Development of the First Day 131 



the head-fold has been illustrated with the 

 cloth model, and is shown in Figs. 37 and 38: 

 it is best seen in sagittal sections, one of 

 which is represented in Fig 45. 



FIG. 44 (A). ANTERIOR PART OF THE PRECEDING FIGURE, 



MORE HIGHLY MAGNIFIED TO SHOW DETAILS. (After Duval.) 



A y anterior end of head. C/i, notochord. i, region where the medullary 

 folds will first fuse to form the medullary canal. 2, lateral limits of medul- 

 lary folds. 3 and 4, posterior regions of medullary folds. 5, lateral limits 

 of head region. 6, limit of entoderm. 



By the formation of the head-fold the an- 

 terior end of the digestive tract, or fore-gut, 

 is also formed (Fig. 45, Pit). The tail-fold 

 and the lateral folds do not begin to form 

 until later. 



In some cases, before the end of the first 



