74 IC. KTSHINOÜTE. 



of the ventrnm, posterior to the pi-octoda?iim, gradmilly thins oif, 

 and after the process of reversion is completed it can not he dis- 

 tinguished from the dorsum (PI. XIV, figs. 31, 31). 



The posterior part of the mesenteron is formed hy an accumla- 

 tion of endoderm cells at the anterior ventral part of the stercoral 

 pocket. It is a wide open funnel-shaped tuhe, resting ahove the 

 mesoderm (fig. 32, Post, mrsent). 



The stercoral pocket produces paired diverticuhi fr(^m its lateral 

 sides (fig. 33). At first, I was inclined to think that these diverti- 

 cula become the Malpighian tubes, as these tubes were formerly 

 th<mght to arise as a pair of outgrowths from the stercoral pocket. 

 But I found that these diverticula give rise to no definite structure 

 in the adult, and that the Malpighian tubes arise in a ditferent way, 

 as will be explained further on. 



At this stage a very important organ is produced, which has 

 been almost entirely neglected by embryologists. I mean the coxal 

 gland, Avhich is f)rmed from an ectodermic invagination at the in- 

 ternal posterior base of the coxal joint of the first ambulatory 

 appendage (PI. XV, fig. 38, Co. gl). The invagination opens into 

 the cœlomic cavity (figs. 35, 36). Its development is traced farther 

 in the next stage. 



After the formation of the circulatory system the cœlomic 

 cavities atrophy, except the one of the anal lobe forming the stercoral 

 pocket, and some part of the thoracic ones in connection with the coxal 

 gland. The so-called body cavity of the adult is not the remnant 

 of the cœlomic cavity ; but it is a secondarily produced blood-space. 

 T'he mesodermic cells which formed the wall of these cavities form 

 The covering of the nervous system, the alimentary canal and other 

 organs. 



Some mesodermic cells at the base of the cephalothoracic appen- 



