120 THE GERM-PLASM 



become worn out, and consequently they possess the power of 

 constantly producing new teeth to replace the old ones. In the 

 mouth of a ray or dog-fish the teeth are arranged in several 

 rows along the edges of the jaws, the outer rows containing 

 those whicli are worn out, and the inner the younger teeth 

 which take their place. Birds, again, possess a very slight 

 power of repairing defects which have arisen accidentally, and 

 lience they are considered to have a very slight capacity for re- 

 generation. But their power of physiological regeneration with 

 respect to certain parts is nevertheless extraordinarily great : — 

 all the feathers are cast off and renewed once a year. Patho- 

 logical regeneration occurs to a very slight extent in mammals; 

 defects in the superficial epithelium, the epithelium of the ducts 

 of glands, the various supporting tissues, including bone, and in 

 nerve-fibres, can be repaired from the elements of the respective 

 tissues ; but in no mammal does a segment of a finger or an 

 eyelid grow again when once it has been cut off. In certain 

 mammals, however, the power of physiological regeneration 

 witli respect to certain parts is unusually marked. Male stags 

 shed their antlers annually, and new ones are formed in four or 

 five months. If we take into consideration the mass of organic 

 tissue which is thus formed in such a short time, this feat out- 

 strips even the regenerative performances of the full-grown 

 salamander. For according to Spallanzani. it takes a salaman- 

 der more than a year to restore an amputated limb to its normal 

 size and strength. Young individuals can, however, certainly 

 reproduce a limb in a few days ; and this gifted experimenter 

 observed in the case of a young Triton that the four limbs and 

 tail when tliev were cut off grew again six times in the space of 

 three summer months ! 



In one respect, however, viz., as regards tJie complexity of 

 the part 7-eplaced, this remarkable regenerative power in stags 

 and birds is far inferior to that which obtains in the Triton. 

 Althougli a bird's feather is a very wonderful structure, it is 

 formed merely from epidermic cells, and a stag's antler is only a 

 dermal bone covered over by the epidermis. But tlie limb of a 

 Triton, on the other hand, consists of every kind of tissue with 

 the exception of endodermal epithelium, — viz., of .skin, muscles, 

 a large number of skeletal parts, connective tissue, blood- 

 vessels, nerves, and so forth ; and all these have a very definite 

 arrangement, number, and form. There is no doubt therefore 



