No. 2.J EMBRYOLOGY OF THE AMERICAN ALLIGATOR. 195 



beginning of the three cerebral vesicles first indicated in Fig. 26 

 are more clearly seen in Fig. 27. Ten days after the fully 

 formed eggs were removed from an alligator, the embryo had 

 reached the stage exhibited in Figs. 29, ventral, and 30, dorsal. 

 There is to be noted here a general increase in the development, 

 a larger number of somites, and a further extension of the head- 

 fold and of the amnion. Fig. 31 is an embryo from the same 

 lot of eggs as that shown in Figs. 29 and 30, but was made four 

 days later ; consequently it was about two weeks old. I have 

 introduced it mainly to show the beginning of the heart. It is 

 a ventral view and this is the oldest stage in which the embryo 

 retains its primitive position, in which the ventral side is turned 

 toward the yolk. 



The change of position begins at the anterior end of the body, 

 first affecting the head and neck region. They are rotated 

 about forty-five degrees on their axis, so that the head rests 

 upon one cheek, more frequently upon the left, but often on the 

 right. It would be strange if the head did not turn to one side, 

 for by the growth posteriorly of the ventral wall of the body the 

 head has come to be suspended, and being relatively very nar- 

 row, it is in an unstable position, and can very easily be swayed 

 to either side. -A movement of the body or a disturbance of 

 the whole egg would probably effect this ; and I would suggest 

 that the pulsation of the now prominent twisted heart lying 

 in the neck region would materially aid in bringing about this 

 change. Once turned, it is well supported by the broad surface 

 of the side of the head. As the body steadily increases its 

 dimensions dorso-ventrally, becoming deep and narrow, with 

 the heavier dorsal side adding to the instability, and as the con- 

 nection with the yolk-sac is constantly decreasing, the strain 

 exerted by the heavy head turned to one side may explain the 

 gradual turning of the entire embryo. 



The opening to the yolk-sac shows well in 34, and the out- 

 lines of the digestive tract ; the position of the future mouth is 

 clearly indicated. The most interesting feature in Fig. 33 is the 

 apparent connection existing between the first and second pha- 

 ryngeal clefts ; the second being a later development, is smaller, 

 and looks as though it might have originated from the first by 

 an outgrowth of the latter's hinder wall. Sections prove that 

 this appearance is due to the inner part of the throat-wall being 



