298 PROGRESS OF SCIENCE IN THE CENTURY. 



embryonic life it arises in the neck and grows down 

 into the chest; it continues to grow after birth, but 

 in adult life it gradually shrivels till its size is in- 

 considerable. It is one of the ductless glands, and 

 is vaguely supposed to have some specific influence 

 on the blood. 



Since Kolliker discovered its endodermic origin 

 in mammals from the epithelium of a gill-pouch, and 

 stated that the original epithelial cells give rise to 

 lymph cells or leucocytes, two views have been held 

 regarding this puzzling organ. " On the one hand, 

 Stieda and His have maintained that the leucocytes 

 which always form integral parts of the thymus soon 

 after its first origin have migrated thither from the 

 exterior, possibly from the mesoblast. In this con- 

 clusion they have been supported by the researches 

 of Dohrn, Gulland, and Maurer, and by almost every 

 text-book of embryology and comparative anatomy 

 published since 1879. On the other hand, Kolliker 

 has stoutly maintained his original position, and the 

 results of his investigations have been emphatically 

 confirmed by Prenant, Oscar Schultze, and Beard." * 



In short, it has been known for a long time that 

 the thymus arises in the neck region of vertebrates in 

 connection with a pair or more of gill-clefts, and that, 

 at an early date in life, it is rich in leucocytes or 

 white blood corpuscles, which some believed to have 

 been born there, while others regarded them as 

 migrants from elsewhere. It was also known that 

 in many mammals, it degenerates after youth is over, 

 being for instance large in the calf, but small in the 

 cow. Generally speaking we may also say that the 

 thymus was known to be more abundantly represented 

 in lower than in higher vertebrates. 



* J. Beard, Lancet, January 21, 1899. 



