Plate XVIII. 



Fig. 1. — A — H-are representations of successive stages of tlie embryo. A,B,C,D, 

 and D' are magnified about 28 diameters ; E, F, G, and H about 13 

 diameters. D' is a view of the otiier side of D. 



Fig. 2.- — Transverse section through the neck of the head-fold of an embryo at about 

 the same stage as that represented in Fig. IC. Zeiss C X 2. 



Fig. 8. — Transverse section from an embryo of a Httle more advanced stage ; the 

 section has passed through ahuost the same region as that of Fig. 2. C X 2. 



Figs. 4 and 5. — Series of transverse sections from an embryo more advanced than 

 that of the last Figure. Fig. 4 is the fourth section anterior to Fig. 5 ; 

 the former section is cut transversely, while the latter has passed 

 through somewhat to the front. C x 2. 



Figs. 6 and 7. — Series of transverse sections through a still more advanced embryo ; 

 the section has passed through an oblique plane indicated by the line a b 

 in Fig. ID'. Fig. 7 is the section next posterior to Fig. G and has been 

 somewhat reconstructed from the section that follows, to make the 

 relative position of the germinal layers intelligible. C x 2. 



Figs. 8, 9, and 10. — These have been selected from a series of transverse sections 

 through the neck of the head-fold of an embrj'O of about the same stage as 

 that of Fig. 1 E. Fig. 8 is the foremost section of the three ; Fig. 9 

 is the fifth, and Fig. 10 the eighth, section after Fig. 8. C X 2. 



Fig. 11. — Transverse section from the embryo of the same age as that of Fig. IE. 

 The section has passed through the same region as in Fig. 9. C X 2. 



Figs. 12, 13, and 14. — Transverse sections from the neck of the head-fold of a little 

 more advanced embryo. Fig. 12 is the most anterior ; Fig. 13 is the 

 fourth and Fig. 14 the eleventh after Fig. 12. C x 2. 



Fig. 15. — Transverse section through the neck of the head-fold of an embryo of the 

 same age as that represented Fig. 16. C X 2. 



Fig. 16. — Transverse section through the neck of the head-fold of an embryo of 

 about the same stage as that in Fig. IH. C X 2. 



