DEVELOPMENT AND LIFE-HISTORIES OF TELEOSTEAN FISHES. 715 



So long as the yolk-ball can be distinguished even in advanced embryonic stages 

 (see PI. VII. figs. 1, 9, &c, cp), it is provided with an envelope of unsegmented proto- 

 plasm especially noticeable round the margin of the disc (per, PI. II. fig. 12), and forming 

 in the early stages of cleavage a thickened peripheral belt. This envelope is the " feuillet 

 vegetatif ou muqueux" of Lereboullet (No. 93, p. 771); the "trophic or glandular layer" 

 of Remak (No. 135, p. 342) ; the "parablast" of Klein (No. 79, p. 116) and His (No. 

 67); the " Korner-zone" of Kupffer (No. 88, p. 217, fig. 1) ; the " lamina mycogastralis" 

 of Haeckel (No. 62); and the yolk-hypoblast of Ryder, (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 egg, — " 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 disc and take part in cleavage, we adopt a theory of its origin which has 

 been questioned by some observers, notably by Agassiz and Whitman (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 Bambeke, as if by anticipation, expressly opposes such a 

 view, and says — " It cannot originate from the disc ; 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 as a 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 T. 

 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 



* Kingslet 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. 



