378 



EMBRYOLOGY OF INSECTS AND MYRIAPODS 



nr 



am 



m 



'ir\ 



:t I I. 



?fi 



t '^ 



-si 



possibly on the fourth segment. The esophageal valve is formed in tht; 

 usual way by the invagination of the smaller esophagus 

 into the larger mid-gut. 



At this time, also, scattered large oval or round 

 neuroblasts separate from the inner walls of the ecto- 

 derm of the head lobes. Ventrally on each side of the 

 neural groove are the neuroblasts which are to develop 

 into the ventral nerve cord (Fig. 334). The segmen- 

 tally arranged neuroblasts at this stage are present in 

 nearly the entire length of the embryo. As develop- 

 ment progresses, the cord separates from the outer 

 ectoderm, and connectives and commissures are estab- 

 lished. The brain and the subesophageal ganglia 

 appear more or less simultaneously with the ventral 

 nerve cord. The nerve cord, until the last third of the 

 developmental period, extends nearly to the caudal end 

 of the body, but at hatching it scarcely reaches the 

 middle. The extreme cephalization found in the 

 mature larva takes place after hatching. 



As has been stated, the posterior mesenteron rudi- 

 ment, i.e., the caudal end of the lower layer, is dis- 

 tinctly differentiated from the middle section, or 

 mesoderm portion, of the lower layer from the begin- 

 ning. With the increasing length of the mesoderm, the 



mesenteron rudiment together with the germ cells is pushed around on 



the dorsal side. Later at least 



some of the germ cells (grc), which 



until this time are found at the 



surface of the enteron rudiment 



(Fig. 3295), pass through the rudi- 

 ment into the interior (Fig. 329i)) . 



At this period the portion of 



mesoderm and the mesenteron 



rudiment which lie on the dorsal 



side of the egg have sunk beneath 



the surface. 



When the embryo has attained 



its greatest length, the stomodaeal 



invagination is first indicated im- 

 pinging upon the anterior mesen- 

 teron rudiment. Escherich (1900) 



especially emphasizes the fact that 



the rudiment is distinctly differentiated before the invagination appears. 



Fig 33.3 — Calli- 

 phora vomitoria. 

 Dorsal aspect. 

 {am) Amniotic in- 

 vagination, {hi) 

 Head lobes. (st) 

 Stigmata. 



proci 



neurg 

 Fig. 334. — Lucilia caesar. Cross section 

 of posterior region, (ent) Entoderm, {mes) 

 Mesoderm, (newr) N euro b 1 ast . {neurg) 

 Neural groove, {prod) Proctodaeum. 



