 1); the “lamina mycogastralis” 
of Harcxen (No. 62); and the yolk-hypoblast of Ryprr (No. 141); but appropriately 
distinguished as the “ periblast”” by many authors. 
We may speak of the periblast as early as the stage of first cleavage, the two primary 
blastomeres constituting the germ proper as distinct from the protoplasmic layer beyond.* 
The distinction, it is true, is more apparent than real, for the protoplasm at the margin 
of the disc is in a state of continual transition, passing into the germ probably during the 
whole cleavage-process, the disc being indeed only a thickened portion of the proto- 
plasmic cortex of the ege,—‘‘a lenticular enlargement of the Rindenschicht,” as 
OELLACHER expresses it (No. 113). In thus regarding the periblast as an aggregation of 
protoplasm which lies outside the germ proper, because it has reached the animal pole too 
late to enter the dise and take part in cleavage, we adopt a theory of its origin which has 
been questioned by some observers, notably by Acassiz and Wurrman (No. 2). These 
observers suggest that the periblast is really a product of the blastoderm ; that, instead of 
being, as we have expressed it, too late to enter the disc, it has already formed part of 
that structure, and has been protruded as a germinal outgrowth all round the margin 
during segmentation. Van Bamseke, as if by anticipation, expressly opposes such a 
view, and says—“ It cannot originate from the dise ; it is coarsely granular, like the cortex 
(le manteau protoplasmique);” but he goes on to state that the cortex wholly disappears 
when the intermediary layer is formed, whereas the cortex persists very much longer, 
though so thin that, as he says, “it is difficult to detect” (No. 20a). 
It is not easy to controvert a view which denies the independent origin of the 
periblast, for its apparent extension outwards from the margin of the disc and the 
continuity of both would seem to favour it. But, if it be correct, then at one stage all 
the superficial protoplasm of the ovum must be collected into the germ-mass; and no 
such complete segregation has been observed—a stratum of cortical protoplasm continuous 
with the germ is always discernible up to the stage when the periblast can be distinctly 
recognised asa nucleated layer. Its extension beneath the disc is implied in the view here 
adopted, for the superficial protoplasm collects beneath the disc as elsewhere, and this 
can be observed by the behaviour of the oleaginous sphere in such an ovum as that of f. 
gurnardus, inasmuch as it passes along beneath the floor of the germinal cavity evidently 
prevented by the layer of continuous protoplasm from entering the chamber. Van 
BAMBEKE, it is true, questions this latter point, saying that at one time no trace of a 
* Krnester and Conn, in mentioning that complete furrows in segmentation pass downward to the vitelline 
globe, except the intermediary layer and peripheral cushion of Van BAMBEKE. We agree with this view. 
