HYMENOPTERA 



329 



arises from the somatic layers. The heart is formed in the usual manner 

 from the cardioblasts. A mass of mesoderm cells, forming the anterior 

 end of the mesoderm and evidently belonging to the primary head seg- 

 ment, closely surrounds the stomodaeum at its appearance and later 

 forms the muscular layer of the fore-gut. A similar mass at the posterior 

 end of the embryo forms the muscular layer of the hind-gut. 



The ovaries are derived from cells of the genital ridges. These 

 ridges are formed from the dorsal portion of the splanchnic layer, in the 

 fifth to the tenth abdominal segments, inclusive. This portion becomes 

 detached from the remainder of the splanchnic layer. During the devel- 



siom 



mge 



Fig. 284. — Apis. Sagittal section of anterior end. (mge) Anterior mid-gut rudiment. 

 (stom) Stomodaeum. {yc) Yolk cell. 



opment of the embryo the genital ridge gradually shortens, finally 

 occupying a position in the seventh to the tenth segments inclusive. 

 Meanwhile it loses its attachment to the dorsal splanchnic layer, at the 

 same time receiving an investment of cells from the splanchnic mesoderm 

 lying immediately ventrad of it. This investiture, composed of flat 

 cells, contracts and adheres to the ventral border of the heart. 



The mid-gut is formed from the anterior and posterior mesenteron 

 rudiments. The discoid anterior rudiment becomes transferred from 

 the ventral to the dorsal side of the cephalic pole of the egg by the 

 lengthening of the embryo. At the same time it increases in super- 

 ficial area, covering the cephalic end of the yolk like a cap. Its caudal 

 margin now extends rapidly caudad over the dorsal surface of the yolk 

 (Fig. 284, mge). Meanwhile the posterior mesenteron rudiment simi- 

 larly transfers to the dorsal side of the caudal end of the egg. It now 

 sends out a thin tongue-like process cephalad over the dorsal surface of 



