ni8 -'REPRODUCTION 



' splice ' may be formed. A broken bone is regenerated by the 

 proliferation of cells of the periosteum, which become bone- 

 corpuscles. Gland cells e.g., liver cells^are also, under certain 

 circumstances, capable of regeneration. Some of the most striking 

 instances of new formation of glandular tissues have already been 

 mentioned in connection with the growth of grafts of certain of the 

 endocrine organs (e.g., thyroid, adrenal cortex), when a physio- 

 logical insufficiency has been created by removal of the greater 

 portion of the organ (p. 648) . There is no evidence that the influence 

 of the nervous system is a factor. It is doubtful whether there is any 

 new formation of nerve-cells in the adult organism, but peripheral 

 nerve-fibres which have been destroyed by accident or operation 

 are readily regenerated, and the end-organs of efferent nerves may 

 share in this regeneration. 



Thus, in a sense, reproduction is constantly going on within the 

 bodies even of the higher animals. But since the whole organism 

 eventually dies, as well as its constituent cells, a reproduction of the 

 whole, a regeneration en masse, is required. 



A cell of the stratum Malpighii can only, so far as we know, 

 reproduce a similar cell, and this is characteristic of cells that have 

 undergone -a certain amount of differentiation, especially in the 

 higher animals. The fertilized ovum, on the other hand, has the 

 power of reproducing not only ova like itself, but the counterparts 

 of every cell in the body. And this is only the highest development 

 of a power which is in a smaller degree inherent in other cells in 

 lower forms. Plants and the lowest animals are far le^s dependent 

 upon reproduction by means of special cells. A piece of a Hydra 

 separated off artificially or by simple fission becomes a complete 

 Hydra, as was shown by Trembley a century and a half ago. A 

 cutting from a branch, a root, a tuber, or even a leaf of a plant, may 

 reproduce the whole plant. It is as if each cell in these lowly forms 

 carried within it the plan of the complete organism, from which it 

 built up the perfect plant or animal. - 



Reproduction in the Higher Animals. In regard to the secretions 

 of the reproductive glands, all that is necessary to be said here is 

 that, unlike other secretions, their essential constituents are living 

 cells. The spermatozoa in the male have, indeed, diverged far from 

 the primitive type. Certain cells (spermocytes) in the tubules of the 

 testicle divide, each forming two daughter spermocytes. Each of 

 the daughter spermocytes in turn divides, so that four cells (sperma- 

 tids) are formed from each spermocyte. In the final division which 

 produces the spermatids a reduction of the chromosomes (p. 1122) 

 occurs, so that the spermatid possesses only one-half the number 

 characteristic of the somatic cells of the species. The spermatids 

 elongate and become spermatozoa, the head of the latter repre- 

 senting the nucleus of the former; and it is this nucleus (with the 



