FORMATION OF THE LAYERS. 259 



to shew that in them, the roof of the segmentation cavity is 

 formed alone of epiblast ; but on account of the great difficulty 

 which is experienced in distinguishing the layers in the blasto- 

 derms of these animals, I still hesitate to accept as conclusive 

 the testimony on this point. 



In the formation a second time of a cellular floor for the 

 segmentation cavity in the third stage, the Elasmobranch embryo 

 seems to resemble that of the Osseous Fish 1 . Upon this feature 

 great stress is laid both by Dr Gotte 2 and Prof. Haeckel 3 : but I 

 am unable to agree with the interpretation of it offered by them. 

 Both Dr Gotte and Prof. Haeckel regard the formation of this 

 floor as part of an involution to which the lower layer cells owe 

 their origin, and consider the involution an equivalent to the 

 alimentary involution of Batrachians, Amphioxus, &c. To this 

 question I hope to return, but it may be pointed out that my 

 observations prove that this view can only be true in a very 

 modified sense ; since the invagination by which hypoblast and 

 alimentary canal are formed in Amphioxus is represented in 

 Elasmobranchs by a structure quite separate from the ingrowth 

 of cells to form the floor of the segmentation cavity. 



The eventual obliteration of the segmentation cavity by cells 

 derived from the yolk is to be regarded as an inherited remnant 

 of the involution by which this obliteration was primitively 

 effected. The passage upwards of cells from the yolk, may 

 possibly be a real survival of the tendency of the hypoblast cells 

 to grow inwards during the process of involution. 



The last feature of the segmentation cavity which deserves 

 notice is its excentric position. It is from the first situated in 

 much closer proximity to the non-embryonic than to the embry- 

 onic end of the blastoderm. This peculiarity in position is also 

 characteristic of the segmentation cavity of Osseous Fishes, as is 

 shewn by the concordant observations of Oellacher 4 and Gotte 5 . 

 Its meaning becomes at once intelligible by referring to the 

 diagrams in my paper 6 on the Early Stages in the Development 

 of Vertebrates. It in fact arises from the asymmetrical character 



1 This floor appears in most Osseous Fish to be only partially formed. Vide 

 Gotte, loc. at. 



2 Loc. cit. 3 Loc. cit. 4 Loc. cit. 

 5 Loc. cit. r > Loc. cit. 



