312 PROCEEDINGS OF THE ACADEMY OF [May, 



(Lam. 2) are shown; each contains a narrow canal communicating 

 posteriorly with the main pulmonary cavity (P. Cav.) and their wall is 

 lined by a very flattened epithelium; where the nuclei are placed there 

 is a heightening of the cytoplasm around them, and the figure shows 

 clearly the biserial arrangement of the nuclei, at certain intervals 

 bicellular pillars occurring between adjacent lamellse. The archiccelic 

 space around these lamellse has become a vascular cavity (Bl. Cav.), 

 shown by stippling. A cross-section of the lung region of this stage 

 is shown in fig. 10; in the vascular cavity (Bl. Cav.) lie the lamellse 

 (Lam. 2) which are seen to be flattened tubes. The last stage I have 

 been interested to examine is that of fig. 11, one of about nine days 

 (hatching usually takes place in the fifteenth day)- Five lamellse are 

 developed, but owing to obliquity of the section none are seen in their 

 full length. 5 Mesial to them (to the right in the figure) there is an ecto- 

 blastic layer (Ect.), from which further lamellse will be formed. The 

 relation of the bicellular pillars to the lamellse and to the blood cavity 

 (Bl. Cav.) is perfectly clear. 



Thus each lung-book arises in the region of the appendage of the 

 second abdominal segment. Immediately behind this appendage devel- 

 ops an ectoblastic invagination, the stigma and pulmonary chamber, 

 and into this the appendage invaginates. There are temporary lamellse 

 on the posterior surface of this appendage, and these come to lie within 

 the pulmonary chamber, but they disappear entirely and from a 

 thickened cell mass of the anterior region of the invagination the 

 secondary or definitive lamellse develop. The appendage thereby 

 forms both operculum and secondary lamellse. 



4. Genetic Relations of the Tracheae and Lung-books 



of Araneads. 



In the Aranese theraphosse there are two pairs of lung-books and no 

 trachese, and this is also the case in the Hypochilidse among the Ara- 

 nese verse. Nothing is known of the development of these organs 

 in these forms, except my observation that in Evagrus the lung-books 

 are derivations of the second and third abdominal segments. All 

 other Aranese verse with two exceptions have one pair of lung-books 



5 It will be noted that all the drawings on Plate XI are made to the same scale, 

 yet in some cases, as on comparison of fig. 3 with figs. 2 and 5, the parts and nuclei 

 of one embryo may be much smaller than in another embryo of approximately 

 the same age. This is because there are great individual differences in the size 

 of eggs, so of the size of the cells that compose them. The number of cells 

 appears to be constant for a particular stage, but the size of the cells depends 

 upon the size of the embryo. 



