EMBRYONIC DEVELOPMENT OF THE TELEOSTEANS. 51 



observations, with the fullest evidence, is this : — That the germ- 

 layers do not proceed exclusively from tlie segmented germ^ that 

 part which at the conclusion of the cleavage-process forms what 

 Lereboullet calls the blastoderm, and what I have denominated, 

 with Haeckel, the hlastodisc. A part of the germ-layers is derived 

 directly from a protoplasmic layer which covers in the deuto- 

 plasmic globe,and takes no part whatever in segmentation. It is this 

 layer which Lereboullet designated under the name of ^'membrane 

 sousjacente au germe,'^ or at other times ''feidllet muquenx.^* 

 It was called "vitelline membrane" by CEllacher (10), " Hinden- 

 schicht" by His (11), "membrane iniermediaire'" by Van Bam- 

 beke. It does not contain before the segmentation of the 

 hlastodisc any trace of cellular elements nor of nuclei; there 

 develops in it, towards the conclusion of the segmentation period, 

 by endogenous generation, a very large number of cells, and 

 their mode of formation recalls in every respect the well-known 

 phenomena of the formation of the blastoderm in Insects. 



This intermediate layer was not recognised by Haeckel, which 

 leads him to declare, " so konnen nur die Furchungszellen einzig 

 und allein die Grundlage des enstehenden I'ischkorpers bilden," 

 and he attributes the formation of the endoderm to the invagina- 

 tion of the margin of the hlastodisc. Is it possible to admit that 

 this layer is absent in the eggs which Hatckel had under his eyes, 

 and that the development of certain layers is brought about by 

 invagination of the margin of ihe hlastodisc in some Osseous 

 Fish, and, on the other hand, in others at the expense of a special 

 layer, which takes no part in the segmentation ? I think not, 

 when the identity of the constitution of the eggs is admitted, 

 and I imagine that Haeckel himself will not feel tempted to 

 adopt such a supposition. There are, moreover, several par- 

 ticulars in the drawings published by Haeckel which appear to 

 me to prove that the intermediate layer existed in his eggs. In 

 figures 55 and 56 of his pi. iv Haeckel figures the segmenta- 

 tion-spheres prolonging themselves outwards by means of a sort 

 of tail applied to the surface of the albuminous globe (Eiweiss- 

 kiigei). If these figures are compared with figure 1 of my plate, 

 it will be recognised that this tail corresponds to that part of the 

 intermediate layer which extends outwards beyond the margin 

 of the hlastodisc. 



This first conclusion from my observations gives support to 

 the results arrived at and maintained in reference to various 

 Osseous Fishes by Lereboullet, by Kupfi'er, by Owsjannikow(12), 

 by Klein (13), by His, and by Van Bambeke. 



I am, however very far from wishing to give any support to 

 the famous " Parablast-theory" of Professor His, which is 

 defended in so far as Osseous Fish are concerned by Owsjannikow. 



