450 



ARACHNIDA. 



but some large cells are shewn in the act of passing into it 

 from the yolk. During a considerable section of the subse- 

 quent development the mesoblast is confined to the ventral 



plate. 



The first important change takes place when about six 

 somites are established ; the mesoblast then becomes divided 



/X ' 4 



FIG. 204. LONGITUDINAL SECTION THROUGH AN EMBRYO OF AGELENA 



LABYRINTHICA. 



The section is through an embryo of the same age as that represented in fig. 

 200 C, and is taken slightly to one side of the middle line so as to shew the relation 

 of the mesoblastic somites to the limbs. In the interior are seen the yolk segments 

 and their nuclei. 



i 16. the segments ; pr.l. procephalic lobe ; do. dorsal integument. 



into two lateral bands, shewn in section in fig. 203 B, which meet 



however in front in the procephalic lobes, and behind in the 



caudal lobes. Very shortly afterwards these bands become 



broken up into a number of parts corresponding to the segments, 



each of which soon becomes divided into two layers, which 



enclose a cavity between them (vide fig. 204 and fig. 207). The 



outer layer (somatic) is thicker and attached to the epiblast, 



and the inner layer (splanchnic) is thinner and mainly, if not 



entirely, derived (in Agelena) from cells which originate in the 



yolk. These structures constitute the mesoblastic somites. In 



the appendage-bearing segments the somatic layer of each of 



them, together with a prolongation of the cavity, is continued 



