126 O. L. Simmons — Development of Lungs of Spiders. 



tion which serves to connect my account with the papers of 

 Locy, Kishinouye, Laurie and others. Here the gill lamellae 

 have slightly increased in number while they have become 

 greatly increased in length. In the figure the pulmonary sac 

 is somewhat funnel-shaped owing to a pulling open of the 

 spiracle in some process of manipulation. 



From this stage the transition to the conditions described by 

 MacLeod and Locy is but slight and although I have studied 

 the later stages up to and even beyond hatching, my observa- 

 tions are but a confirmation of theirs and so I do not repeat 

 them here. The lungs are well developed and apparently 

 ready to function as respiratory organs at the time of hatching. 

 With the growth of the young spider the principal changes 

 are an increase in the number of lamellae and a corresponding 

 increase in the size of the pulmonary organ, the new lamellae 

 being formed at the inner end of the sac. 



Tracheae. 



The study of these has been a matter of considerable diffi- 

 culty and I have been able to follow with certaint} T only the 

 earlier stages. The tracheae arise behind the appendage on 

 somite ix, which, in its earlier stages has exactly the same 

 history as appendage vni. There is the same inpushing be- 

 hind the limb, which results in its taking a position not point- 

 ing outward but towards the median line and backward. In fig. 

 8 is seen the first differentiation of the tracheae. The inpush- 

 ing has given rise to the spiracle as before, but the sac which 

 results does not show so markedly those infoldings of the 

 appendicular wall which occur in the case of the lungs. There 

 is at most but a slight undulation of this surface. At the 

 inner end, however, two ingrowths are seen, the earliest indi- 

 cations of the formation of the tracheal twigs. It is, however, 

 easy to see that these inpushings are to be compared with the 

 infoldings which produce the lamellae, while the undulations 

 just referred to admit of the interpretation that they are 

 aborted lung leaves. 



After the reversion of the embiyo the same parts can be 

 recognized (fig. 9). The inpushing has been carried to a greater 

 extent, and sections in other planes show that this ingrowth is 

 tubular in character. The cells lining its walls are elongate 

 and are already taking the character shown in the tracheae of 

 the adult. At the inner end of the tracheal trunk thus formed, 

 the nuclei are arranged in a radiating or bush-like manner, 

 apparently indicating that here is the place where the trunk is 

 about to divide into the tracheal twigs, but I have not been 

 able to trace any tracheal lumina between these cells. I have 

 not followed the later history of the tracheal system with any 



