FORMATION OF THE LAYERS. 261 



the rest of the germ, but by the conversion of the lower layer 

 cells into hypoblast having been carried far further towards the 

 centre of the germ in the axial line than in the lateral regions of 

 the rim. 



The most anterior of the series of transverse sections (PI. 7, 

 fig. 8^:) I have represented, is especially instructive with reference 

 to this point. Though the embryonic rim is cut through at 

 the sides of the section, yet in these parts the rim consists 

 of hardly more than a continuity between epiblast and lower 

 layer cells, and the lower layer cells shew no trace of a division 

 into mesoblast and hypoblast. In the axis of the embryo, how- 

 ever, the columnar hypoblast is quite distinct ; and on it a small 

 cap of mesoblast is seen on each side of the medullary groove. 

 Had the embryonic rim resulted from a projecting growth of the 

 blastoderm, such a condition could not have existed. It might 

 have been possible to find the hypoblast formed at the sides 

 of the section and not at the centre ; but the reverse, as in these 

 sections, could not have occurred. Indeed it is scarcely necessary 

 to have recourse to sections to prove that the growth of the 

 embryonic rim is towards the centre of the blastoderm. The 

 inspection of, a surface view of a blastoderm at this period 

 demonstrates it beyond a doubt (PI. 8, fig. B). The embryo, 

 close to which the embryonic rim is alone largely developed, 

 does not project outwards beyond the edge of the germ, but 

 inwards towards its centre. 



The space between the embryonic rim and the yolk (PI. 7, 

 fig. 7 #/.) is the alimentary cavity. The roof of this is therefore 

 primitively formed of hypoblast and the floor of yolk. The ex- 

 ternal opening of this space at the edge of the blastoderm is the 

 exact morphological homologue of the anus of Rusconi, or 

 blastopore of Amphioxus, the Amphibians, &c. The importance 

 of the mode of growth in the embryonic rim depends upon the 

 homology of the cavity between it and the yolk, with the alimen- 

 tary cavity of Amphioxus and Amphibians. Since this homology 

 exists, the direction of the growth of this cavity ought to be, 

 as it in fact is, the same as in Amphioxus, etc., viz. towards the 

 centre of the germ and original position of the segmentation 

 cavity. Thus though a true invagination is not present as in 

 the other cases, yet this is represented in Elasmobranchs by the 



