124 0. L. Shnmons — Development of Lungs of Spiders. 



heated to 80° C. and hardened in alcohol, beginning with 50 

 per cent. The staining was usually in toto with alum cochineal. 

 The sections described, unless otherwise specified, are sagittal. 



In the first stage studied, corresponding in general to Locy's 

 fig. 6, somite VII is cut off from somite Yin. Somites VIII 

 and IX are still united (fig. 1) and the unsegmented mesoderm 

 extends farther back. The ectoderm is a single layer deep 

 except a portion over somite VII (possibly the anlage of an 

 appendage) and between VIII and ix. The infolding of ecto- 

 derm shows the first differentiation of external segmentation. 



In the next stage (fig. 2) which is about midway between 

 Locy's figures 6 and 7, the second abdominal somite is differ- 

 entiated, and to a less extent the line between somite ix and 

 somite x, which has developed, is marked off. The ectoderm 

 has become thickened from somite VII to somite IX. It is to 

 be noticed that the ccelomic pouches are flattened in all except 

 somite VIII. 



The succeeding stage (fig. 3) shows the same features carried 

 still farther. The XI somite has appeared. This stage corre- 

 sponds to Locy's figure 7 or a stage a little earlier. I may note 

 here that I have found at least as many ccelomic pouches as 

 are described bv Kishinouye in his " Note on the Ccelomic 

 Cavity of the Spider," 1894. 



After the stage just mentioned the appendages begin to be 

 formed ; no detailed account need be given of the external 

 appearance of these, as in the main, my observations are but 

 the repetition of those of various authors, from Claparede to 

 Kishinouye. They grow out, one on either side of somites 

 vin-x, as rounded knobs. 



Pulmonary organs. 



In figure 4, which represents somites VII and VIII, the early 

 appearance of the appendage is seen. In somite VII the 

 ccelome is already greatly reduced and no trace of an append- 

 age is to be seen. In the next somite (viii) the appendage is 

 plainly visible. It is marked off from somite vn by a slight 

 groove, while the groove separating it from somite IX is deeper 

 and directed forward, giving the limb a markedly backward 

 direction, a tendency which is even more pronounced in later 

 stages. Its outer wall is formed of several layers of cells 

 while the ccelomic pouch sends into the budding appendage 

 an outgrowth like that described by various authors. 



With farther growth the conditions just described become 

 more strongly emphasized ; the anterior demarkation of the 

 appendage becomes more and more faint, while behind, the 

 inpushing becomes more and more marked, so that eventually 

 a pit is formed, actually extending into the general body sur- 



