STRUCTURE OF FOUR-DAY CHICKS 



143 



farther and farther caudad until at the end of the fourth day 

 it lies at the level of the anterior appendage buds. Being un- 

 attached to the body, the ventricular region of the heart is 

 carried farthest caudad (Cf. Figs. 19, 29, 34, and 40). 



The changes which take place in the heart wall can be seen 

 best in sections. The endocardium in the heart of a four-day 



C32 HOURS 

 12 somitei 



38 HOURS 



16 somites 



K 



100 HOURS 

 45 somite* 



PIG. 50. Dextral views of the same series of hearts shown in ventral view 

 in Pig. 49. The heart drawings in Figs. 49 and 50 should be compared with 

 actual specimens or with drawings of entire embryos of corresponding age for 

 the relation of the heart to the body of the embryo. 



chick is still a single cell layer lining the lumen. The original 

 epi-myocardium at this stage can be differentiated into an 

 inner myocardial portion and an outer epicardial portion. The 

 myocardium has become greatly thickened and the cells in it 

 are elongated and beginning to show the histological character- 



