EMBRYONIC DEVELOPMENT OF THE TELEOSTEANS. 49 



ceding stage consists in the differentiation of the superficial cells. 

 These have become flattened, and form a sort of simple pavement 

 epithelium, which limits the blastodisc externally. In section 

 these cells appear lenticular, their external face is nearly plane, 

 their deep face is regularly convex or presents facettes, by which 

 these cells are moulded to the subjacent elements. The deepest 

 cells present nothing particular. 



The blastodisc has then divided itself by delamination into an 

 enveloping lamella (Umhiillungshaut of Reichert (7), epidermoid 

 layer of Vogt (8) and LerebouUet, Deckschicht of Golte (9), 

 and a deeper mass, which is the primitive external layer. 



The intermediate layer has not changed ; the only peculiarity 

 which I observed, was that in optical section, there may be dis- 

 tinguished, both in the peripheral welt and in the median lens, 

 certain rounded cells which appear to be definitely individualised, 

 though immersed in the protoplasmof theintermediate layer (fig-7). 

 I believe that these cells form the origin of a layer, which in the 

 next stage will be found to have made its appearance between 

 the blastodisc and the intermediate layer. 



To the stage just described succeeds that represented in fig. 6. 

 The eggs of this mass, which in the morning about ten o'clock 

 presented the characters above indicated (figs. 4 and 5), had 

 arrived in the afternoon about three o'clock at the stage which I 

 am about to describe. 



The blastodisc has extended considerably, and forms now a cap 

 fitted to the truncated portion of the deutoplasmic globe, which 

 has become slightly convex. The blastodisc has become flattened 

 out and notably thinner. Between it and the intermediate layer 

 has appeared an eccentrically placed cavity, and in the disc itself 

 we can distinguish two regions : the one, central and thinner, 

 forms the roof of the germinal cavity ; the other, peripheral and 

 thicker, forms a marginal welt to the blastodisc. This welt is in 

 the form of a ring, broader at one side than the other, and thus 

 brings about the eccentricity of the cavity. 



The cellular constitution of the blastodisc has become notably 

 modified, and first of all it is observable that the cells have mul- 

 tiplied and become much more voluminous. We may distinguish 

 in the disc (assigning to it all which is placed out of the region 

 of the intermediate layer, properly so- called) : 



1st. An enveloping lamella formed by a single layer of flat 

 cells. 



2nd. A layer constituted by polyhedric cells, pressed one 

 against the other, clear and transparent, with a nearly homo- 

 geneous protoplasm and nuclei devoid of nucleoli. This layer 

 forms the greater part of the blastodisc; it has its maximum 



VOL. XVII. NEW SER. D 



