0. L. Simmons — Development of Lungs of Spiders. 125 



face, the outer wall of the pit being formed by the appendage 

 whose growth we are tracing. This pit forms the pulmonary 

 sac and the opening of the inpnshing persists as the respiratory 

 stigma. At no time do the appendages rise to any considerable 

 distance above the general abdominal surface. 



The changes described can be seen by a glance at fig. 5 

 which, besides the points already mentioned, shows some other 

 features worthy of notice. The ccelome of somite vii still 

 persists. That of vin has become divided into two portions, 

 one of whicli remains in the appendage, while the other por- 

 tion, reduced in size, has been pushed backwards by the ingrow- 

 ing pulmonary sac. The sac itself is irregular in outline, its 

 inner wall being slightly undulating, while its outer wall, i. e. 

 the morphologically posterior surface of the appendage, has its 

 ectoderm thrown into folds, the anlagen of the leaves of the 

 lung-book. The ectoderm lining the inner wall of this sac is 

 but a single cell in thickness, but that of the appendicular side 

 is thicker, the nuclei being rather irregularly arranged, the 

 pulmonary ingrowths forcing their way between them. In 

 this stage but two lung leaves are outlined, as shown in the 

 hgure. 



In eggs of the same lot as the last, a stage apparently a little 

 older was found ; and from it the section figured in 6 was 

 drawn. In its general features the changes are slight but there 

 are some details of importance. From the fact that the plane 

 of this section is not the same as that of the last, the appendicu- 

 lar ccelome is not shown, while the ccelomic cavity of the 

 body is here much larger. So, too, the inner wall of the pul- 

 monary cavity is shown to be thicker, a fact probably due to 

 the obliquity of the section. In this stage four gill leaves are 

 shown, the most developed ones being the most distal ones. 

 In these, too, the nuclei are already arranged with their major 

 axes parallel to the plane of the leaves. Proximally the leaves 

 are much shorter and the nuclei are irregularly arranged. 

 These facts place it beyond a doubt that the growing point of 

 the organ is at the base of the appendage, a point of no little 

 importance in comparison with the Xiphosures. 



Figure 10 shows the ventral view of the embryo at the stage 

 which figures 5, 6 and 8 represent in sections. This stage is 

 about two or three days before the reversion of the embryo. 

 Changes during this period are very rapid. In 4—5 days after 

 this stage the lungs are almost fully developed and have about 

 the same appearance as in hatched specimens, except in size 

 and number of lamellae. The embryo hatches in from 7-8 

 days after the stage figured in 10. 



With the reversion of the embryo, the changes rapidly pro- 

 ceed toward the adult condition. In fig. 7 I insert an illustra- 



