264 



EMBRYOLOGY OF INSECTS AND MYRIAPODS 



invagination to the first thoracic segment and posteriorly in the last three 

 abdominal segments. 



Before the differentiation of the inner layer has been completed, 

 the beginnings of the stomodaeal and proctodaeal invaginations appear 

 almost simultaneously. At this time the germ band has already become 

 greatly elongated and flexed into an S shape. The stomodaeal invagina- 

 tion is clothed on the ventral side with mesodermal cells which, however, 

 do not cover the blind end. Near the blind end are entodermal cells 

 which Hirschler is convinced are not derived from the ectodermal stomo- 



daeum. Later, after revolution, 

 when the stomodaeum has increased 

 in length until it has nearly reached 

 the abdomen, the blind end is in 

 contact with the mesenteron rudi- 

 ment, an entodermal cell strand 

 that lacks a lumen. In a similar 

 manner the proctodaeal invagina- 

 tion impinges upon the posterior 

 mesenteron rudiment which is com- 

 posed of few cells and which 

 Hirschler maintains cannot have 

 arisen from ectodermal cells of the 

 bhnd end of the proctodaeum. As 

 development proceeds, the blind 

 ends of the two invaginations are 

 nearly in contact, with only a small 

 entodermal strand composed of few 

 cells and lacking a lumen lying 

 between them. This strand repre- 

 sents the fused anterior and pos- 

 terior rudiments or immature mid- 

 gut. Later still the blind ends of the stomodaeum and proctodaeum 

 come in contact, with no evidence of the mid-gut rudiment remaining. 

 Since there is no evidence of their degeneration, Hirschler ventures 

 the opinion that the few cells of the mid-gut rudiment have migrated 

 on to the stomodaeum and proctodaeum and later form a suspen- 

 sorium. Subsequently the ectodermal cells at the blind ends of the 

 invagination break down, thereby establishing continuity of the alimen- 

 tary canal. The posterior part of the stomodaeum by this time has 

 enlarged, forming a functional mid-gut. Hirschler therefore concludes 

 that the yolk cells take no part in the formation of the embryo, that the 

 gut owes its origin chiefly to the ectodermal stomodaeum and procto- 



ser. ceph 



Fig. 189. — Aphis pelargonii. Section of 

 embryo through the germ cells {gc). {am) 

 Amnion, {ent) Entoderm, (gc) Germ 

 cells, (mes) Mesoderm, (mst) Median 

 nerve cord, (ser) Serosa, (ser. ceph) 

 Cephalic serosa ( = serosa and amnion) . 

 iy) Yolk. {Adapted from Will.) 



