REPRODUCTION 865 



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 less 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. But the special bias or trend of growth 

 characteristic of each form is not a rigid rule. It can be modified ; 

 it is modified in every garden and pond by influences coming from 

 without. The inborn rule of life for many plants is to grow straight 

 up ; but this rule is often traversed by circumstances by differences 

 in the amount of sunshine, for example, caught by one side or the 

 other, or by the position of neighbouring objects which hinder or 

 help a vertical growth. And in animals Pfliiger has shown that the 

 direction of the lines of cleavage of the ovum of a frog depends on 

 the direction in which gravity acts, although Driesch and Hertwig 

 find that the nucleus can even be made artificially to change its place 

 with reference to the yolk, without hindering the development of a 

 normal animal. Artificial mouths, surrounded by tentacles, can be 

 formed in Cerianthus, an animal belonging to the same group as the 

 sea-anemones, merely by making a cut in the body-wall and prevent- 

 ing it from closing. In an Ascidian, too (the Cynone intestinalis\ 

 artificial openings in the branchial sac, surrounded by numerous pig- 

 menied points similar to the eye-spots around the natural mouth and 

 anus, have been produced (Loeb). Even in Amphioxus, the lowest 

 of the vertebrates, the eggs have been broken up by shaking, and a 

 complete animal evolved from as little as one-eighth of an ovum. 

 If the separation was incomplete a kind of Siamese twins, or even 

 triplets, could be obtained (Wilson and Mathews). 



Reproduction in the Higher Animals. In all the higher animals 

 reproduction is sexual, and the sexes are separate. 



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 

 (spermatogenous) cells in the tubules of the testicle divide so as to 

 form spermatoblasts. Each spermatoblast becomes a spermatozoon, 

 the head of the latter representing the nucleus of the former ; and it 

 is this nucleus which is the essential contribution of the male to the 

 reproductive process. The tail of the spermatozoon is simply, from 

 the physiological point of view, a motile arrangement, whose function 

 it is to carry the nucleus of the spermatoblast, freighted with all that 

 the father can transmit to the offspring, into the neighbourhood of 

 the female reproductive element or ovum. 



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