DEVELOPMENT OF THE EXTERNAL FORM. 



9 



flu. 



of development, the yolk becomes absorbed by the entoderm, prob- 

 ably after the yolk -cells scattered throughout the former have 

 brought about its liquefaction. The en- 

 teron formed by the circumcrescence of the 

 yolk by the entoderm becomes connected 

 with the stomodaeum, which is formed 

 between the cephalic lobes (Figs. 6 and 

 13 B, m). The proctodaeum also arises 

 from a depression of the ectoderm situated 

 on the ventral surface of the telson 

 (according to Kowalevsky and Schulgin 

 on the penultimate segment). The meso- 

 derm also extends with the growth of 

 blastoderm over the yolk, and, starting 

 from the ventral surface, grows upward 

 between the ectoderm and the entoderm. 

 The development of the embryo thus 

 progresses from the ventral surface, which 

 was formed early, towards the dorsal 

 surface, until at last this also is com- 

 pletely covered. 



It appears as if the extension of the 

 embryonic rudiment over the yolk was 

 accompanied by an outward displacement 



of the primitive attachment of the embryonic envelopes, so that these 

 finally surround the whole embryo. According to Metschxikoff, 

 they become entirely detached from the embryo, evidently after 

 the enclosing of the yolk, and then form an isolated bilaminar 

 envelope around it. Metschxikoff also confirms Ganin's view, 

 that between the inner envelope and the embryo another fine 

 cuticular membrane is secreted by the latter, and becomes detached. 

 This would represent a larval integument, such as occurs in the 

 Araneae and Acarina. The embryo is born surrounded by the 

 embryonic envelopes, and escapes from them only after birth 

 (Metschxikoff). 



The limbs of the embryo, up to this point, are merely truncated 

 appendages. The pedipalps now become forked, and thus attain 

 their chelate character (Fig. 7 D) ; the chelicerae undergo the same 

 development. Both these pairs of limbs shift towards the mouth 

 and lie at its sides. At the base of each of the pedipalps and of 

 the four pairs of limbs there soon appears an outgrowth, which at 



Fig. 6.— Gerin-bands of Euscor- 

 pi us italkus (after Laurie), ch, 

 chelicerae ; g, ganglia of the 

 cheliceral segment ; Kl, ceph- 

 alic lobes ; in, mouth ; p, am- 

 bulatory limbs ; pab, the post- 

 abdomen bent forward ; ped, 

 pedipalps ; 1, 2, 3, h, first four 

 limb3. 



