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EXPLANATION OF PLATE VI. 



Reference letters. 



a x -a xx . Appendages. s. Spiracle. 



c 7 -c 11 . Ccelonric cavities. so. Somatopleure. 



e. Ectoderm. sp. Splanchnopleure. 



m. Mesoderm. t. Trachea. 



pc. Pulmonary cavity. y. Yolk. 

 pi. Pulmonary leaves. 



All figures except fig. 10 are magnified 125 diameters. 



Fig. 1. A sagittal section showing the coelomic cavities of the vn, viii, 

 and lx somites. 



Fig. 2. A sagittal section through the same region as the preceding at 

 a little later stage. The coelomic pouch of the x somite is seen 

 as well as the inpushings of ectoderm behind somites vii and viii. 



Fig. 3. A section cut under conditions similar to those of the two pre- 

 ceding sections. The stage is a little later than fig. 2, and shows 



