TIII: OBGAN8 Of i m: r 



319 



whole ma.-.s of the thyroid gland, whereat His n.aintains that they 



become in Man the voluminous lateral loho, and that the unpaired 

 fundaim lit hrcoiii! > the small middle part of t 1. 



The further development of the thyroid inland is accomplished 

 in a vei v similar manner in all Vertebrates. Two stages are 



distinguishable. 



During th" first Map- the whole fundament grows out into 

 numerous cylindrical cords, which in turn push out lateral buds 

 (tig. 17'.'). ry the union of these \\ith one another then- is formed a 

 network, into the interstices of which are distributed branches of the 

 blood-vessels 

 together with 

 embryonic con- 

 nective tissue. 

 In the case of 

 the Chick it is 

 found that the 

 thyroid gland 

 has reached 

 this stage of de- 

 velopment on 

 the ninth day 

 of incubation, 

 in the Babbit 

 embryo when 

 it is about .six- 

 teen days old, 

 in Man in the 

 second month. 



During the second .stage the network of epithelial cords is resolved 

 into the characteristic follicles of the thyroid gland. The cords 

 acquire a narrow lumen, around which the cylindrical cells are 

 regularly arranged. Then there are formed on the cords at short 

 intervals enlargements, wliii-h are separated by slight con>tric- 

 tions oig. 180). By the deepening of the constrictions the 

 whole network is linally subdivided into numerous, small, hollow 

 pithelial vesicles or follicles, which are separated from one another 



* [The elevation caused by the mid-brain may l><- apex or crown 



(_Selu'itrl). In later stages the distance between cn.wn and rump is ir: 

 than that between neck and rump, hence the measurement is made from the 

 crown. Compare foot-note, p. - 



Fig. 179. Right half of the thyroid gland of an embryo Pig 21 5 mm. 



long, crown-rump measurement,* i SO 



di.tiiieters. 

 The lateral () and median (1C S) thyroid glands are in process of 



fusion, g, Blood-vessels; tr, trachea. 



