234 



EMBRYOLOGY OF INSECTS AND MYRIAPODS 



meiit of the abdominal somites differs from that of the thorax only in so 

 far as the former are associated with the development of the gonads and 

 the genital ducts. The genital ducts are differentiated in the median 

 walls of the coelomic sacs from the second to the tenth segments but sub- 

 sequently undergo a concentration so as to become restricted to the third 

 to the sixth abdominal segments. A greater part of the dorsal portions 

 of the abdominal somites form myoblast plates and the fat body, as in the 

 thorax. The ventral portions of segments three, four, and six to ten form 

 coelomic ampullae which are comparable to the muscle-forming mesoderm 

 of the thoracic legs. 



In the 52-hour stage, rudiments of the labrum appear as paired swell- 

 ings. Soon, however, the paired nature is no longer evident. The bifid 



dc 



coel 



am 



procf 



Fig. 157. — Locusta. Cross section of eleventh abdominal segment, (am) Amnion. 

 {coel) Coelomic cavity, (dc) Provisional dorsal closure, (eps) Epineural sinus, (fid and 

 mes) ectoderm and mesoderm of the proctodaeum {prod). 



nature of the labrum is of secondary origin. The presence of an inde- 

 pendent pair of coelomic cavities lying in the labrum suggests the 

 appendicular nature of the labrum, although it is not so regarded by most 

 raorphologists. The antennary rudiments arise in the 50-hour stage. 

 At first behind the oral aperture; subsequently, owing to the backward 

 shifting of the latter, they come to lie in front of the mouth. Five days 

 after blastokinesis the segmentation is established for the entire antenna. 

 The intercalary appendages are represented by a thickening of the ecto- 

 derm between the antennae and the mandibles (Fig. 152), but no definite 

 evagination is formed. The thickening is first seen in the 75-hour stage 

 but disappears before hatching. The mandibles, maxillae, and labium 

 make their appearance at the 52-hour stage and acquire their definitive 

 form during the course of the next two or three days. 



The legs are first in evidence about the time the mouth parts arise 

 and lie on the outer edge of the primary lateral sternite. In the 100-hour 



