46 



ARACHNIDA. 



the surface of the Araneid egg, this comparison being apparently 

 suggested by the band just described, together with the primitive 

 cumulus (Fig. 23). The band soon widens at the end furthest from 

 the primitive cumulus, and it becomes still broader as the thickening 

 of the blastoderm extends out laterally from this region. 



Such a lateral extension of the blastodermic thickening, starting 

 from the band, implies that we regard the band itself, as well as the 

 primitive cumulus, as thickenings of the blastoderm, which have 

 arisen by active increase of cells at these points. According to 

 Salensky, a depression appears in front of the primitive cumulus ; 

 this soon closes again, and is regarded by him as the blastopore. 

 We are disposed to attribute the same significance to that thickening 

 of the blastoderm which was mentioned above in the description of 

 the formation of the germ-layers. We thus assume that the primitive 



-5*. 



Cfzrr 



C/ltt 



Fig. 23. — Superficial aspect of three early stages in the development of an Araneid, showing 

 the embryonic rudiment {A and B, Agalena labyrinthica, after Balfour; C. Theridium, 

 after Morin). c.pr, primitive cumulus ; /(, posterior ; v, anterior. 



cumulus lies at the future posterior end, and that the band runs 

 out from it anteriorly. Its position therefore indicates the ventral 

 surface. The latter is clearly recognisable as such at a somewhat 

 later stage, the blastodermic thickening extending further, and 

 finally becoming evident on the surface of the egg as a region shaped 

 somewhat like an isosceles triangle (Fig. 23 C). The basal part of 

 this triangle seems to appear first (Fig. 23 B), and then by degrees 

 the parts nearer the apex. The base of the triangle corresponds to 

 the rudiment of the cephalic lobes, the point to the posterior end 

 of the embryo. According to this account, the primitive cumulus 

 would occupy the apex of the triangle, and must be looked for in 

 the posterior region (Fig. 23 B), and the band which developed at 

 first and proceeded from the primitive cumulus (Fig. 23 A) would 



