212 Vertebrate Embryology 



second day the thyroid body originates as 

 a pit from the floor of the pharynx, opposite 

 the first pair of visceral arches (Fig. 63, Th). 

 This pit deepens and elongates, and gradually 

 closes up, by the fusion of its sides, to form a 

 solid rod of entoblast lying in a longitudinal 

 position under the floor of the pharynx, just 

 in front of the truncus arteriosus. By the 

 sixth day, it separates from the pharynx, and 

 lies freely in the mesoblast of that region. It 

 now becomes bilobed, and the lobes send out 

 solid rods of tissue, which become hollowed 

 out to form the vesicles of the adult thyroid. 

 The thyroid gradually shifts its position back- 

 wards, and becomes surrounded with a sheath 

 of vascular connective tissue. 



Changes in the mesoblast. If a tranverse 

 section through the middle region of a second- 

 day chick be compared with a similar section 

 of a chick at the end of the third day, a 

 marked difference in the depth, in a dorso- 

 ventral direction, will be seen (Figs. 70 and 

 71). This increase in depth is due to three 

 chief causes : to the greater slope of the sides, 

 on account of the formation of the side folds; 

 to the increase in the mesoblast between the 

 notochord and the digestive tract ; and to the 



