VIII 



ARTHROPODA 



225 



rudiments of appendages appear as paired outgrowths from the seg- 

 ments, developing in order from before backwards, and a pouch from 

 the corresponding coelomic sac extends into each of them. 



The abdominal appendages grow slowly and only form rounded 

 knobs ; they do not grow nearly as quickly as the appendages on the 

 other segments, and are absent on the seventh to tenth abdominal 



O * 



segments. The first abdominal segment gradually disappears ; it 

 possesses indications of a pair of appendages, but they never develop 

 beyond the condition of slight elevations and soon disappear altogether. 

 Very occasionally indications of a pair of appendages are found on 

 the sixth segment. These also soon disappear. 



The cephalic lobe, the origin of which has already been described, 

 becomes divided into two semicircular lobes, in each of which a 

 semicircular groove -like invagination is formed, which is almost 



Fiu. 172. Two views of embryos of Agelena at the period of maxima fti extension of the 

 ventral plate (i.e. before reversion has begun). The coelomic cavities are seen by their 

 degree of transparence, and are represented by lighter shading. (After Kishinouye. ) 



A, lateral view of an embryo. B, dorsal view of a slightly older embryo than that represented in 

 A. aM 5 , the abdominal appendages belonging to fourth and fifth abdominal segments ; c.gr, cerebral 

 groove ; ch, rudiment of chelicera ; c.l, caudal lobe ; coe 1 ' 15 , the coelomic sacs, from coe 1 , the sac in the 

 prae-oral lobe, to coe 15 , the sac in the seventh abdominal segment ; P~ 4 , rudiments of the legs ; i:<l, 

 rudiment of the pedipalp ; stem, stomodaeum. 



certainly homologous to some extent with the " cerebral grooves " of 

 Peripetias and Astacus. This invagination is the chief source of the 

 cells of the brain ; but there are also formed two invaginations at the 

 posterior lateral border of the cephalic lobe, which become closed from 

 the exterior and form little vesicles, and they also contribute to the 

 building up of the brain. The rest of the nervous system appears 

 as a series of paired thickenings of the ectoderm near the mid- ventral 

 line, one pair appearing in each segment. The thickening in each 

 segment is, however, only a more prominent portion of a continuous 

 ridge-like thickening which extends through the whole length of the 

 ventral plate and is continuous with the thickening which forms 

 one side of the brain. 



The stomodaeum is now formed as an invagination just behind 

 the cephalic lobes but in front of the chelicerae, and the proctodaeum 



VOL. i Q 



