528 



PISCES FISHES. 



the rocks ; that of a neighbour species (C. gateatiis} has reduced spirals 

 ending in a couple of tendrils, which may be 90 in. in length, and 

 serve very effectively to entangle the egg among sea- weed. 



The segmentation is meroblastic, being confined to the disc 

 of formative protoplasm. From the edge of the blastoderm, 



or segmented area, some 

 nuclei (so-called " mero- 

 cytes ") are formed in the 

 outer part of the subjacent 

 yolk (Fig. 255, .). It 

 seems most probable that 

 these are hypoblast ele- 

 ments which assist in the 

 preparation of the yolk 

 for absorption, and event- 

 ually degenerate in the 

 empty external yolk-sac. 



At the close of seg- 

 mentation 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 segmentation 



si 



ys 



FIG. 256. Embryo dogfish in egg-case 

 ( " Mermaid's purse") which has been 

 cut open to show contents. 



e.g., External gills; d.f., dorsal fin fold; 

 y.s., yolk-sac; s/., stalk of yolk-sac; T., 

 tendrils, prolongations of egg-case by 

 means of which it is moored to sea-weed, 

 an "adaptation " in a dead substance. 



cavity (Fig. 255, sg.c.). 



At the embryonic end 

 the outer layer or epiblast 

 undergoes a slight invagi- 

 nation (Fig. 255, x.\ be- 

 ginning to form the roof 

 of the future gut (g.) ; in other words, establishing the hypo- 

 blast. This inflected arc of the blastoderm corresponds to the 

 blastopore or mouth of the gastrula, which is much disguised 

 by the presence of a large quantity of yolk. As the invagina- 

 tion 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 



