56 



ARACHNIDA. 



larger abdominal limb-rudiments are still distinctly visible (a. 2 -ab 5 ). 

 This figure further shows very distinctly the presence (already alluded 

 to) of an abdominal segment lying in front of that bearing the 

 first provisional limb (a& x ). This segment, as well as the one which 

 follows the last limb-bearing segment, also takes part in the growth 

 towards the dorsal side. From this figure and others taken from 

 Locy, we gather that the segmentation of the dorsal side is, in this 

 part of the body, a true segmentation, and that the segmentation 

 observed by Barrois on the dorsal side of an Epeirid embryo is to> 

 be regarded in the same way. It seems advisable to point this out,. 



Fig. 31. — Two older embryos of Agahna labyrinfhica (after Balfour) A, embryo seen from 

 the side with the large ventral yolk-prominence. The angle formed by two lines, one drawn 

 through the points of origin of the thoracic limbs, and the other through those of the 

 abdominal limbs, indicates the degree of ventral curvature. C, embryo shortly before- 

 hatching. The abdomen, which has not yet fully attained its permanent form, is pressed 

 against the ventral side of the thorax, ch, chelicerae ; cl, caudal lobe ; pel, pedipalps ; pr.l, 

 cephalic lobes ; pr.p, provisional abdominal appendages. 



because the embryo of Epeira has such a characteristic form even in 

 later stages. The external segmentation is the expression of the 

 dorsal extension of the primitive segments, which arise in the 

 Araneae as in the Scorpiones, by the segmentation of the mesoderm- 

 band (pp. 23 and 85). 



The further development of the external form of the body, apart 

 from the reduction of the caudal region just described, is determined 

 by the union in the middle line of the halves of the segments that lie 

 on each side. During this process, the principal mass of the yolk is 

 withdrawn into tin- abdomen, and the cephalo-thorax and abdomen 



