The Organization of the Egg 193 



Regeneration occurs to a greater or less extent in all the 

 great groups of animals and plants. If Hydra be cut into 

 several pieces each will develop into a new animal. Earth- 

 worms and flatworms can regenerate either head or tail if 

 these be removed. The starfish can have a new arm made to 

 order ; the lobster a claw ; the snail may acquire a new head, 

 and the sea cucumber a new stomach. In higher plants a 

 piece of leaf or root may give rise to an entire new plant. 



Among animals the regenerative power decreases with in- 

 creasing specialization. The relative size of a piece of Hydra 

 necessary to produce a new animal is much less than that 

 of a crab or a frog. In vertebrates the amphibians have 

 been most used for regeneration experiments. There are 

 many salamanders which can regenerate legs or tail, but 

 there appear to be differences in the regenerative ability of 

 different forms. Age has an influence, as well as degrees 

 of specialization, for while the tadpole will readily regenerate 

 a lost limb the frog is unable to do so. 



In man the power of regeneration is relatively slight, 

 although skin, muscles, bone and other tissues show this power 

 to some extent in the healing of wounds, while the lens of 

 the eye may occasionally regenerate. There are some remark- 

 able cases on record of regeneration of internal organs in 

 mammals, although these are sometimes merely cases of hyper- 

 trophy of part of an organ, in compensation for the loss of 

 another part of that same organ, rather than instances of true 

 regeneration. Thus the removal of one-half or even three- 

 fourths of the liver of a dog or rabbit may result in the 

 enlargement of the remainder, without any replacement of 

 the lost part. It is well known that in man injury to a 

 lung or kidney may be compensated by increased growth and 

 activity of its opposite. There are recorded instances how- 

 ever of true regeneration of internal organs in mammals. 

 In the rabbit removal of as much as five-sixths of a salivary 

 gland may be followed by complete regeneration, and the 

 kidney of a rat or a rabbit may develop new tissue to a 

 certain extent, after part has been removed. 



The regeneration of the lens in vertebrates has been a bone 

 of contention among zoologists for many years. In the 

 development of the normal eye there first arises an evagina- 

 tion of the primary forebrain forming a primary optic cup 

 or vesicle, which is shortly followed by an invagination of the 

 adjacent ectoderm to form a secondary cup or vesicle, from 

 which is formed the lens. The question at issue is: Is the 

 lens dependent upon the presence of the primary vesicle for 

 its development, or may it arise independently of the latter? 

 Many ingenious experiments have been performed, prin- 

 cipally on amphibian larvae, in the attempt to solve this 



