In one respect male and female gametes are alike. Both have the same num 

 ber of chromosomes, and this number is half that found in the tissue cells of 

 the same species (see illustration, p. 388). Now the chromosomes are evi- 

 dently related to the qualities or characteristics developed by the new in- 

 dividual. It is probably true, as Hertwig surmised about seventy years ago, 

 that the mother-cell and the father-cell contribute in equal measure to the 

 constitution of the offspring (see page 374). 



From Water to Land As we compare different classes of plants or differ- 

 ent classes of animals, from the simplest to the more and more complex, we 

 find the male and female gametes increasingly different. Moreover, the or- 

 gans that bear the two kinds of gametes, and various accessory organs and 

 structures, also become more and more differentiated. 



When the egg and sperm unite during fertilization, their chromosomes 

 combine, so that the zygote has the number "normal" for the species, called 

 the diploid or double number. In distinction from this, we speak of the gametes 

 having the haploid number (from a Greek word meaning "single"). 



In the mosses, for example, the male plant and the female plant look very 

 much alike, except for the archegonia, or egg-organs, and the antheridia, or 

 sperm organs, formed at the tip of the leafy gametophytes. After fertiliza- 

 tion, which takes place inside the archegonium, the new individual grows out 

 from the tip of the mother-plant, upon which it depends for nourishment. 

 This new individual, as we saw, is a sporophyte, and it has the diploid, or 

 double, number of chromosomes in its cells. 



Among the ferns (which attained the size of great trees in former times) 

 and among the seed-bearing plants, the familiar and conspicuous generation 

 is the sporophyte, or spore- bearing, stage. In such types fertilization still 

 takes place in water, although the plants seem to be high and dry above the 

 soil. In most species of ferns the sexual generation, or prothallium, is rather 

 inconspicuous (see page 387). In fact, Linnaeus classed all seedless plants as 

 "cryptogamous" — that is, having hidden, or secret, marriage. That means 

 merely that in his time we did not know how conjugation, or fertilization, 

 takes place in the nonflowering plants, and we had only some guesses in regard 

 to the seed plants. Although we consider the ferns farther advanced than the 

 mosses, we find that both the egg-organs and the sperm-organs are borne on 

 the same individual. As we come to the more complex fio\^'ering plants, how- 

 ever, maleness and femaleness become more sharply differentiated. Yet the 

 gametophyte, or sexual, generation is so far reduced — especially in contrast 

 with the sporophyte — that we have found the actual structures and processes 

 only in modern times. 



Among some of the lower orders of animals there are many species in which 

 each individual bears both eggs and sperms. Such animals are sometimes 

 spoken of as hermaphrodite, after a mythical Greek character having both 



386 



