( 804 ) 



concentrates, the superficial layer is still more clearly visible as a 

 definite enveloping layer of cells. It is just the synchronic differen- 

 tiation of the superficial layer, which shuts off the blastoderm from the 

 surrounding medium and is the only way by which the developing 

 cells may get the oxygenium from the perivitelline fluid, on one side, 

 and of the periblast, by means of which the blastoderm is nourished 

 by the yolk, on the other side, which seems to me to be important ; 

 by this synchronic differentiation a new phase in the developmental 

 process is initiated, and the series of changes have begun that lead 

 to gast filiation. 



Very soon the blastoderm-disc flattens, at first only because the 

 superficial layer contracts a little, and the blastoderm sinks a little 

 into the yolk-sphere (fig. 8) but after that because the blastodisc 

 itself spreads out, flattens (fig. 9). The cells come closer together, 

 and soon the unilateral thickening that forms the first outwardly 

 recogni>able beginning of the building of the embryo, becomes visible. 



During these changes it is of no account whether a blastula-cavity 

 is formed, or not. As I have described elsewhere, in different murae- 

 noids during this stage a distinct blastula-cavity is formed, which may 

 be seen in the living egg. Afterwards follows the flattening of the 

 blastodisc and the disappearance of the cavity as such. The closer 

 study of young stages of the eggs of muraena N u . 7 ') showed me 

 however, that in these eggs no blastula-cavity is formed, and that 

 in this case the blastoderm, that takes just the same conical shape 

 as the hollow blastoderm in the other muraenoid eggs, remains solid 

 and is built up of a ma>s of loosely arranged cells. The further 

 development is the same as in the other series (c.f. fig. 1 — 3 on 

 plate 1). 



This flattening of the blastodisc. following on the stage just described, 

 coinciding with the concentration of the cells of the blastoderm 

 tow anls the side where in later stages the embryo is formed, and 

 coming before the invagination (and partial delamination) of the 

 blastoderm cells, that leads to the formation of the chorda and the 

 mesodermic plates, is already a part of the gastrulation process and 

 must be compared with the "cli\ age gastruléen'" of the amphibian egg. 



Immediately on this "cÜvage gastruléen" follows the formation of 

 the prostomal thickening (that is the "blastopore reel" of Bkachetj, 

 there where the superficial layer or pavement layer is connected 

 with the periblast, out of the surperficial cells of the periblast 2 ) (c.f. 



! ) Gomp. Petrus Camper, Vol. II p. 150. 



2 ) Sumner (1. c. page 145) saw evidences for this mode of origin in the egg of 

 Salvelinus, but not in that of Noturus or Schilbeodes. On these two forms I can* 



