PRELIMINARY ACCOUNT. 



FIG. 10. 



N.C. 



[CHAP. 



F.So. 



DIAGRAMMATIC LONGITUDINAL SECTION THROUGH THE Axis OF 

 AN EMBRYO. 



The section is supposed to be made at a time when the head- 

 fold has commenced but the tail-fold has not yet appeared. 

 F. So. fold of the somatopleure. 

 F. Sp. fold of the splanchnopleure. 



The line of reference F. So. is placed in the lower bay, outside 

 the embryo. The line of D is placed in the upper bay inside 

 the embryo ; this will remain as the alimentary canal. Both 

 folds (F. So., F. Sp.) are parts of the head- fold, and are to be 

 thought of as continually travelling onwards (to the left) as de- 

 velopment proceeds. 

 pp. space between somatopleure and splanchnopleure : pleuro- 



peritoneal cavity. 



Am. commencing (head) fold of the amnion. 

 A fuller explanation is given under Fig. 29. 



figures), the level is maintained for some distance, and 

 then there is a sudden descent, the blastoderm bending 

 round and pursuing a precisely opposite direction to its 

 previous one, running backwards instead of forwards, for 

 some distance. It soon, however, turns round again, and 

 once more running forward, with a gentle ascent, regains 

 the original level. As seen in section, then, the blasto- 

 derm at this spot may be said to be folded up in the 



