ALTERNATION OF GENERATIONS. 



189 



process is not one of division, or of budding. It is a degenerate process of 

 parthenogenetic reproduction in early life. The facts may be again summed up 

 in a formula, which does not take account of the occasional division of the 

 "sporocyst." 



Fig. 65. — A, asexual larva; ; S, sexual fluke ; the upp r circles 

 represent the special germ-cells ; fertilized ova at the base. 



The germ-cells, which behave like ova, and yet do not rise to that level, 

 appear sometimes in a central mass within the asexual individual, sometimes 

 simply in the epithelium lining the body walls. There may be a long series of 

 generations producing and produced in this way, and these are often unlike one 

 another. Fluke, embryo, sporocyst, redia, and cercaria, are all markedly 

 different in structure, though embryo changes into sporocyst, and cercaria into 

 fluke. 



This alternation between sexual reproduction with the usual fertilization, and 

 reproduction by means of special cells which yet require no fertilization, prevails 

 in many plants, for example, ferns and mosses. From a fertilized egg-cell the 

 ordinary fern-plant, with which everyone is familiar, develops. But this is quite 

 asexual, if we mean by that that it is neither male nor female, and that it 

 produces neither male nor female elements. At the same time it produces 

 special reproductive cells, — not egg-cells exactly, any more than those within 

 the sporocyst were, but yet able to develop of themselves into a new organism. 

 This is not another fern-plant, however, but an inconspicuous green organism, 

 much less vegetative, and sexual. The so-called "spore" formed on the 

 leaves of the sexless fern-plant falls to the ground, develops a " prothallus," 

 which bears male or female organs, or both. An egg-cell is fertilized by a male 

 element, and the conspicuous vegetative fern-plant once more arises. The 

 formula is therefore as follows : — 



Fig. 66 —Where A= sexless vegetative fern-plant; 



sp. = the parthenogenetic special reproductive cell or spore; 

 S = the sexual inconspicuous "prothallus," with male and female organs. 



Now take the history of a moss. Unlike the fern, the more conspicuous 

 " moss plant " is sexual. It bears male and female organs, and an egg -cell is 

 fertilized by a male element. The fertilized egg-cell, however, does not lose its 

 hold of the mother plant, but grows like an encumbering parasite upon it. 

 Obviously, then, it does not give rise to another " moss-plant." The result of 

 the fertilized egg-cell is a tiny sexless stalk, which bears on its apex the special 

 reproductive cells or spores with which we are now familiar. In other words, 



