i68 COMPARATIVE ANATOMY 



formed as a pair of bands which are divided into as many pairs 

 of somites as there are true segments in the body of the embryo. 

 The somites are solid at first, but soon become hollow vesicles 

 in much the same manner as do the somites of the earthworm, 

 and as the germ cells are formed from the walls of sorne of the 

 abdominal somites there can be no doubt that the cavities of the 

 vesicles are true coelomic spaces homologous with those of 

 AnneUd worms. 



In all, nineteen pairs of somites are formed in the embryo 

 of the cockroach, in addition to an anterior and a posterior 

 section in which no coelomic sacs are 4eveloped. The anterior 

 section is bi-lobed, the mouth opens on its ventral surface, and 

 it differs from the true segments of the body both in the 

 absence of coelomic sacs and in the fact that it never bears 

 any trace of limbs or appendages. It will be distinguished as 

 the cephalic lobe. The posterior section of the ventral plate 

 is known as the caudal lobe or telson, and is similarly character- 

 ised by the absence of coelom sacs and appendages and by the 

 opening of the anus on its ventral surface. Each of the inter- 

 vening nineteen segments contains a pair of coelomic sacs, and 

 each, with the exception of the second, bears a pair of lateral 

 limb rudiments in the form of short conical outgrowths. 

 Thus we see that the embryo insect resembles a crustacean, 

 and differs from the adult in having a pair of appendages on 

 each segment. We find also that a pair of ganglia is formed 

 from epiblastic thickenings in the cephalic lobe and in every 

 successive segmept, just as in Astacus. The further fate of the 

 cephalic lobe and segments deserves the closest attention, for it 

 affords the clue which enables us to homologise the insect with 

 the crustacean body. 



The cephalic lobes and the first five embryonic segments go 

 to form the head of the adult insect. The appendages of the 

 first segment, which are at first post-oral, are shifted forward in 

 front of the mouth and become the antennse. Their ganglion 

 fuses with the ga:nglion of the cephalic lobes and becomes the 

 deuto-cerebrum of the adult brain. The second segment is 

 known as the intercalary segment ; it does not bear limb 

 rudiments in any winged insect, but a pair of such rudiments 

 has been discovered in the embryo of one of the primitive 

 wingless insects belonging to the family Podurids. Its 

 ganglion becomes incorporated into the brain and forms the 



Digitized by Microsoft® 



