228 



EMBRYOLOGY OF INSECTS AND MYRIAPODS 



the small intersegmental anteromedian mesoderm masses (blood-cell 

 lamella) give rise largely to the blood cells. A primary median mesoderm 

 band is not present in Locusta but is formed secondarily by the extension 

 of the blood-cell lamellae medially and by cells derived from the lower 

 end of the median wall of the coelomic sacs (Fig. 147). The antennary 



am 



Fig. 146. — Locusta. 



Cross section of second maxillary segment. 

 Ectoderm, (il) Inner layer. 



iam) Amnion, (ect) 



mesoderm is the first to be differentiated into a more or less- discrete 

 segment. 



In the formation of the coelomic cavities of the head and thorax the 

 inner layer spreads laterally and becomes in large part one-layered. The 

 ectoderm at the lateral edges of the germ band then curves dorsomedially, 

 and the mesoderm in each segment follows suit. The free ends of the 



mes 



coel 



bl 



""^^li^i 



am 



ft 



«*^i 





^*< 





^W 



Fig. 147. — Locusta. Cross section of extreme anterior end of second thoracic segment 

 of 75-hour embryo, (am) Amnion, (bl) Blood cell, (coel) Dorsoanal pouch of first 

 thoracic coelome. (dc) Provisional dorsal closure, (ect) Ectoderm, (mes) Mesoderm. 

 (p2) Second thoracic leg. 



mesoderm layer in each lateral half of a segment then approach each other 

 and unite to form a coelomic sac (Fig. 148). At the time of their first 

 appearance the cavities lie inside the rudiments of the appendages belong- 

 ing to their segment and may therefore be called "appendicular coelomic 

 cavities," each pair being formed distinctly and separately. 



