346 EMBRYOLOGY OF INSECTS AND MYRIAPODS 



and conspicuous, extending from the first gnathal head segment to the 

 last abdominal segment. On each side of the neural groove the neuro- 

 blasts are already distinct in each segment. Of these there are from two 

 to four pairs in transverse section of the intermediate body segments 

 (Fig. 304^), though they are not so regularly arranged as described by 

 Wheeler (1889) for the Orthoptera. At 50 hours, daughter cells may be 

 found in mitotic division and more or less continuous longitudinally. 

 Laterad and dorsad is the mesoderm, which in the intermediate abdomi- 

 nal segments now shows well-developed coelomic sacs. Since the inner 

 layer at the beginning consisted of a single layer of cells and not of a 

 gastrula tube, the coelomic sacs are formed by a folding back of the 

 lateral margin in each segment (Fig. 304A, coeZ). These sacs, though 

 never large, have reached their fullest development at 60 hours and are 



neur 

 neurg neur neurg 



A B 



Fig. 305. — Diacrisia. A, cross section of head between mandibles and maxillae of 

 60-hour embryo. B, through maxillary segment of 64-hour embryo, (am) Amnion. 

 (coel) Coelomic cavity, (mes) Mesoderm, (mx) Maxilla, {neur) Neuroblast, (neurg) 

 Neural groove. 



apparent in the protocephalic region, in the gnathal and thoracic seg- 

 ments, and in the abdominal segments, with the exception of those toward 

 the posterior extremity adjacent to the proctodaeum. The thoracic 

 and abdominal legs are at this stage conspicuous evaginations of the 

 body wall and are filled with yolk. 



The caudal pouch, so conspicuous and distinctive at 30 hours, becomes 

 smaller and more tubular as the embryo lengthens. At 48 hours, the 

 tubular part at the caudal end of the embryo measures about 60 microns 

 in length to the point where it opens to the yolk cavity on the dorsal 

 side. Except on the middorsal line, it is lined with mesoderm. At this 

 time, very near the tip of the caudal pouch on its dorsal (yolk) side, the 

 proctodaeal invagination appears. A few hours later the blind end of the 

 proctodaeum becomes free from mesodermal cells, but proliferating mid- 

 gut epithelial cells appear in the tip (Fig. 303 A). With the increase in 

 depth of the proctodaeum the dorsal side of the caudal pouch shortens 

 (Fig. 303 A). Between the proctodaeum and its envelope (the caudal 

 pouch) there are two layers of mesoderm, which, however, at this stage 



