Chap. XII.] THE CRAYFISH. 255 



Thence it gradually spreads, first over the sternal and eventually 

 over the tergal aspect of the embryo. 



The blastopore completely closes. The archenteron gives 

 rise to the mid-gut, from which the digestive gland arises as a 

 diverticulum. The mouth and fore-gut arise as a stomodaeal in- 

 vagination (79, E., st.) ; the vent and hind-gut as a proctodaeal 

 invagination (pr.). These two invaginations do not at first com- 

 municate with the archenteron. 



The anterior end of the embryo becomes marked out by the 

 formation of procephalic lobes (Fig. 79, F.,^>r. /.), in the midst of 

 each of which is an optic pit (op.). The posterior end is marked 

 out by a rounded elevation, the rudiment of the abdomen. 

 Between these two is a narrow groove, in the centre of which is 

 the stomodaeal invagination (m.). The sides of this groove grow 

 inwards to form two solid cords of cells of epiblastic origin. At 

 its front end a mass of cells grows inwards and connects the 

 two cords in front of the mouth. The anterior mass gives rise 

 to the cerebral ganglion ; the two cords which partially coalesce 

 in the mid-line differentiate into the post-cesophageal chain of 

 ganglia. Thus the central nervous system has an epiblastic 

 origin. 



The first appendages to appear are the rudiments of the 

 antennae (2), antennules (3), and mandibles (4). In certain 

 other Crustacea these are greatly developed as embryonic oar- 

 like appendages, and the embryo at this stage becomes free. 

 Such an embryo is called a nauplius. In the crayfish these 

 appendages do not thus develop into embryonic organs, nor does 

 the embryo become free till a much later stage. But the forma- 

 tion of a delicate cuticle after the appearance of these three 

 appendages seems, in the crayfish or allied forms, to symbolise 

 the nauplius condition. The other thoracic appendages appear 

 as bud-like protuberances in regular order. All are similar at 

 first, but subsequently differentiate. 



When it is hatched, the young crayfish differs considerably 

 from the adult. The cephalothorax is more convex and larger. 

 The rostrum is short and bent down. The sterna of the thorax 

 are relatively wider. The chelae are more slender, and their 



