32 



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. j>) within the chorion (ib. he). 



The formation of the blastoderm accords with that of freshwater Gammari and ol 

 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 fii'st recognizable, as such, by jmrial grou2)s of these cells (cut, fig. 4) 

 on the surface of the protoderm (ib. ^w^; "amnion," Brandt). These groups, by their 

 gradational difference of size, indicate, what later development shows, that they are the 

 beginnings of the cephaletral Hmbs. Between and a little in advance of the smaller 

 pair a round speck appears, which denotes the mouth. Portions of yolk (ib. x, x) 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. 



Fis. 6. 



Embryo, with ccphaletron and thorac- 

 etron defined. (Pkd. pi. iv. fig. 10.) 



Embryo ju.st before batching, 

 (i-kd. pi. y. fig. 24.) 



Embryo at the same stage, from belo\y. 

 (Pkd. pi. iy. fig. 19 a.) 



Newly hatched young. 

 (Pkd. pi. V. fig. 25 a.) 



A similar heaping up of cells, as a ridge, marks out the hinrl margin of the cephal- 



