Development of the Third Day 167 



middle of the day the body is twisted so that, 

 on looking down upon the embryo, the anterior 

 end is seen in profile while the posterior end is 

 still seen from the dorsal side (Fig. 57). By 

 the end of the third, or early in the fourth day 

 the embryo has completed the change in posi- 

 tion. At the same time that the embryo is 

 turning over to its left side, a marked curva- 

 ture of the body begins and becomes so great, 

 at a later stage, that the head and tail almost 

 touch each other. 



As the embryo turns to its left side, the vi- 

 telline vein of that side becomes much larger 

 than the other vein, and the right vitelline vein 

 dwindles in size and at last disappears. 



The cranial flexure increases very markedly, 

 during this period, so that the fore-brain now 

 lies ventral to the hind-brain, and the mid- 

 brain is often the most anterior part of the 

 head, along a straight axis through the centre 

 of the embryo (Fig. 57). 



The amnion. As the amnion becomes prac- 

 tically complete during this day, its entire his- 

 tory will be given so that it need not be more 

 than referred to again. 



At the end of the second day, it will be re- 

 membered, the head-fold of the amnion covered 



