TRACE EAT A. 



373 



of the subsequent 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 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 into the appendage (fig. 203 C). 

 Since the cavity of the mesoblastic somites is part of the body cavity, 

 all the appendages contain prolongations of the body cavity. Not only 

 is a pair of mesoblastic somites formed for each segment of the body, 

 but also for the procephalic lobes (fig. 205). The mesoblastic somites 

 for these lobes are established somewhat later than for the true seg- 

 ments, but only differ from them in the fact that the somites of the 

 two sides are united by a median bridge of undivided mesoblast. The 

 development of a somite for the procephalic lobes is similar to what 

 has been described by Kleinenberg for Lumbricus (p. 281), but must 

 not be necessarily 

 supposed to indicate 

 that the procephalic 

 lobes form a segment 

 equivalent to the seg- 

 ments of the trunk. 

 They are rather equi- 

 valent to the prse- 

 oral lobe of Chseto- 

 pod Iarva3. When 

 the dorsal surface of 

 the embryo is esta- 

 blished a thick layer 

 of mesoblast becomes 

 formed below the epi- 

 blast. This layer is 

 not however derived from an upgrowth of the mesoblast of the 

 somites, but from cells which originate in the yolk. The first traces of 

 the layer are seen in fig. 204, do, and it is fully established as a layer 

 of large round cells in the stage shewn in fig. 206. This layer of 

 cells is seen to be quite independent of the mesoblastic somites 

 (me.s). The mesoblast of the dorsal surface becomes at the stage 

 represented in fig. 201 B divided into splanchnic and somatic layers, 

 and in the abdomen at any rate into somites continuous with those 



re .v 



FIG. 205. SECTION THROUGH THE PROCEPHALIC LOBES 

 OF AN EMBRYO OF A&ELENA LABYEINTHICA. 



The section is taken from an embryo of the same age 

 as fig. 200 D. 



st. stornodasum ; gr. section through semi-circular 

 groove in procephalic lobe ; ce.a. cephalic section of body 

 cavity. 



