494 FISHES. 



wards share in the making of the embryo. On the other 

 hand, it has been said that they do nothing, even that they 

 are the heads of numerous spermatozoa which have suc- 

 ceeded in entering the ovum. 



At the close of segmentation the blastoderm is a lens- 

 shaped disc with two strata of cells. It is thicker at one 

 end where the embryo begins to be formed. Towards the 

 other end, between the blastoderm and the yolk, lies a seg- 

 mentation cavity (Fig. 164, sg.c.). 



At the embryonic end, the outer layer or epiblast under- 

 goes a slight invagination (Fig. 164, x.), beginning to form 

 the roof of the future gut (^.), in other words establishing 

 the hypoblast. This inflected arc of the blastoderm corres- 

 ponds to the blastopore or mouth of the gastrula, which is 



sg.c. 



FIG. 164. Elasmobranch development. (After BALFOUR.) 



Uppermost figure shows blastoderm at an early stage. Ep., 

 epiblast ; sg-.c., segmentation cavity ; n., yolk nuclei. 



Middle figure shows the invagination which forms the gut. x., 

 blastopore; g. , archenteron. Mesoderm dark. 



Lowest figure, a longitudinal section at a later stage. Ep. , 

 epiblast; n.c., neural canal ; ne.c., neurenteric canal ; g., gut ; ., 

 notochord. Mesoderm dark. 



much disguised by the presence of a large quantity of yolk. 

 As the invagination proceeds, the segmentation cavity is 

 obliterated. The floor of the gut is formed by infolding of 

 the lateral walls. 



Along the mid dorsal line of the epiblast a medullary 

 groove appears the beginning of the central nervous 

 system. Its sides afterwards arch towards one another, and 

 meet to form a medullary canal (Fig. 164, n.c.}. A posterior 



