Yolk in the Eggs of Osseous Fishes. 3 



of an extra-blastodermic layer of protoplasm (figs. 1, 9, and 

 10, c.p. and perih., PL II.) must not be ignored. The very 

 fact, however, that such an area or periblastic ring exists 

 supports the view here propounded. If the protoplasm inter- 

 fused amongst the yolk becomes, by a physical process of 

 separation and superficial transference, concentrated at the 

 animal pole, as represented in the diagram fig. 9, PI. II., it is 

 easy to see that some of it may be left at the margin as a 

 peripheral ring. The process is slow, and much protoplasm 

 may continue to pass towards the animal pole, even after the 

 germinal disk is defined and segmentation is in progress. 

 Such, in fact, is the case, and this is the explanation of the 

 extra-germinal area, appropriately called periblast. Mr. G. 

 Brook aptly expressed the condition of this area when he said * 

 that the germinal protoplasm is for the most part included 

 in the first two cells of the blastodisk, and, " as if not to waste 

 any material, the remainder collects around this disk and is 

 afterwards developed into the periblast." Further away 

 from the disk the periblast (figs. 9 and 10, perih., PI. IE.) 

 thins out and gradually passes into a filmy protoplasmic 

 layer, uniformly investing the remaining surface of the 

 yolk and known as the cortical layer (PI. II. figs. 9 and 10, 

 per lb.). 



Kingsley and Conn afiirm f that in the earliest stages 

 the periblast is not present ; and, paradoxical as it may seem, 

 they are right, for the periblast, as such, does not exist until a 

 later period — until, in fact, the limits of the disk are indicated 

 with some precision by the progress of segmentation (compare 

 figs. 1 and 9, PL II.). The protoplasmic cortex, of which 

 the periblast forms merely a thickened annular portion, is 

 really present from the moment that superficial segregation 

 begins, and so long as the process continues the cortical layer 

 persists, and even in advanced embryos it is distinguishable, 

 passing beneath the embryonic trunk, between the hypoblast 

 and the remnant of the yolk (PL II. fig. 11, c.p.). Segrega- 

 tion is not only superficial but, as stated elsewhere :j:, tliere is 

 also a subgerminal transference, and Mr. Brook has shown § 

 that in Clupea these deep-seated tracts form definite ramifica- 

 tions amongst the yolk. The periblast is simply germinal 

 matter which has not yet entered the disk, and that it gradu- 



* Quart. Journ. Microsc. Sci., Jan. 1885, p. 4. 



t Mem. Boston Soc. Nat. Hist., April 1883, p. 202. 



X " Develop, of the Food-Fishes," Ann. & Mag. Nat. Hist. 1886, vol. 

 xvii. p 447. 



§ ' Fourth Annual Report of Fishery Board for Scotland,' 1885, App. F, 

 no. i. pp. 34, 35. 



1* 



