TRACHEATA. 443 



cause the whole embryo to be bent in so as to have the ventral 

 surface concave. But instead of this the flexure is at first con- 

 fined to the two bands which form the ventral plate. These 

 bands, as shewn in fig. 201 A, acquire a true ventral flexure, but 

 the yolk forms a projection a kind of yolk sack as Barrois 

 (No. 441) calls it distending the thin integument between the 

 two ventral bands. This yolk sack is shewn in surface view in 



FlG. 201. TWO LATE STAGES IN THE DEVELOPMENT OF AciELENA LABYRINTHICA. 



A. Embryo from the side at the stage when there is a large ventral protuberance 

 of yolk. The angle between the line of insertion of the permanent and provisional 

 appendages shews the extent of the ventral flexure. 



B. Embryo nearly ready to be hatched. The abdomen which has not quite 

 acquired its permanent form is seen to be pressed against the ventral side of the 

 thorax. 



pr.l. procephalic lobe; pd. pedipalpi ; ch. chelicene ; c.l. caudal lobe; pr.p. pro- 

 visional appendages. 



fig. 20 1 A and in section in fig. 206. At a later period, when 

 the yolk has become largely absorbed, the true nature of the 

 ventral flexure becomes quite obvious, since the abdomen of the 

 young Spider, while still in the egg, is found to be bent over so 

 as to press against the ventral surface of the thorax (fig. 201 B). 

 The general character of the changes which take place 

 during this period in the development is shewn in fig. 201 A and 

 B representing two stages in it. In the first of these stages 

 there is no constriction between the future thorax and abdomen. 



