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THE POPULAR SCIENCE MONTHLY 



of the female, while if it were fertilized by a sperrn 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 



.4 scoria Type 



KcdvtUon 

 Division 



Malare Egg and 

 Polar Body 



Fertilized 

 Egg 



6 



Spe ran s 



Fig. 38. Diagrams of Sex Diffeeentiation in the Thread Worm, 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 fer- 

 tilized by one type of sperm produce females, those fertilized by the otner type pro- 

 duce 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 sperm- 

 atozoa 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 



