190 



THE EVOLUTION OF SEX. 



the fertilized egg-cell develops into a parasitic spore-bearing generation. The 

 "spores" fall into the ground, as they did in the fern, and there grow into a 

 usually thread-like structure, from which the sexual moss-plants are budded off. 

 If we do not emphasize the transitional thread-like stage, — the protonema as it 

 is called, — the formula is as follows (see also fig. p. 185):— 



Fig. 67. — Where A = inconspicuous sexless parasitic generation upon the "moss-plant." 

 sp. = the special parthenogenetic reproductive cell or spore produced by A. 

 S = the conspicuous sexual "moss-plant," budded from the threads developed 

 from the spore. 



If we do emphasize the "protonema " stage (/>), and regard the moss-plants 

 as asexually budded from it, the formula runs : — ■ 



^JL 



Fig. 68 



In the fern, the vegetative sexless generation was the more conspicuous ; in 

 mosses, the sexual generation. In a way this recalls the contrast between the 

 life-history of many a zoophyte, and that of the common jellyfish Aurelia. The 

 asexual hydroid colony is more conspicuous than the usually small swimming- 

 bell, but the sexual jellyfish is much more conspicuous than the minute asexual 

 "hydra-tuba." The common comparison between medusoid and hydroid on 

 the one hand, and prothallus and fern-plant on the other, is rather misleading, 

 simply because the hydroid merely buds off the medusoid, while the fern-plant 

 produces the prothallus from a special reproductive cell or spore. In some ferns 

 and mosses, however, a more exact parallel is occasionally exhibited. The 

 production of "spores" may be suppressed, and from the place where they 

 should be formed a (sexual) fern-prothallus or a new (sexual) moss-plant is 

 vegetatively developed, just as medusoid from hydroid. This exceptional 

 occurrence is technically called afiospoty. The very opposite of this also 

 occurs, the suppression not of the spore-bearing, but of the sexual generations. 

 The fern-plant then arises vegetatively from the prothallus; and this would be 

 paralleled if we supposed the sporocyst of the fluke to bud off redi:e (as it some- 

 times does), and these to continue the species without ever becoming really 

 sexual, solely by means of the special cells above described. 



IV. Combination of both these Alternations.— The asexual hydroid buds 

 off a medusoid, the fertilized ovum of which develops into a hydroid. Here 

 there is simple alternation between sexual and asexual reproduction. 



