VIII 



AETHEOPODA 



255 



appear m these, and when the yolk in the mid-gut is finally absorbed 

 a cavity appears there also. The former cavities then open into the 

 mid-gut cavity, and it is then seen that a secretion has been developed 

 m each rounded pocket, and that these pockets are in fact glandular 

 outgrowths of the mid-gut. Hirschler compares these outgrowths to 

 the liver diverticula of Arachnida and Crustacea, they are transitory 

 structures and soon disappear in the larva (Fig. 203). The inner 

 end of the stomodaeum is enlarged and develops a ring-like thickening 

 of ectoderm at its inner end ; in later stages endoderm cells become 

 closely pressed against this end, and individual cells wander in 



amongst the ectoderm cells; 

 thus when the cavity of the 

 stomodaeum finally coalesces 

 with the mid-gut it is im- 

 possible to tell where ectoderm 

 ends and endoderm begins. 



Fig. 202. — Sagittal sections through the stomodaeum 

 and proctodaeum of Doncuda crassipes. (After 

 Hirschler.) 



A, through stomodaeum. B, through proctodaeum. 



a, anus ; ect, ectodermal cells ; &nd, endodermal cells ; 



mes, mesodermal cells ; o, mouth. 



Fig. 203. — Diagrammatic 

 representation of mid-gut 

 of the embryo of Donacia 

 crassipes showing its 

 lateral pouches. (After 

 Hirschler.) 



gl, lateral pouches. 



We must now consider the further development of the mesoderm. 

 As the coelomic sacs increase in size they shrink away from the yolk 

 and in this way there arises a space on each side which is the lateral 

 portion of the epineural sinus. The two lateral spaces in the 

 abdominal region, where coelomic sacs of right and left sides are in 

 contact with one another, form from the beginning a continuous cavity; 

 but in the thoracic region they are converted into a single cavity 

 by the disintegration of the mid- ventral plate of endoderm described 

 above, thus giving origin to a median sinus by the joining of those 

 of the right and left sides. 



The walls of the coelomic sac now begin to undergo differentia- 

 tion. The outer wall where it abuts on the ectoderm is composed of 

 a single layer of closely apposed cells; this is the rudiment of the 



