94 F. M. BALFOUR. 



spiders are descended from ancestors with chelate chelicerae. 

 The four embryonic post-ambulatory appendages are now at 

 the height of their development. 



The stomod^um (PL VIII, fig. 7, and PI. IX, fig. 17, ^0 

 is a deepish pit between the two procephalic lobes, and dis- 

 tinctly in front of segment of the chelicerae. It is bordered 

 in front by a large, well-marked, bilobed upper lip, and be- 

 hind by a smaller lower lip. The large upper lip is a tem- 

 porary structure, to be compared, perhaps, with the gigantic 

 upper lip of the embryo of Chelifer (cf. Metschinkoff). On 

 each side of and behind the mouth two whitish masses are 

 visible, which are the epiblastic thickenings which constitute 

 the ganglia of the chelicerae (PI. VIII, fig. 7, ch. g). 



The procephalic lobes {pr. l) now form two distinct 

 masses, and each of them is marked by a semicircular groove, 

 dividing it into a narrower anterior and a broader posterior 

 division. 



In the region of the trunk the general arrangement of the 

 germinal layers has not altered to any great extent. The 

 ventral ganglionic thickenings are now developed in all the 

 segments in the abdominal as well as in the thoracic region. 

 The individual thickenings themselves, though much more 

 conspicuous than in the previous stage (PI. IX, fig. 16, v. c), 

 are still integral parts of the epiblast. They are more widely 

 separated than before in the middle line. The mesoblastic 

 somites retain their earlier constitution (PI. IX, fig. 16). 

 Beneath the procephalic lobes the mesoblast has, in most 

 respects, a constitution similar to that of a mesoblastic somite 

 in the trunk. It is formed of two bodies, one on each side, 

 each composed of a splanchnic and somatic layer (PI. IX, 

 fig. 17, sp. and so), enclosing between them a section of the 

 body-cavity. But the cephalic somites, unlike those of the 

 trunk, are united by a median bridge of mesoblast, in which 

 no division into two layers can be detected. This bridge 

 assists in forming a thick investment of mesoblast round the 

 stomodseum {st). 



The existence of a section of the body cavity in the 

 prseoral region is a fact of some interest, especially when 

 taken in connection with the discovery, by Kleinenberg, of a 

 similar structure in the head of Lumbricus. The procephalic 

 lobe represents the praeoral lobe of Chaetopod larvae, but the 

 prolongation of the body cavity into it does not, in my opinion, 

 necessarily imply that it is equivalent to a post-oral segment. 

 The epiblast of the procephalic lobes is a thick layer 

 several cells deep, but without any trace of a separation of 

 the ganglionic portion from the epidermis. 



