4 Mr. E. E. Prince on the Significance of the 



ates into the yolk below is not surprising, for its protoplasm is 

 continually in transit^. 



Now the yolk in the Amphibian ovum becomes divided by 

 cleavage into large nucleated yolk-cells, just as in the Qg^ of 

 Petromyzon (PI. II. fig. 2, y), and enters more or less inti- 

 mately into the formation of the embryo. The ventral lining 

 of the mesenteron is really yolk-hypoblast, and arises directly 

 from the yolk-cells proper, as Mr. Shipley shows in Petro- 

 myzon : the dorsal wall '' is composed of columnar cells re- 

 sembling those of the general epiblast ; the cells forming the 

 floor have the same characters as the yolk-cells " * (PI. II. 

 fig. 7,3/). Nothing like this is seen in the Teleostean egg, 

 though Mr. Brook, relinquishing the view referred to on a 

 prior page, has adopted the conception that Teleostean and 

 Amphibian ova are similar even in the details of their deve- 

 lopment " the derivatives of the animal and vegetative poles 

 are in both cases practically identical." If the mesenteron 

 in Osseous Fishes does not arise as a slit in the thickened 

 median hypoblast, as the greater part of it really seems to do, 

 but is largely built up out of nucleated periblast, as Mr. Cun- 

 ningham has suggested +, the yolk is still not directly con- 

 cerned in the process, the periblast being, as Klein says, a 

 continuation of the germ, both are " one and the same sub- 

 stance "J. Kupffer^s vesicle, which arises as a sub-embryonic 

 chamber, is not ventrally limited by the yolk, but by the 

 periblast. Throughout the embryonic period in Teleostean s 

 the periblast intervenes as a continuous layer between the 

 yolk and the germ (as shown in PI. II. tigs. 7 and 11, c.p.). 

 Oellacher speaks of the germ as feeding on the yolk §, and 

 Kingsley and Conn say that particles of yolk seem to be taken 

 in after segmentation has begun ||, while Klein expresses the 

 view, which Mr. Brook adopts, that the periblast performs the 

 digestive function, so that, as the last-named author says, 

 " large masses of yolk are incorporated within its substance 

 and assimilated "Tj. The formation of the disk and early pro- 

 toplasmic cortex is due, it is granted, to a kind of physical 

 transference, mainly superficial segregation : At what point, 

 it may be asked, does such segregation cease and digestion 

 begin ? No such point can be determined. The yolk, in fact, 

 does not diminish to such an extent as the theory of digestion 

 plus segregation would imply, as we see by comparing the 



* Quart. Journ. Microsc. Sci., Jan. 1887, p. 329. 



t Ibid., Jan. 1885, p. 7, and Nov. 1885, pp. 20, 21. 



X Ibid. vol. xvi. 187(3, p. 118. 



§ Zeitschr. f. Wiss. Zcol. Bd. xxii. p. 4. || Loc. cit. p. 127. 



«i[ ' Report of Fishery Board for Scotland,' 1885, p. 35. 



