tional egg while the other three are small rudimentary eggs which are called polar bodies and which take no further part in development (Fig. 23, D-F). The fertilization of the egg usually takes place coincidently with the formation of the polar bodies—and so we come back once more to the stage from which we started, thus completing the life cycle.
4. Sex Determination
In the formation of the sex cells one can frequently distinguish at an early stage, differences between the larger oogonia and the smaller and more numerous spermatogonia; this difference is the first visible distinction in the development of the two sexes. In the case of the human embryo this distinction can be made as early as the fifth week, and it is evident that the real causes of this difference must be at a still earlier period of development.
The cause of sex has been a favorite subject of speculation for thousands of years. Hundreds of hypotheses have been advanced to explain this perennially interesting phenomenon. The causes of sex determination have been ascribed to almost every possible external or internal influence and the world is full of people who think they have discovered by personal experience just how sex is determined. Unfortunately these hypotheses and rules are generally founded upon a few observations of selected cases. Since there are only two sexes the chances are that any hypothesis will be right half the time, and if only one forgets the failures of a rule and remembers the times when it holds good it is possible to believe in the influence of food or temperature or age, of war or peace or education on the relative number of the sexes, or on almost any other thing. By statistics it has been shown that each of these things influences the sex ratio, and by more extensive statistics it has been proved that they do not.
This was the condition regarding the causes of sex determination which prevailed up to the year 1902. Immediately preceding that year it had been found that the kinds of spermatozoa were formed in equal numbers in certain insects; one of these kinds contained a peculiar "accessory" chromosome, and the other lacked it. The manner in which these two types of spermatozoa were formed had been carefully worked out by several investigators without any suspicion of the real significance of the facts. It was shown that an uneven number of chromosomes might be present in the spermatogonia of certain insects and that when maternal and paternal chromosomes united in pairs in synapsis one "odd" chromosome was left without a mate (Fig. 34 B). Later, in the reduction division, when the synaptic pairs separated, the odd chromosome went entire into one of the daughter cells, and the spermatozoa formed from this cell contained one chromosome more than those formed from the other daughter cell (Fig. 34 C and D).
