xviii EXPLANATION OF PLATES II AND 12. 



Fig. i. Section through the anterior part of an embryo belonging to stage C. 

 The section is mainly intended to illustrate the formation of the ventral wall of the 

 alimentary canal from cells formed around the nuclei of the yolk. It also shews the 

 shallowness of the medullary groove in the anterior part of the body. 



Figs. ia,ib,ic. Three sections from the same embryo as fig. 2. Fig. 2 a is the 

 most anterior of the three sections and is taken through a point shortly in front of 

 fig. i. The figures illustrate the general features of an embryo of stage C, more 

 especially the complete closing of the alimentary canal in front and the triangular 

 section which it there presents. 



Fig. 3. Section through the posterior part of an embryo belonging to stage D. 

 (x 86 diameters.) 



It shews the general features of the layers during the stage, more especially the 

 differentiation of somatic and splanchnic layers of the mesoblast. 



Figs. 3 a, 3^, 3^, 3^, 3*, 3/ Sections of the same embryo as fig. 3. ( x 60 dia- 

 meters.) Fig. 3 belongs to part of the embryo intermediate between figs. 3 e and $f. 



The sections shew the features of various parts of the embryo. Figs. 3 a, 3 b and 

 3 c belong to the head, and special attention should be paid to the presence of a cavity 

 in the mesoblast in 3 b and to the ventral curvature of the medullary folds. 



Fig. 3 d belongs to the neck, fig. 3 e to the back, and fig. 3/to the tail. 



Fig. 4. Section through the region of the tail at the commencement of stage F. 

 ( x 60 diameters.) 



The section shews the character of the tail -swellings and the commencing closure 

 of the medullary groove. 



Fig. 5- Transverse section through the anterior part of the head of an embryo 

 belonging to stage F. (x 60 diameters.) It shews (i) the ventral curvature of the 

 medullary folds next the head. (2) The absence of mesoblast in the anterior part of 

 the head, hy points to the extreme front end of the alimentary canal. 



Fig. 6. Section through the head of an embryo at a stage intermediate between F 

 and G. (x 86 diameters.) 



It shews the manner in which the medullary folds of the head unite to form the 

 medullary canal. 



Fig. 7. Longitudinal and vertical section through the tail of an embryo belonging 

 to stage G. 



It shews the direct communication which exists between the neural and alimentary 

 canals. 



The section is not quite parallel to the long axis of the embryo, so that the proto- 

 vertebriE are cut through in its anterior part, and the neural canal passes out of the 

 section anteriorly. 



Fig. 8. Network of nuclei from the yolk of an embryo belonging to stage H. 



I XI'LANATION OF PLATES u AND 12. (X. p. 315.) 



COMPLETK LIST OF REFERENCE LETTERS. 



a/. Alimentary tract, an. Point where anus will be formed, ao. Dorsal aorta. 



ar. Rudiment of anterior root of spinal nerve, b. Anterior fin. c. Connective-tissue 



, cav. Cardinal vein. ch. Notochord. df Dorsal fin. ep. Epiblast. ge. 



