418 ZOOLOGY. 



enemies they do not lose much ground in the struggle for 

 life. The oviparous forms such as certain sharks, skates, 

 and ChirtKBra, lay large eggs enclosed in tough, leathery, 

 purse-shaped cases. The other Elasmobranchiates are vivip- 

 arous, bringing forth their young alive. In Mustelus and 

 Garcharias a rudimentary "placenta" analogous to that of 

 Mammals is developed from the yolk. The following ac- 

 count of the development of the dog-fish {Mustelus), which 

 is condensed from Balfour, may be found to be applicable to 

 sharks in general : 



The blastoderm or germinal disk is a large round spot 

 darker than the rest of the yolk, bordered by a dark line 

 (really a shallow groove). Segmentation occurs much as de- 

 scribed in the bony fishes, reptiles, and birds. The upper 

 germ-layer (epiblast) arises much as in the bony fishes, the 

 Batrachians and birds, while the two inner germ-layers are 

 not clearly indicated until a considerably later stage. The 

 segmentation-cavity is formed nearly as in the bony fishes. 

 There is no invagination of the outer germ-layer to form the 

 primitive digestive cavity, as in Amphioxus, the lamprey, 

 , sturgeons, and Batrachians, but the Selachians agree with 

 the bony fishes, the reptiles, and birds, in having the alimen- 

 tary canal formed by an infolding of the innermost germ- 

 layer, the digestive track remaining in communication with 

 the yolk for the greater part of embryonic life by an 

 umbilical canal. This mode of origin of the digestive cav- 

 ity, Balfour regards as secondary and adaptive, no "gas- 

 trula" (Hseckel) being formed as in Amphioxus, etc. The 

 embryo now rises up as a distinct body from the blastoderm, 

 just as in other Vertebrates, and there is a medullary groove 

 along the middle line, and by the time this has appeared the 

 middle and inner germ-layers are clearly indicated. After 

 this development continues in much the same manner as in 

 the chick. 



At this time the embryo dog-fish externally resembles the 

 trout ; the chief difEerence is an internal one, the outer 

 germ-layer not being divided into a nervous and epidermal 

 sublayer as in the bony fishes. 



