96 



ARACHNIDA. 



band, but later it breaks up into two symmetrical halves, a ridge 

 of yolk pressing outwards in the median line. Here also there is 

 agreement with the Araneae. The germ-band soon becomes segmented 

 (Fig. 50 .4). The cephalic lobe, which, in Myobia as in the Araneae, 

 curves over to the dorsal side (Fig. 50 B), and the caudal lobe 

 become segmented off from the trunk. The part lying between 

 theni, which corresponds to the cephalo-thorax, is divided up into a 

 number of segments, the truncated rudiments of the mouth-parts 

 and limbs soon appearing on these (Fig. 50 B). This segmentation 

 is less distinct in other Acarina, and, as is well known, eventually 



disappears. The abdomen is still 

 comparatively large in such an 

 embryo ; in many Acarina it is 

 much reduced, or is united to 

 the cephalo-thorax. 



Before development has pro- 

 gressed thus far, a delicate struc- 

 tureless integument separates, in 

 At ax, from the embryo, and 

 surrounds it, like a second egg- 

 integument, in the form of a 

 closed envelope (Fig. 51, dm). 

 In other Acarina, this process 

 only takes place later, when the 

 limbs are already present, so that 

 these are found on the envelope 

 in the form of sheaths surround- 

 ing the actual limbs (Fig. 52, dm). 

 This delicate envelope, though separated from the embryo, is thus 

 seen to be a true larval integument. 



The embryo is now enclosed in a double envelope, and the dorsal 

 surface which, up to this period, showed little signs of development, 

 being covered only by a thin cell-layer, now commences to develop 

 by the growth of the mesodermal elements towards this surface. 

 The yolk for some time longer retains its former appearance (Figs. 

 50-53), but we must no doubt assume that the formation of entoderm 

 has already begun. Nothing certain is as yet known of the develop- 

 ment of the germ-layers and the rudiments of the organs in the 

 Acarina. The limbs of the embryo lengthen (Figs. 51 and 53 A) 

 and become segmented (Fig. 52). In the stage depicted in Fig. 51, 

 and more especially in the following stages, the embryos of many 



Fio. 51.— Embryo of Atax Bonzi surrounded 

 by the deutovum and the egg-shell (after 

 Claparede). eh, chelicerae ; d, yolk ; dm, 

 deutovum ; eh, egg-shell ; kl, cephalic 

 lobe ; 2'!-j),,, the three pairs of limbs ; ped, 

 pedipalps ; si, caudal lobe. 



