48 FORMATION OF THE LAYERS. 



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 

 hiyer cells, and the lower layer cells show no trace of a divi- 

 sion into mesoblast and hypoblast. In the axis of the embryo, 

 however, 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. 

 VI. 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. iv. 

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

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

 nal 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 alimentary 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 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 continuous con- 

 version of lower layer cells into hypoblast along a line leading 

 towards the centre of the blastoderm. 



In the parts of the rim adjoining the embryo, the lower layer- 

 cells, on becoming continuous with the epiblast cells, assume a 

 columnar form. At the sides of the rim this is not strictly the 

 case, and the lower layer cells retain their rounded form, though 



