THE ORGANS OF THE INNER GERM-LAYER. 



319 



Avhole mass of the thyroid gland, whereas His maintains that they 

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

 fundament becomes the small middle part of the organ. 



The further development of the thyroid gland is accomplished 

 in a very similar manner in all Vertebrates. Two stages tire 

 distinguishable. 



During the first stage the whole fundament grows out into 

 numerous cylindrical cords, which in turn push out lateral buds 

 (fig. 179). By the union of these with one another there is formed a 

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

 blood-ve s s e 1 s 

 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 Rabbit 

 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, which are separated by slight constric- 

 tions (fig. 180). By the deepening of the constrictions the 

 whole network is finally subdivided into numerous, small, hollow 

 epithelial vesicles or follicles, which are separated from one another 



* [The elevation caused by the mid-brain may be called the apex or crown 

 (Scheitel). In later stages the distance between crown and rump is greater 

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

 crown. Compare foot-note, p. 283.] 



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



long, crown-rump measurement,* after BORN. Magnified 80 



diameters. 

 The lateral (LS) and median (MS) thyroid glands are in process of 



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



