ZOOLOGY 



SECT. 



blast 



ser 



airm 



end 



amn.f 



posterior ends of the embryo give rise to the stomodseum and 



proctodaeum. 



Each of the two mesoderm 

 bands becomes divided trans- 

 versely into a series of seg- 

 ments, which become hollow, 

 and then become closely ap- 

 plied to one another, eventu- 

 ally coalescing, so that the 

 cavities of all of them unite 

 to form the body cavity of 

 the adult, the outer walls 

 becoming applied to the 

 ectoderm to form a somato- 

 pleure, or lamina consisting 

 of somatic layer of mesoderm 

 and of ectoderm ; the inner 

 becoming applied to the en- 

 doderm to form a splanclino- 

 plcurc, or lamina consisting 

 of splanchnic layer of meso- 

 derm and of endoderm. 



The ventral plate gradually 

 grows upwards at the sides, 

 and eventually its borders 

 meet and unite along the 

 dorsal middle line, the entire 

 yolk thus becoming enclosed 

 by it. 



The ventral nerve-chain 

 is developed from a groove 

 of the ectoderm, bounded by 

 thickenings which become 

 detached from the surface 

 ectoderm, and form the chain 

 of ganglia. The brain is de- 

 veloped from a pair of ecto- 

 dermal thickenings. That 

 part which is developed in 

 the prostomial region - 

 the archi-cercbrum becomes 

 united with that developed 

 in the following two seg- 

 ments to form the completed 

 brain or syn-mrbn/m. 



It can hardly be said that 

 the Cockroach undergoes a metamorphosis, the young Insect when 



blast 



ect 



anui.cav. 



end 



blast 



Fin. 46(3. A C, transverse sections through the 

 developing ovum nf an Insect at successive 

 stages t" show the mode of development of the 

 germinal layers and of the amnioii. num. 

 amnion ; nmn.f. fold of the ainnion ; num. <-nr. 

 cavity of the amnion ; &Zosi. blastoderm cover- 

 ing the yolk ; <.<-i. ectoderm; mil. endoderm; 

 iinnt, invaginaticiii of ventral plate; . .,-. sernsa; 

 vent. ill. ventral plate ; ///,-. yolk. (After 

 and Heider.) 



