NOTES ON THE DEVELOPMENT OF THE ARANEINA. 91 



differentiated. The caudal lobe, though less broad than the 

 procephalic lobe, is still a "widish structure. The most 

 important internal changes concern the mesoblast, which is 

 now imperfectly though distinctly divided into somites, 

 corresponding with segments visible externally. Each meso> 

 blastic somite is formed of a distinct somatic layer closely 

 attached to the epiblast, and a thinner and less well-marked 

 splanchnic layer. In the appendage-bearing segments the 

 somatic layer is continued up into the appendages. 



The epiblast is distinctly thinner in the median line than 

 at the two sides. 



The next stage figured (PI. VIII, figs. 5 and 6) is an im- 

 portant one, as it is characterised by the establishment of 

 the full number of appendages. The whole length of the 

 ventral plate has greatly increased, so that it embraces 

 nearly the circumference of the ovum, and there is left 

 uncovered but a very small arc between the two extremities 

 of the plate (PL VIII, fig. 6 ; PI. IX, fig. 15). This arc 

 is the future dorsal portion of the embryo, which lags in its 

 development immensely behind the ventral portion. 



There is a very distinctly bilobed procephalic region [pr. T) 

 well separated from the segment with the chelicerse {ch). It 

 is marked by a shallow groove opening behind into a circular 

 depression [st.) — the earliest rudiment of the stomodseum. 

 The six segments behind the procephalic lobes are the six 

 largest, and each of them bears two prominent appendages. 

 They constitute the six appendage-bearing segments of the 

 adult. The four future ambulatory appendages are equal in 

 size : they are slightly larger than the pedipalpi, and these 

 again than the chelicerae. Behind the six somites with pro- 

 minent appendages there are four well-marked somites, each 

 with a small protuberance. These four protuberances are 

 provisional appendages. They have been found in many 

 other genera of Araneina (Claparede, Barrels). The segments 

 behind these are rudimentary and difficult to count, but 

 there are, at any rate, five, and at a slightly later stage 

 probably six, including the anal lobe. These fresh segments 

 have been formed by the continued segmentation of the anal 

 lobe, which has greatly altered its shape in the process. The 

 ventral groove of the earlier stage is still continued along 

 the whole length of the ventral plate. 



By the close of this stage the full number of post-cephalic 

 segments has become established. They are best seen in the 

 longitudinal section (PI. IX, fig. 15). There are six anterior 

 appendage-bearing segments, followed by four with rudimen- 

 tary appendages (not seen in this figure), and six without 



