366 ARANEINA. 



supra-cesophageal ganglia derived from the procephalic lobes. (3) The 

 pointed form of the caudal lobe. In Pholcus (Claparede, No. 442) 

 the caudal lobe forms a projecting structure which, like the caudal 

 lobe of the Scorpion, bends forward so as to face the ventral surface of 

 the part of the body immediately in front. In most Spiders such a 

 projecting caudal lobe is not found. While the embryo still retains its 

 dorsal flexure considerable changes are effected in its general consti- 

 tution. The appendages (fig. 200 D and E) become imperfectly 

 jointed, and grow inwards so as to approach each other in the middle 

 line. Even in the stage before this, the ventral integument 

 between the rudiments of the ganglia had become very much 

 thinner, and had in this way divided the ventral plate into two 

 halves. At the present stage the two halves of the ventral plate are 

 still further separated, and there is a wide space on the ventral side 

 only covered by a delicate layer of epiblast. This is shewn in surface 

 view (fig. 200 D) and in section in fig. 203 C. 



The stomodaeum (st) is much more conspicuous, and is bounded 

 in front by a prominent upper lip, and by a less marked lip behind. 

 The upper lip becomes less conspicuous in later stages, and is per- 

 haps to be compared with the provisional upper lip of Chelifer. 

 Each procephalic lobe is now marked by a deep semicircular groove. 



The next period in the development is characterised by the 

 gradual change in the flexure of the embryo from a dorsal to a ven- 

 tral one ; accompanied by the division of the body into an abdomen 

 and cephalo-thorax, and the gradual assumption of the adult characters. 



The change in the flexure of the embryo is caused by the elon- 

 gation of the dorsal region, which has hitherto been hardly developed. 

 Such an elongation increases the space on the dorsal surface between 

 the procephalic and caudal regions, and therefore necessarily sepa- 

 rates the caudal and procephalic lobes; but, since the ventral plate does 

 not become shortened in the process, and the embryo cannot straighten 

 itself in the egg-shell, it necessarily becomes ventrally flexed. 



If there were but little food yolk this flexure would naturally 

 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 fig. 201 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 sur- 

 face 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 represent- 

 ing two stages in it. In the first of these stages there is no con- 



