CHAPTER XVI 

 THE DUCTLESS GLANDS 



IN the developing embryos of vertebrate animals the endodermal 

 epithelium of the pharynx is invaginated into a number of paired or 

 median clefts or pits which, in most cases, are later cut off from communi- 

 cation with the digestive lumen by the constriction and atrophy of the 

 connecting ducts. These structures may be classified as ductless glands, 

 although in some of them a duct persists. They play an important part 

 in the animal's economy, as can be readily, although negatively, demon- 

 strated by experiment. Their structure also shows that there must be 

 considerable specialization in this function. 



The most anterior of these structures is partly derived from a pit 

 in the dorsal wall of the mouth. This invagination unites with a corre- 

 sponding invagination of the wall of the diencephalon or mid brain to 

 form the hypophysis or pituitary gland. Further back are to be seen the 

 five gill clefts which occur in all vertebrate embryos as five invaginations 

 of the pharyngeal epithelium. They sometimes do, and in other cases 

 do not, open through the sides of the neck to the exterior. In the mam- 

 mals they do not so open and, while the first pair form no glands, the 

 second pair form the paired palatine tonsils, whose ducts persist; the 

 third pair form another ductless gland called the thymus gland. From 

 a median invagination of the epithelium on the base of the tongue is 

 derived the thyroid gland, and the fourth and fifth gill clefts probably 

 give rise in the same way to the parathyroid bodies or glands. 



The adrenal gland originates in an entirely different manner from two 

 kinds of tissues, and is a ductless gland in the body cavity. It, too, is 

 of vital importance, as is attested by the death of the subject from which 

 the adrenals have been removed. The carotid gland and coccygeal gland 

 are two small ductless glands found on the carotid artery, and ventrad 

 of the middle sacral artery respectively, in man and some of the mam- 

 mals. 



All these glands discharge their secretions into the blood as well as 

 take their food materials from it. They therefore have a very incom- 

 plete lumen which rarely remains in communication with the exterior 

 (as in the tonsils) and is usually cut off from it (as in the thyroid, thymus, 



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