NOTES ON THE DEVELOPMENT OF THE ARANEINA. 69! 



present stage, and that both the stomach in the thorax and the 

 intestine in the abdomen are products of the mesenteron. 



The yolk retains its earlier constitution, being divided into 

 polygonal segments, formed of large yolk vesicles. The nuclei 

 are more numerous than before. In the thorax the yolk is 

 anteriorly divided into two lobes by the vertical septum, which 

 contains the vertical muscle of the suctorial pouch. In the 

 posterior part of the thorax it is undivided. 



I have not yet been able clearly to make out the eventual 

 fate of the yolk. At a subsequent stage, when the cavity of the 

 abdomen is cut up into a series of compartments by the growth 

 of the septa, described above, the yolk fills these compartments, 

 and there is undoubtedly a proliferation of yolk cells round the 

 walls of these compartments. It would not be unreasonable to 

 conclude from this that the compartments were destined to form 

 the hepatic caeca, each caecum being enclosed in a layer of 

 splanchnic mesoblast, and its hypoblastic wall being derived 

 from the yolk cells. I think that this hypothesis is probably 

 correct, but I have met with some facts which made me think it 

 possible that the thickenings at the ends of the septa, visible in 

 PI. 32, fig. 22, were the commencing hepatic caeca. 



I must, in fact, admit that I have hitherto failed to work 

 out satisfactorily the history of the mesenteron and its append- 

 ages. The firm cuticle of young spiders is an obstacle both in 

 the way of making sections and of staining, which I have not 

 yet overcome. 



General Conclusions. 



Without attempting to compare at length the development 

 of the spiders with that of other Arthropoda, I propose to point 

 out a few features in the development of spiders, which appear 

 to shew that the Arachnida are undoubtedly more closely re- 

 lated to the other Tracheata than to the Crustacea. 



The whole history of the formation of the mesoblast is very 

 similar to that in insects. The mesoblast in both groups is 

 formed by a thickening of the median line of the ventral plate 

 (germinal streak). 



