NOTES ON THE DEVELOPMENT OF THE ARANEINA. 187 
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 wdll 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 
stomodaeum. 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 proctodaeuin, 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 stomodaeum 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 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 thick- ^ 
enings at the ends of the septa, visible in PI. XXI, fig. 22, 
w’ere 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. 
