NOTES ON THE DEVELOPMENT OF THE ARANEINA. 6// 



a very well-marked swelling at each extremity. These swellings 

 are the earliest traces of the appendages. Of the three succeed- 

 ing segments, only the first is well differentiated. The caudal 

 lobe, though less broad than the procephalic lobe, is still a 

 widish structure. The most important internal changes con- 

 cern the mesoblast, which is now imperfectly though distinctly 

 divided into somites, corresponding with segments visible ex- 

 ternally. Each mesoblastic 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 (PL 30, figs. 5 and 6) is an important 

 one, as it is characterized by the establishment of the full num- 

 ber 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 30, fig. 6; PL 31, 

 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. /) 

 well separated from the segment with the chelicerae (ch}. It is 

 marked by a shallow groove opening behind into a circular 

 depression (sf.) the earliest rudiment of the stomodaeum. The 

 six segments behind the procephalic lobes are the six largest, 

 and each of them bears two prominent appendages. They con- 

 stitute 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 prominent appendages 

 there are four well-marked somites, each with a small protuber- 

 ance. These four protuberances are provisional appendages. 

 They have been found in many other genera of Araneina (Clapa- 

 rede, Barrois). 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 



