and is usually many thousands of times smaller than the egg. In most animals, and in all vertebrates, it is an elongated, thread-like cell with an enlarged head which contains the nucleus, a smaller middle piece, and a very long and slender tail or flagellum, by the lashing of which the spermatozoon swims forward in the jerking fashion characteristic of many monads or flagellated protozoa." 4

Its tail

"At the time of fertilization, when the spermatozoon touches the surface of the egg, the egg pushes out a cone of protoplasm at the point of contact, and, lending a helping hand, as it were, draws it into the egg. In a few minutes the head of the sperm has entered. is often left outside. The head absorbs fluid from the egg and becomes the sperm nucleus, which passes towards the center of the egg. Here it comes to lie by the side of the egg nucleus, and the two fuse. The walls of the combined nuclei dissolve and the chromosomes appear. Half of these are derived from the father through the nucleus of the sperm, and half from the mother through the egg nucleus." 5

The chromosomes do not immediately fuse, and there is reason to believe that they keep their identity through life. The fertilized cell quickly begins to divide and thus inaugurates the growth of the individual. It is important to note that from each parent comes one-half of the necessary number of chromosomes and that throughout life every cell of the body thus draws one-half of its chromosomes from either sex. It is small wonder that biologists now are inclined to consider the chromosomes as "the carriers of heredity." Just what rôle is played by the cytoplasm of the cell is unknown but neither the nucleus

5 MORGAN, T. H. Heredity and Sex, p. 39 ff.

nor the cytoplasm can long function or exist without the other nor can the one create the other. Moreover, the cytoplasm of different animals is as distinct as are the chromosomes.

The fact that the sperm introduces half of the necessary chromosomes indicates that it does more than stimulate the development of the egg. Inasmuch as both sperm and egg originally possess the full number of chromosomes we are puzzled to explain the necessity of sex. Why, for example, may not an egg develop into a complete organism? As a matter of fact such development (parthenogenesis) occurs rarely in nature and has been brought about in the laboratory by the use of certain salts. The eggs of worms, mollusks and even frogs have been stimulated into growth. In some cases the nucleus of the egg has been replaced by the nucleus of the sperm and the cell made to grow. So far as I know none of those has been brought to maturity but whether this is due to some failure to provide for them properly or to some inherent weakness resulting from the lack of fertilization is not known. Among the bees the unfertilized eggs regularly produce males, the fertilized eggs as regularly producing females. Recognizing frankly then that there are many unsolved questions and many apparent contradictions in the present state of knowledge there is a growing opinion that the trail has been found which must ultimately lead to an understanding of the process of reproduction.

In the nineties a Dutch botanist, Hugo de Vries, found some plants of the evening primrose (Enothera Lamarckiana) which had escaped from a neighboring garden and were growing wild. He began to cultivate them and found, to his surprise, that although they had a common ancestry certain new types appeared. One very large

form appeared three times though its ancestors for three generations had been the ordinary Lamarckiana. In 1897 the self-fertilized seeds from these produced over 450 plants which, with one exception, were like the giant form. The exception is best described as a dwarf whose seeds reproduced the dwarf form. The commonest new type produced by Lamarckiana was one with red veins and brittle stems, which appeared 66 times and also bred true. Here, then, were new species apparently springing into existence in a single generation and breeding true thereafter. The existence of these sudden variations had long been recognized under the name of "sports," but it had been assumed that they quickly reverted to the old type. But the primrose did not revert. Hence De Vries came to the belief that the process of evolution had been a series of sudden changes or mutations rather than the slow unbroken curve pictured by Darwin. The contrast in the two viewpoints may be illustrated by the following diagram.

CONTINUOUS VARIATION

DISCONTINUOUS VARIATION

From the standpoint of final result there is no difference. The discovery of the chromosomes, however, makes it possible for us to guess at the cause of sudden variations and to explain their persistence.

Gregor Mendel (1822-1884), an Austrian monk, in 1866 published in an obscure journal an account of a

long series of experiments he had made with sweet peas. Few students had even heard of this work and it had been ignored by Nägeli, the only prominent botanist to whom it was known. In the light of later discoveries, it suddenly assumed great importance. In the year 1900 the scientific world was surprised to learn that three botanists, De Vries, Correns and Tschermak, working independently and in different countries had come to similar conclusions and had all rediscovered the earlier studies of Mendel.

Mendel found that instead of thinking of the entire plant as a unit he had to consider each plant as made up of various units. He found in the pea seven of these unit characters on which he could depend: (1) the form of the ripe seeds; (2) the color of the substance of the pea; (3) the color of the seed-coat; (4) the form of the ripe pods; (5) the color of the unripe pods; (6) the position of the flowers and (7) the length of the stem. Moreover these characters appeared to go in pairs; the seeds were either smooth or wrinkled, in color they were either yellow or green. If two plants differing with reference to any of these points were crossed all the individuals of the next generation showed one, and only one, of the two characters. To the character that appeared he gave the name dominant," to the one that disappeared, the name recessive." Thus Mendel made 58 crosses on ten plants and found that yellow was dominant over green in every instance. He found that the smooth, round seed coat was always dominant over the wrinkled. Here was a puzzle. What had become of the character that did not show? Why was it that all the seeds showed only one character? These hybrid plants are allowed to selffertilize, and behold 8,023 seeds, of which 6,022 were yellow and 2,001 were green. This it will be noted is a

66

Five hundred and nineteen

ratio of about three to one. of the yellow coated seeds were allowed to grow and again self-fertilize. It was found that 166 produced yellow only, while 353 produced both yellow and those that could produced yellow and green. The hybrids of the round and wrinkled sorts produced 5,474 round and 1,850 wrinkled; those of the tall and short varieties, 787 tall and 277 short; in both cases approximately 3 to 1.

Correns found that if he crossed the white variety of the "four o'clock" with the red variety all the hybrids were pink but that self-fertilized seeds from these pink produced white, pink and red flowers in the ratio of 1:2:1. At first sight this seems to be different from the results obtained by Mendel but in reality it is the same, the principle being brought out more clearly by the fact that the hybrid is of different color than the pure dominant. Before seeking the explanation let us examine a few other

COLOR INHERITANCE IN GUINEA PIGS

If a black guinea pig of black ancestry be mated with a white of white ancestry all the offspring will be black. If this hybrid generation is interbred the next generation will show three black and one white. This white mated with white never again produces black. The black will be found like the second hybrid generation of Mendel's peas. One-third will produce only black if mated with pure black, while the other two-thirds if interbred will