EMBRYONIC nEVELOPMENT OF THE TELEOSTEANS. 53 



retains its characters and remains undivided. The fact of a greater 

 indisposition to segment on the part of the cells destined to give 

 rise to the endoderm is very frequent, not only among the Verte- 

 brates but also in the other divisions of the animal kingdom. It is 

 exhibited always in the case of the formation of agastrula by epi- 

 boly ; and, indeed, it is after this mode that the develoj)ment of the 

 gastrula of tlie Teleosteans is carried out. The " discogastrula''^ 

 has, then, no existence as the result of a special mode of gastrula- 

 tion ; " discoidal segmentation" differs in no essential respect 

 from the ^'inequal segmentation''^(Inequale llirchung) of Haeckel. 

 An objection to these views may perhaps be made on the score 

 of the peculiar mode of formation of the cells of the interme- 

 diate layer, which do not result from segmentation. I may, in 

 reply, call to mind that Strasburger (19) has demonstrated that 

 the endogenous generation of cells is not a primordial form of 

 cell-generation but a secondary mode derived from division pure 

 and simple. It is connected to division by a complete series of 

 intermediate forms. The simultaneous formation by the endo- 

 genous mode of a great number of cells in a single cell is an 

 abbreviation or condensation of successive divisions which have 

 become less and less complete, It is clear that the formation of 

 the blastoderm of Insects nmst be interi)reted in this manner, as 

 well as the existence in the group of the Arthropods, chiefly 

 among the Crustacea, of a series of transitional forms. This 

 view receives in the present case a new application ; the formation 

 of the endoderm in the Osseous Fish has the same relation to the 

 development of the layers by regular segmentation, such as is 

 presented, for instance, by the Acrania, by the Cyclostoma and 

 Batrachia, which the formation of the blastoderm in insects has 

 to the total cleavage observed in many Crustacea. 



II. On comparing my observations on the constitution and 

 development of the intermediate layer with those of my prede- 

 cessors I find that Kupffer is the only one who has recognised 

 and described in Osseous Fishes the development of cells on the 

 surface of the deutoplasmic globe, beyond the area of the seg- 

 mented germ. The "nuclear zone" of Kupffer, observed in 

 the genera Gasteroseus and Spinachia evidently corresponds 

 to that part of my intermediate layer which lies outside the blas- 

 todisc. The part of this layer situated under the blastodisc cor- 

 responds to the '' membrane formed of vitelline gramdes" to the 

 " membrane underli/ing the blastoderm,'^ or to the " mucous layer" 

 of Lereboullet. I find in it the essential characters attributed by 

 Van Bambeke to his intermediate layer. The peripheral welt, tri- 

 angular in section, on which rests the margin of the blasto- 

 disc, was perfectly described by Van Bambeke. As to the 



