80 VESSELS. AKTEKIES. VEINS. CAPILLARIES, ETC. 



the gland is represented by a large cavity (cortical 

 substance) filled with globules, which are imbedded 

 in a vascular network, from which they extract mate- 

 rial for elaboration. On one side, this cavity is in 

 communication with the afferent lymphatic vessels, 

 and on the other, with the efferent vessels, into which 

 latter it pours the organized products which after- 

 wards become white, and perhaps, also, red globules 

 of the blood. 



? e e 8 Iei8 pmefit of Arteries, veins, and lymphatic vessels are alike 

 developed from embryonic cells. They are at first 

 recognisable in the shape of rows, or columns, of cells. 

 Of the cells which correspond to the axis of the 

 column, one portion liquifies, and the remainder 

 becomes transformed into blood globules. Those 

 which lie on either side of the axis undergo various 

 changes, and finally form the three coats which con- 

 stitute the walls of the vessel. 



The development of capillary vessels is accom- 

 plished in the following manner : cells of oval shape 

 become united to each other end to end, and then the 

 partitions which separate them are absorbed and dis- 

 appear ; so that each series of cells is thus trans- 

 formed into a minute canal, in the structureless walls 

 of which nuclei are to be recognised, at regular inter- 

 vals, as in the fully formed capillaries of the adult. 

 Anastomoses are effected by means of minute prolon- 

 gations, like canaliculi, which take their origin from 

 the walls of the vessel, and, pushing out in different 

 directions, unite finally with each other. 



Kolliker has also demonstrated that branching 

 plasmatic cells not unfrequently form a connexion, by 



