ON THE DEVELOPMENT OF ARANEINÂ. 7l 



The third and fourth pairs of the ahdoiniual appendage are 

 modified into spinning niammillœ (PL XV, fig. 34). At the distal 

 end of each of tliese appendages a solid proliferation (sp. gl) of ecto- 

 derniic cells is for?ned. This becomes the spinning gland. Spiders 

 have generally three pairs of spinning mammillas ; two of which 

 are modified abdominal appendages, while the remaining one is added 

 very late, after the hatching of the embryo. The primitive spider 

 must have had only two pairs of spinning mammiilœ. Some tetrap- 

 neumonous spiders have only two pairs. 



The two semicircular halves of the cephalic lobe, between which 

 there is at first a deep median notch (PI. XIII, fig. 20), now fuse 

 with each other at the median line above the stomoda^um, so that tlie 

 notch becomes much shallower (fig. 21). The grooves formed 

 along their anterior margin during the preceding stage separate from 

 the ectoderm beginning from their external end and sink down 

 beneath the body surface. They are cut off from the ectoderm latest 

 at the hindermost j)ai'ts of their inner limbs (PL XVI, fio-. 48). 

 The lumina in the two separated semicircular grooves come to com- 

 municate with each other at the anterior median part (PL XV, fio-. 

 45). 



At the last point of separation there is left a shallow in\a<'ina- 

 tion or rather sac on the sui-face. The invagination is ])aired. The 

 openings of these sacs are directed towards the mouth of the embryi^, 

 and the invaginations are directed anteriorly. They are the first 

 traces of the posterior median eyes f^ee below) or the 'Hauptaugen' 

 of Bertkau* (PL XV, figs. 41-46, 48, P. M. E.). The anterior wall 

 of the sac is thicker than the posterior, the former being two t(j 

 several cells deep, the latter only one cell deep. The formation of 



* Bertlvau— Beiträge zur Kenatniss der Siuuesoi'y;iue dur Spiuneu. Arch. f. Mik. Anat 

 XXVII. 



