320 



EMBRYOLOGY. 



by highly vascular embryonic tissue. Subsequently the follicles 

 increase in size, especially in the case of Man ; this results from the 

 epithelial cells secreting a considerable quantity of colloid substance 

 into the cavity. 



A few further details concerning the thyroid gland of Man, for which we are 

 indebted to His, may be of interest. First, it is to be noted that the lateral 

 fundaments are considerably more voluminous than the middle part, and that 

 the future fundamental form of the organ is thus from the beginning pre- 

 determined. Secondly, some rare anatomical conditions (His) are explained 

 by the development, such as the ductus lingualis, the ductus thyroideus, and 

 the glandula suprahyoidea and praehyoidea. As was previously stated, the 

 unpaired fundament of the thyroid gland is connected with the root of the 

 tongue by means of the ductus thyreoglossus. When the thyroid gland moves 



from its place of origin farther 

 down, this duct becomes elon- 

 gated into a narrow epithelial 

 passage, whose external orifice 

 remains permanently visible as 

 the foramen coscum at the base 

 of the tongue. The remaining 

 part usually undergoes degene- 

 ration, but occasionally some 

 parts of it also persist. Thus 

 the foramen caecum is some- 

 times elongated into a canal 

 (ductus lingualis) 2J cm. long, 

 that leads to the body of the 



Fig. 180. Section through the thyroid gland of an 

 embryo Sheep 6 cm. long, after W. MULLER. 



sch, Sac-like fundaments of the gland ; f, glandular 

 follicles in process of formation ; b, interstitial 

 connective tissue with blood-vessels (g). 



hyoid bone. In other instances 

 the middle part of the thyroid 

 gland is prolonged upward in 

 the form of a horn, which is 

 continued as a tube (ductus 



thyroideus) to the hyoid bone. Finally, according to His, the glandular vesicles 

 now and then to be observed in the vicinity of the hyoid bone the accessory 

 thyroid glands, as well as the glandula supra- and prae-hyoidea are to be 

 interpreted as remnants of the ductus thyreoglossus. 



(3) Lung and Larynx. 



The lung with its outlet (larynx and trachea) is developed, like 

 a lobed gland, out of the oesophagus in a tolerably uniform manner, 

 as it appears, for all amniotic Vertebrates. Immediately behind the 

 unpaired fundament of the thyroid gland (fig. 181 Sd) there arises on 

 the ventral side of the oesophagus a groove (KK), which is slightly 

 enlarged at its proximal end. It is to be seen in the Chick at the 

 beginning of the third day, in the Rabbit on the tenth day after 

 fertilisation, and in the human embryo when it is 3'2 mm. long. 



