II.] 



THE HEAD-FOLD. 



33 



Lastly, in N the yolk sac (?/s) is shewn being withdrawn into the cavity 

 of the body of the embryo. The allantois is as before, for the sake of 

 simplicity, omitted ; its pedicle would of course lie by the side of ys in the 

 somatic stalk marked by the usual dotted shading. 



It may be repeated that the above are diagrams, the various spaces being 

 shewn distended, whereas in many of them in the actual egg the walls have 

 collapsed, and are in near juxtaposition. 



c 



FIG. 9. 



JVC. 



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 com- 

 menced 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 develop- 

 ment proceeds. 



pp. space between somatopleure and splanchnopleure: pleuroperitoneal cavity. 

 Am. commencing (head) fold of the amnion. 



A fuller explanation is given under Fig. 16. 



forward, with a gentle ascent, regains the original level. As 

 seen in section, then, the blastoderm at this spot may be said to 

 be folded up in the form of the letter 8. This fold we shall 

 always speak of as the head-fold. In it we may recognize 

 two limbs : an upper limb in which the curve is directed for- 

 wards, and its bay, opening backwards, is underneath the 

 blastoderm, i. e. as we shall see, inside the embryo (Fig. 9, D] 

 and an under limb in which the curve is directed backwards, 

 and its bay, opening forwards, is above the blastoderm, i.e. 

 outside the embryo. If an j like the above, made of some 

 elastic material, were stretched laterally, the effect would be 

 to make both limbs longer and proportionally narrower, and 

 E. 3 



