II.] THE CRAYFISH AND LOBSTER. 187 



the ventral surfaces of the abdominal somites and bases of 

 the last pair of swimmerets of the female. A Lobster with 

 eggs thus attached, is said by the fishermen to be 'in berry.' 

 The segmenting egg of the Crayfish differs in some im- 

 portant respects from that of the Frog. The yolk material 

 is aggregated in the central protoplasm, and not at one pole 

 as in the case of the latter animal, and segmentation is 

 restricted to the superficial least yolk-laden protoplasm ; 

 the egg is at no period completely cleft into two parts, 

 segmentation is from first to last partial or merohlastiq a 

 cellular investment being as it were formed around a central 

 yolk-bearing mass. The investing cells become invaginated 

 at one point to form a small sac, which remains for a time 

 in open communication with the exterior. The embryo at 

 this stage is to be resolved into a double-walled sac or gas- 

 triila^ the interspace between the two walls being filled with 

 yolk. The outer layer or epiblast eventually gives rise to 

 the epidermis and its derivatives — the nervous system and 

 sensiferous epithelia; the inner one or hypoblast forms the 

 lining membrane of the mid-gut, and from it the digestive 

 gland is formed later as a paired outgrowth — it is from first 

 to last the true digestive epithelium. The cavity enclosed 

 by the hypoblast in its simple sac-like condition is the 

 primitive alimentary canal or archenteron, its aperture of 

 invagination being termed the blastopore. The remaining 

 constituents of the body are derived from cells which are 

 budded off from one or both of the above layers. Early 

 traces of the embryo are obvious in the development of the 

 cephalo-thoracic appendages, which arise as paired out- 

 growths of a relatively small area of the surface of the egg 

 known as t\iQ gerfninal area; there appear at the same time, 

 at opposite ends of this, two median papillate outgrowths — 

 an anterior one which gives rise to the labrum, and a pos- 

 terior one which, by subsequent elongation and segmenta- 



