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. 



tch 1 , scli,*, First and sixth visceral pockets ; 

 th, fundaments of the thymus ; sd, 

 hyroid gland ; nsd, accessory thyroid 

 gland. 



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 M HURON. 



ch\ tch', First and second visceral pockets ; jrf, 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 



