92 F. M. BALFOUR. 



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 PL IX, fig. 14) have by this 

 stage undergone a further development. The mesoblastic 

 somites are more fully developed. The general relations of 

 these somites is shown in longitudinal section in PI. IX, 

 fig. 15, and in transverse section in PI. IX, fig. 14. In the 

 tail, where they are simplest (shown 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 splanclmic mesoblast. The mesoblastic somites do not 

 extend outwards beyond the edge of the ventral plate, and 

 the corresponding mesoblastic somites of the two sides do 

 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. 



The epiblast at this stage has also undergone important 

 changes. Along the median ventral groove it has become 

 very thin. On each side of this groove it exhibits in each 

 appendage-bearing somite a well-marked thickening, which 

 gives in surface views the appearance of a slightly raised 

 area (PI. VIII, fig, 5), between each appendage and the 

 median line. These thickenings are the first rudiments of 

 the ventral nerve ganglia. The ventral nerve cord at this stage 

 is thus formed of two ridge-like thickenings of the epiblast, 

 widely separated in the median line, each of which is con- 

 stituted of a series of raised divisions — the ganglia (fig. 14, vg) 

 — united by shorter, less prominent divisions. The nerve 

 cords are formed from before backwards, and are not at this 

 stage found in the hinder segments. There is a distinct 



