68 



K. KISHTNOUYE. 



out gradually more and more prominent, until the stage represented 

 in fig. 27 (surface view, fig. 21) is reached. After this, the tail 

 process gradually shortens (figs. 28, 29) until after a while there is 

 no tail projecting from the general body surface (fig. 32). 



At about the same time with the increase of cells of the dorsum, 

 the two nerve cords begin to diverge from each other. They are 

 most widely separated from each other at the anterior part of the 

 abdomen and gradually approach each other anteriorly and posteriorly 

 until they meet in the cephalic and tail lobes (fig. 21). Their 

 divergence together with the expansion of the dorsum makes the 

 embryo assume the ventral flexure. 



The cœlomic cavity of the caudal lobe now becomes gradually 

 conspicuous. This unpaired cavity is transformed into the so-called 

 stercoral pocket (Rectalblase, Kloake) of the adult spider. Hence 

 the stercoral pocket does not arise from the swelling of the internal 

 end of the proctodaßum, as has been supposed by other authors. This 

 organ is purely mesodermic in origin and nothing more than a 

 remnant of cœlomic cavities. This may be understood by examining 

 figs. 24-32, PI. XIV. From these figures it will be seen that the 

 proctodseum is formed in the caudal lobe later than the stercoral 

 pocket. 



The fact that any part of the adult alimentary canal should be 

 derived from the cœlom seemed to me so remarkable that I have 

 repeatedly examined my series of sections and am convinced of the 

 correctness of the observation. I do not know how to interpret this 

 fact unless it be that the stercoral pocket is a part of the primitive 

 excretory system — a supposition which is strengthened by its peculiar 

 relation to the remaining part of the digestive tube (PI. XVf, fig. 

 55) and by the fact that the Malpighian tubes open into it. 



At this period the mesodermic somites and the ganglia of the 



