22 



ARACHNIDA. 



D. The Mesodermal Derivatives. 



Our information as to the origin of the mesodermal structures is 

 very slight ; but from what is known of the differentiation of the 

 mesoderm-bands, it appears to be very primitive. The two niesoderm- 

 bands break up into a number of somites, corresponding to the 

 segmentation of the body, these somites developing from before: 



Fig. 13.— Sagittal sections through the embryo of Euscorpius italicus (after Laurie), to show 

 the dorsal curvature of the germ-band. A, near the middle line ; B, median section. 

 ab, abdomen ; ch, chelicera ; d, yolk ; cut, entoderm ; h, cavity of the primitive segments ; 

 AY, cephalic lobe; m, mouth fstomodaeum) ; mes, mesoderm; j',-2' 4 , first four limbs; 

 pab, post-abdomen ; ped, pedipalp. 



backward (Fig. 13, h and mes), those of the post-abdomen being the 

 last to form. A well-developed pair of these primitive mesodermal 

 segments occurs in the (primary) cephalic region (Fig. 13 ^4). At 

 first the primitive segments lie at a little distance from the ventral 

 middle line (Fig. 3 B), but they grow in towards it later. 



