216 Mr. 0. L. Simmons on the 



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

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

 and IX are still united (PL VI. fig. 1), and the unsegmented 

 mesoderm extends further back. The ectoderm is a single 

 layer deep, except a portion over somite vil (possibly the 

 incipient foundation of an appendage) and between VIII and 

 IX. The infolding of ectoderm 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 diffe- 

 rentiated, 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 coelomic pouches are flattened in all except 

 somite VIII. 



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

 still further. 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 coelomic pouches 

 as are described by Kishinouye in his " Note on the Coelomic 

 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 Claparfcde to 

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

 vm-x, as rounded knobs. 



Pulmonary Organs. 



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

 appearance of the appendage is seen. In somite vil the 

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

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

 is plainly visible. It is marked off from somite vil 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 coelomic pouch sends into the budding 

 appendage an outgrowth like that described by various 

 authors. 



"With further growth the conditions just described become 

 more strongly emphasized; the anterior demarcation of the 

 appendage becomes more and more faint, while behind the 

 inpushing becomes more and more marked, so that eventually 



