418 ZOOLOGY. 
enemies they do not lose much ground in the struggle for 
life. The oviparous forms such as certain sharks, skates, 
and Chimera, lay large eggs enclosed in tough, leathery, 
purse-shaped cases. ‘The other Hlasmobranchiates are vivip- 
arous, bringing forth their young alive. In Mustelus and 
Carcharias 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” (Heckel) 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 difference is an internal one, the outer 
germ-layer not being divided into a nervous and epidermal 
sublayer as in the bony fishes. 
