NOTES ON THE DEVELOPMENT OF THE ARANEINA. 103 



ends blindly on the side adjoining the rectum, but is open 

 anteriorly towards the yolk, and there can be very little doubt 

 that it owes its origin to cells derived from the yolk. On 

 its outer surface is a layer of mesoblast. 



From the condition of the mesenteron at this stage there 

 can be but little doubt that it will be formed, not on the sur- 

 face, hut in the interior of the yolk. I failed to find any 

 trace of an anterior part of the mesenteron adjoining the 

 stomodseum. In the posterior part of the thorax (vide fig. 

 20 d), there is undoubtedly no trace of the alimentary tract. 



The presence of this rudiment shows that Barrois is 

 mistaken in supposing that the alimentary canal is formed 

 entirely from the stomodaeum and proctodseum, which are 

 stated by him to grow towards each other, and to meet at 

 the junction of the thorax and abdomen. My own impres- 

 sion is that the stomodseum and proctodaeum have reached 

 their full extension at the 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 compart- 

 ments were destined to form the hepatic cseca, each ceecum 

 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 thick- 

 enings at the ends of the septa, visible in PI. X, 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 appen- 

 dages. 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. 



