SEXUAL REPRODUCTION. I2 q 



CHAPTER XI. 

 SEXUAL REPRODUCTION. 



I. Different Modes of Reproduction. — It is well known that 

 a starfish deprived of an arm can replace this by a fresh growth; that 

 crabs can renew the great claws which they have lost in fighting; and 

 that, even as high up as the lizards, the loss of a leg or a tail can be 

 made good. In a great variety of cases a kind of physiological for- 

 giveness is shown in the reparation of even serious injuries. Now this 

 "regeneration," as it is called, is in a certain degree a process ot 

 reproduction. By continuous growth the cells of a persistent stump 

 are able to reproduce the entire member. We know, too, that a 

 sponge, a hydra, or a sea-anemone, may be cut into pieces, with 

 the result that each fragment grows into a new organism. The same 

 is done with many plants; and though the division is artificial, the 

 result shows how very far from unique is the process which we usually 

 speak of as reproduction. In fact, as Spencer and Haeckel said long 

 ago, reproduction is but more or less discontinuous growth. So 

 again, we pass onward insensibly from cases of continuous budding, 

 as in sponge or rosebush, to discontinuous budding in hydra, zoophyte, 

 and tiger-lily, where the offspring, vegetatively produced, are sooner 

 or later set free. Similarly in the Protozoa an almost mechanical 

 breakage begins the series. This becomes more definite, in the pro- 

 duction of several buds at once, or of only one. Budding leads on to 

 deliberate and orderly division, both multiple and binary; while finally, 

 in colonial forms, the liberation of special reproductive units may be 

 observed. 



We shall afterwards have to discuss the relations of these and other 

 processes; but just as we began the study of sex with the familiar 

 contrast of male and female, so we shall begin our investigation of the 

 reproductive processes with the most obtrusive mode, known as sexual 

 reproduction. 



II. Facts Involved in Sexual Reproduction. — It is necessary 

 at the outset, to be quite clear as to the concurrence of several distinct 

 facts in any ordinary case of sexual reproduction among many-celled 

 organisms, (i) There is, first of all, the fact that special reproductive 

 cells are present in more or less marked contrast to the ordinary cells 

 making up the body. To this antithesis we have already given due 

 prominence. (2) Then there is the further fact, that these special 



