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. 



f 



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



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



A, anterior end of head. C6, 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, PJi). The tail-fold 

 and the lateral folds do not begin to form 

 until later. 



In some cases, before the end of the first 



