THE DEEPENING OF PHYSIOLOGY. 299 



The last impression has been made more exact by 

 the zoological embryologists who have shown that 

 there are 28 thymus rudiments in the lamprey, 14 in 

 the shark, 10 in the skate, 6 in the lizard, 2 in birds 

 and mammals. This diminished representation in 

 the higher vertebrates suggested the idea that the 

 thymua might be an organ specially adapted for the 

 phagocytio protection of the gills from the invad- 

 ing bacteria, or from the effects of other parasites 

 or of injuries. If this be so, we can understand 

 why the thymus should be less represented in the 

 higher vertebrates Reptiles, Birds, and Mammals 

 in which there is no trace of gills, in which, more- 

 over, other structures, such as the palatal and pharyn- 

 geal tonsils have, according to some authorities 

 (Stb'hr, Killian, Gulland) become garrisons of pro- 

 tective phagocytes, most strategically disposed. 



At the beginning of 1899, however, Dr. John 

 Beard published a short paper, announcing his dis- 

 covery that leucocytes appear in the thymus rudi- 

 ments of the skate (Raid batis) at a time when the 

 spleen has no existence, when there are, apart from 

 the thymus, no lymphoid structures of any sort. 

 Cradled in the thymus, the leucocytes soon begin to 

 emerge and migrate elsewhere. 



The conclusion that the thymus is the original 

 cradle of the white blood corpuscles of the body re- 

 quires to be confirmed and extended, but it is at least 

 a good illustration of the way in which comparative 

 study may throw welcome light on the physiological 

 puzzles of the human body. 



Experimental. More generally it should be noted 

 as characteristic of the second half of the nineteenth 

 century that physiological investigation became more 

 and more experimental in its method. We allude 



