6/8 NOTES ON THE DEVELOPMENT OF THE ARANEINA. 



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. 31, fig. 15). There are six anterior 

 appendage-bearing segments, followed by four with rudimentary 

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

 behind. There are, therefore, sixteen in all. This number 

 accords with the result arrived at by Barrois, but is higher by 

 two than that given by Claparede. 



The germinal layers (vide PI. 31, fig. 14) have by this stage 

 undergone a further development. The mesoblastic somites are 

 more fully developed. The general relations of these somites 

 is shewn in longitudinal section in PI. 31, fig. 15, and in trans- 

 verse section in PI. 31, fig. 14. In the tail, where they are 

 simplest (shewn on the upper side in fig. 14), each mesoblastic 

 somite is formed of a somatic layer of more or less cubical cells 

 attached to the epiblast, and a splanchnic layer of flattened cells. 

 Between the two is placed a completely circumscribed cavity, 

 which constitutes part of the embryonic body-cavity. Between 

 the yolk and the splanchnic layer are placed a few scattered 

 cells, which form the latest derivatives of the yolk-cells, and are 

 to be reckoned as part of the splanchnic mesoblast. The meso- 

 blastic somites do not extend outwards beyond the edge of the 

 ventral plate, and the corresponding mesoblastic somites of the 

 two sides clo not nearly meet in the middle line. In the limb- 

 bearing somites the mesoblast has the same general characters 

 as in the posterior somites, but the somatic layer is prolonged as 

 a hollow papilliform process into the limb, so that each limb 

 has an axial cavity continuous with the section of the body- 

 cavity of its somite. The description given by Metschnikoff 

 of the formation of the mesoblastic somites in the scorpion, 

 and their continuation into the limbs, closely corresponds with 

 the history of these parts in spiders. In the region of each 

 procephalic lobe the mesoblast is present as a continuous layer 

 underneath the epiblast, but in the earlier part of the stage, 

 at any rate, is not formed of two distinct layers with a cavity 

 between them. 



