188 



THE COCKROACH 



which lies on the outer side of the hypoblast, and an outer 

 somatic or parietal layer, beneath the epiblast. There are 

 accordingly four layers in the Cockroach-embryo viz., (1) epi- 

 bfast, from which the integument and nervous system are 

 developed ; (2) somatic layer of mesoblast, mainly converted into 

 the muscles of the body-wall ; (3) splanchnic layer of mesoblast, 

 yielding the muscular coat of the alimentary canal ; and (4) 

 hypoblast, yielding the epithelium of the mesenteron. 



Scattered yolk-cells associate themselves with the mesoblast 

 cells, so that the constituents of the mesoblast have a two-fold 

 origin. Fig. 109 shows that the yolk-cells are large, finely 



Fig. 109. Transverse sections of Embryo of B. yermanica, with rudimentary nervous 

 system (Oc. 4, Obj. D.D. Zeiss). N, nervous system ; M, mesoblastic somites. 



granular, and provided with many (3-6) nuclei and nucleoli. 

 They send out many branching protoplasmic threads, which 

 connect the different cells together, and thus form a cellular 

 network. Certain cells separate themselves from the rest, apply 

 themselves to the walls of the somites, and form a provisional 



