8 



ARACHNIDA. 



is followed by another series of six pairs of abdominal limbs. [In 

 front of these is a small limbless segment (Brauer).] The first 

 pair is specially small, and soon degenerates, the slight prominence 

 covering the genital aperture (genital operculum) taking the 

 place of these limbs, while the second pair gives rise to the large 

 combs (Fig. 7 C, pe). The four posterior pairs also abort, but first 



become connected with the forma- 

 tion of the lung-sacs, as will be 

 described later (Fig. 8, ap z -ap { '). 

 While the number of segments 

 and limbs increases in the germ- 

 band, the position of the latter in 

 relation to the yolk changes some- 

 what. By continuous growth, it 

 finally covers almost half of the 

 oval yolk. As the anterior end 

 grows specially large, the cephalic- 

 lobes extend round the yolk, and 

 the germ-band now appears bent 

 round the anterior pole of the egg 

 (Fig. 13 A and B, p. 22). The 

 posterior end of the germ-band, on 

 the other hand, grows out from 

 the egg and bends downwards 

 and forwards, and the post- 

 abdomen continues to grow with 

 its ventral surface turned to the 

 ventral surface of the germ-band. 

 On its ventral surface a furrow is 

 apparent; this is the continuation of the neural groove (Fig. 7 B). 

 Five segments form in it at a later period, the unsegmented telson 

 remaining as the terminal joint (Fig. 7 C). In the germ -band 

 proper, the ventral chain of ganglia now appears as a series of 

 distinct segmental thickenings (Fig. 7 A and B). Long before 

 the germ-band has developed to this extent, the germ-layers 

 outside it have extended further over the yolk ; this extension of 

 the blastoderm consists not only of the ectoderm, but also of the 

 entoderm, which is composed of large cells underlying the former, 

 and in this manner the yolk is gradually enclosed.* In the course 



* Cf. the detailed account of the formation of the intestinal canal, given 

 below (p. 19). 



Fig. 5.— Embryo of FAiscovpius italicus en- 

 veloped in its membranes, between which 

 fine filamenls stretch (after Metschnikokf, 

 from Balfour's Text -book). The germ- 

 band is seen in profile, lying upon the 

 yolk, ab, the post-abdomen bent round 

 anteriorly; eh, chelicerae ; pd, pedipalps ; 

 Pi-Pi, the four ambulatory limbs, and, 

 behind them, the abdominal limbs. 



