196 THE FOUETH DAY. [CHAP. 



all traces of the junction of its folds are by this time 

 lost. As yet there is very little fluid in the amniotic 

 sac proper, so that the true amnion lies close upon the 

 embryo. 



The folding off of the embryo from the yolk sac has 

 made great progress. The splanchnic stalk, which on 

 the third day was still tolerably wide, inasmuch as about 

 one third of the total length of the alimentary canal 

 was as yet quite open to the yolk sac below, now be- 

 comes so much constricted by the progressive closing in 

 of the splanchnopleure folds, that the alimentary canal 

 may be said to be connected with the yolk sac by a very 

 narrow neck only. This remnant of the splanchnic 

 stalk we may now call the vitelline duct; though narrow, 

 it is as yet quite open, affording still free communica- 

 tion between the inside of the yolk sac and the interior 

 of the alimentary canal. 



The somatic stalk, though narrowing somewhat, is 

 much wider than the splanchnic stalk, so that a con- 

 siderable ring-shaped space exists between the two. 



Another very prominent feature is the increase in 

 the cranial flexure. During the third day, the axis of 

 the front part of the head was about at right angles to 

 the long axis of the body; the whole embryo being still 

 somewhat retort-shaped. On this day, however, the 

 flexure has so much increased that the angle between 

 the long axis of the body and that of the front segment 

 of the head is an acute one. 



The tail-fold, which commenced to be noticeable 

 during the third day, has during this day increased very 

 much, and the somewhat curved tail (Fig. 67) forms 

 quite a conspicuous feature of the embryo. The general 



