OF THE AMERICAN KING-CRAB. 489 
the hyaline nucleolus, a. Round this is the mass of germ-yolk, c, in which are recog- 
nizable granules round other centres definable as yolk-cells, d. Impregnation of such 
ovum is followed by denser blastodermal aggregates (cut, fig. 2, c). Peripheral differen- 
tiation and condensation next define upon the blastoderm (e, cut, fig. 3) a protoderm 
(ib. p) within the chorion (ib. Ac). 
The formation of the blastoderm accords with that of freshwater Gammari and of 
Arachnids, the yolk not undergoing segmentation. It is not uniformly diffused, but the 
seat of development is localized in an aggregate of more numerous and smaller blasto- 
dermal cells (ib. e). 
The embryo is first recognizable, as such, by parial groups of these cells (cut, fig. 4) 
on the surface of the protoderm (ib. pm; “amnion,” Brandt) These groups, by their 
gradational difference of size, indicate, what later development shows, that they are the 
beginnings of the cephaletral limbs. Between and a little in advance of the smaller 
pair à round speck appears, which denotes the mouth. Portions of yolk (ib. z, z) are 
detached before the formation of the blastodermic skin. This advance takes from one to 
two weeks after exclusion under ordinary favourable influences, and is accompanied 
by secession of the protoderm. 
Fig. 5. 
Embryo, with me and thorac- ` 
etron defined, (Pkd. pl. iv. fig. 19.) Embryo at the same stage, from below. 
(Pkd. pl. iv. fig. 19 4.) 
Embryo just before hatching. 
(Pkd. pl. v. fig. 24.) Newly hatched young. 
(Pkd. pl. v. fig. 25 a.) 
A similar heaping up of cells, as a ridge, marks out the hind margin of the cephal- 
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