TRACHEATA. 



451 



into the appendage (fig. 203 C). Since the cavity of the meso- 

 blastic 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 

 segments, but only differ from them in the fact that the somites 

 of the two sides are united by a median bridge of undivided meso- 

 blast. The development of a somite for the procephalic lobes 

 is similar to what has been described by Kleinenberg for Lum- 

 bricus (p. 339), 

 but must not be 

 necessarily sup- 

 posed to indicate 

 that the procepha- 

 lic lobes form a 

 segment equiva- 

 lent to the seg- 

 ments of the trunk. 

 They are rather 

 equivalent to the 

 prae-oral lobe of 

 Chaetopod larvae. 

 When the dorsal surface of the embryo is established a thick- 

 layer of mesoblast becomes formed below the epiblast. 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 (ine.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 of the ventral part of the 

 mesoblast. At the lines of junction of successive somites the 

 splanchnic layer of mesoblast dips into the yolk, and forms a 

 number of transverse septa, which do not reach the middle of 

 the yolk, but leave a central part free, in which the mesenteron 

 is subsequently formed. At the insertion of these septa there 



FIG. 205. SECTION THROUGH THE PROCEPHALIC 

 LOBES OF AN EMBRYO OF AGELENA LABYRINTHICA. 



The section is taken from an embryo of the same age 

 as fig. 200 D. 



sf. stomodaeum ; gr. section through semi-circular 

 g roove m procephalic lobe ; ce.s. cephalic section of body 

 cavity. 



29 2 



