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EXPLANATION OF THE FIGURES. 

 Reference Letters. 



a 1 . -a 11 . Appendages. s. Spiracle. 



c'.-c 11 . Ccelomic cavities. so. Somatoplure. 



e. Ectoderm. sp. Splanchnoplure. 



in. Mesoderm. t. Trachea. 



pc. Pulmonary cavity. y. Yolk. 

 pi. Pulmonary leaves. 



All figures except fig. 10 are magnified 125 diameters. 



EXPLANATION OF THE PLATE. 



Fig. 1. — A sagittal section showing the coelomic cavities of the vir, vni and ix 

 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 vn and vni. 



Fig. 3. — A section cut under conditions similar to those of the two preceding 

 sections. The stage is a little later than fig. 2 and shows the ccelomic 

 cavity of somite xi and further inpushings of the ectoderm. In all 

 of the preceding sections the somatopluric wall is the thicker. 



Fig. 4. — In this figure the ccelome of somite vn is much reduced in size. The 

 ccelome of vin is beginning to thrust itself into the appendage, which 

 is marked off by a shallow groove in front and by a deeper inpushing, 

 which has a forward direction, between somites vin and is. 



Fig. 5. — In this section the inpushing, seen in fig. 4, has gone much farther, 

 dividing the ccelome of somite vni into two parts. On the inner 

 wall of the appendage are two inpushings which are the beginnings 

 of the lamellae of the lung book. 



Fig. 6. — A section of the appendage of somite Tin cut at a different angle from 

 the preceding. Here four lamellae are seen. 



Fig. 7. — A sagittal section from a stage just after the reversion of the embryo. 



Fig. 8. — A section of the appendage of the ix somite, showing, at the inner end 

 of the inpushing, the beginning of the tracheal tubes. 



Fig. 9. — A section of the tracheal region after the reversion of the embryo. 



Fig. 10. — This figure is a representation of the ventral surface of an embryo 

 which is about the same age as those from which sections seen in figs. 

 5, 6 and 8 were cut. 



