Page:Popular Science Monthly Volume 85.djvu/134

130 of the female, while if it were fertilized by a sperm without an accessory there would be present in the zygote only the accessory derived from the egg (Fig. 35 E and F, Fig. 36).

In other cases Miss Stevens as well as Wilson discovered that two accessory chromosomes, differing in size, might be present in the male whereas in the female they are of equal size (Fig. 37). In such cases two types of spermatozoa are produced in equal numbers, one containing a large and the other a small accessory chromosome, whereas every egg contains one large accessory chromosome. If such an egg is fertilized

 Ascaris. The sex chromosomes are here joined to ordinary chromosomes, there being two in the egg mother cell and one in the sperm mother cell. All eggs contain one of these sex chromosomes, while half of the spermatozoa have it and half do not. Eggs fertilized by one type of sperm produce females, those fertilized by the other type produce males. (From Wilson.)

by a sperm containing a large accessory (the X chromosome) it gives rise to a female, if by a sperm containing a small accessory (the Y chromosome) it gives rise to a male (Fig. 37).

In other animals one may not be able to distinguish separate X or Y chromosomes and yet such structures may be joined to one or two ordinary chromosomes. This is the case in the thread worm, Ascaris (Fig. 38), where two such accessory elements are present in the female, each being joined to the end of an ordinary chromosome, whereas in the male only one such element is present. Here also two classes of spermatozoa are found one with and the other without the accessory element, whereas all ova have this element, and in this case also sex is probably determined by the type of spermatozoon which enters the egg (Fig. 38).

Even in man sex is determined in the same manner according to several recent investigators. There are in the spermatogonia of man 47 chromosomes, accordingly to Winiwarter, one of which is the X or accessory chromosome (Fig. 39 A). These unite in synapsis into 23