DEVELOPMENT. 187 



two mesoblastic bands, but also, in its central region, to the 

 hypoblast. This makes its appearance, however, not as a 

 continuous layer, but as two hourglass-shaped rudiments, one at 

 the anterior, the other at the posterior end of the ventral plate. 

 These rudiments have their convex ends directed away from 

 each other, while their edges are approximated and gradually 

 meet so as to form a continuous hypoblast beneath the meso- 

 blast. Although I have not been able completely to satisfy 

 myself as to the mode of formation of the hypoblast in the 

 Cockroach, I have observed stages of development which lead 

 me to suppose that it proceeds in this Insect in a manner 

 similar to that observed by Kowalewsky in Muscidce. The 

 hourglass-shaped rudiments of the hypoblast become pushed 

 upwards by those foldings-in of the epiblast which form 

 towards the anterior and posterior ends of the embryo, and give 

 rise to the stomodceum and proctodocum.* 



The stage of development in which the germinal groove 

 appears, by the folding inwards of the epiblast, has been 

 observed in many other animals, and is known as the Gastroca- 

 stage. In all higher types (Vertebrates, the higher Worms, 

 Arthropoda, Echinodermata) the mesoblast and hypoblast are 

 formed in the folded-in part of the Gastraea in a manner 

 similar to that observed in Insects. 



The yolk-cells, which some observers have supposed to form 

 the hypoblast, are believed by Kowalewsky to have no other 

 function except that of the disintegration and solution of the 

 yolk. I can, however, with confidence affirm that in the Cock- 

 roach these cells take part in the formation of permanent 

 tissues (see below). 



Each of the two mesoblastic bands which lie right and left of 

 the germinal groove divides into many successive somites, and 

 each of these becomes hollow. Every such somite consists of 



* 



an inner (dorsal) one-layered and an outer (ventral) many- 

 layered wall, the latter being in contact with the epiblast. The 

 cavities of all the somites unite to form a common cavity, the 

 coelom or perivisceral space of the Cockroach. The ccelorn, like 

 the cavities in which it originates, is bounded by two layers of 

 mesoblast an inner, the so-called sp/auc/tnic or visceral layer, 



* These terms are explained on p. 115. 



