Development of the Lungs of Spiders. 217 



a pit is formed, actually extending into the general body 

 surface, 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 inpushing 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 coelome of somite VII 

 still persists ; that of VIII has become divided into two por- 

 tions, one of which remains in the appendage, while the other 

 portion, reduced in size, has been pushed backwards by the 

 ingrowing 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 rudiments 

 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 figure. 



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 

 appendicular ccelome is not shown, while the coelomic cavity 

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

 the pulmonary 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. Proxi- 

 mally the leaves are much shorter and the nuclei are irregu- 

 larly 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 Xipho- 

 sures. 



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 four to five 

 days alter this stage the lungs are almost fully developed and 

 have about the same appearance as in hatched specimens, 



