314 



EMBRYOLOGY. 



(1) The Thymus 



has been for several years a favorite object of embryological investiga- 

 tion, since the time when KOLLIKER 

 made the interesting discovery that 

 in mammalian embryos it takes 

 its origin from the epithelium of a 

 visceral cleft. This discovery has 

 since then been corroborated, and 

 at the same time extended ; for also 

 in such animals as persistently 

 breathe by means of gills the 

 thymus is developed out of epi- 

 thelial tracts of the open and func- 

 tionally active gill-clefts. 



Let us first examine the original 

 condition as exhibited by Fishes. 

 As stated by DOHRN, MAURER, and 

 DE MEURON, the thymus (th} of the 

 Selachians (fig. 175) and the Bony 

 Fishes has a multiple origin and is 

 derived from separate solid epithelial 

 growths, which take place at the 

 dorsal ends of all the gill-clefts, and, 



indeed, to a greater extent on the anterior than on the posterior ones. 



Fig. 175. Diagram to show the develop- 

 ment of the thymus, the thyroid 

 gland, and the accessory thyroid 

 glands, and their relations to the 

 visceral pockets in a Shark embryo, 

 after DE MEURON. 



ich 1 , sch', First and sixth visceral pockets ; 

 Hi, fundaments of the thymus ; ad, 

 hyroid gland ; nsd, accessory thyroid 

 gland. 



tl 



nsd 



Fig. 176. Two diagrams [ventral aspect] of the development of the thymus, the thyroid gland 

 and the accessory thyroid glands, and their relations to the visceral pockets in a Lizard 

 embryo (A) and a Chick embryo (B), after DE MEURON. 



*ch l , sch", First and second visceral pockets ; sd, thyroid gland ; nsd, accessory thyroid gland ; 

 th, fundament of thymus. 



In the Bony Fishes the separate fundaments at an early period, even 

 before they have detached themselves from their matrix, fuse together 



