178 
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 stomodseum (PI. XIX, fig. 7, and PI. XX, fig. 17, st) 
is a deepish pit between the two procephalic lobes, and dis- 
tinctly in front of segment of the chelicerse. 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. XIX, fig. 7, ch. g). 
The procephalic lobes {p7\ 1) now form two distinct 
masses, and each of them is marked by a semicircular groove, 
dividing them 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. XX, 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. XX, fig. 16). 
Beneath the procephalic lobes the raesoblast 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. XX, 
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 
stomodeeum [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, 
