INVERTEBRATA, CRYPTOGAMIA, MICROSCOPY, ETC. 623 



portion, and. a caudal lobe ; the ventral thickening is important, 

 inasmuch as it is the point at which two rows of cells are tirst 

 developed, and is therefore the first indication of the future mesoderm. 

 The already mentioned intermediate portion consists of three indis- 

 tinct segments ; the first of these appears to be the seat of origin of 

 the first pair of ambulatory limbs. The character of the ventral 

 thickening is to be noted ; first, because it shows that in this form, 

 at any rate, the cells are continuous across the middle line, and do 

 not exhibit any bilateral arrangement ; and secondly, because it is 

 very similar to the arrangement described by Kowalevsky as obtain- 

 ing in an insect, and is sujiported by the observations of Barrois, who 

 has already noted the presence of " a continuous ventral plate of 

 mesoblast." 



Shortly after this stage, there is found one in which there are six 

 segments intermediate between the procephalic and caudal lobes ; the 

 first two are more indistinct than the succeeding ones, owing pro- 

 bably to their later formation. The increase in segments is regular, 

 and each new one appears between the one last formed and the 

 caudal lobe ; the appendages do not begin to appear until some nine 

 or ten segments have been formed, and there is, at this period a 

 distinct median ventral groove ; at this stage the procephalic region 

 is " distinctly bilobed," but the first segment is still without append- 

 ages (" chelicerjB ") ; this is in keeping with its late appearance as a 

 segment. Some of the succeeding appendages are indicated by 

 swellings. An imperfect, though distinct, division of the mesoblast 

 into somites is now api^arent. 



In the next stage the ventral plate extends over nearly the whole 

 circumference of the ovum, the procephalic region is distinctly bilobed, 

 the stomodoeum is commencing, each of the six segments behind the 

 lobe bears prominent appendages; and behind these there are four 

 somites with small protuberances. The latter are provisional ap- 

 pendages, and their presence has been already noted by Clajjarede and 

 Barrois. Just a little later, sixteen post-cej)halic segments may bo 

 detected. 



The cpiblast exhibits no less interesting characters ; it is very thin 

 along the median groove, but on either side there is well-marked 

 thickening forming the first rudiments of the ventral ncrvc-ganglia. 

 It is of importance to note that tlio clielicera) have a ganglionic- 

 thickening independent of the procephalic lobes. In these latter tho 

 cpiblast is much thickened, but this is tho part which goes to form 

 the supra-cesophagcal ganglia. 



Later on, the appendages begin to be jointed, and primitively 

 these joints are five in number ; this is, as Mr. Balfour points out, an 

 interesting character, for " this number is permanent in Insects and 

 in Pcripatus." 



Next, we find that tho limbs nearly meet in the middle lino ; tho 

 two-jointed cheliccrfc ajipear to teruiiuate in rudimentary chclic, 

 and, so far, indicate that the spiders had anccstdrs with chelate 

 chelicorro. A largo upi)er and n small lower lip have bocomo 

 diveldind at the entrance to the stoiiiodiiim ; the priv'cphalic lobes 



