FERTILIZATION 23 



term. From these facts Conklin and Loeb argue that.the cytoplasm is really the embryo 

 in the rough, the nucleus, through Mendelian heredity, adding only the finer details. 

 Morgan, among others, refuses to admit the validity of this interpretation. 



The Determination of Sex. The assumption that the chromosomes are the carriers 

 of hereditary tendencies is borne out by experimental breeding (Morgan) and by the corre- 

 lated observations of cytologists on the germ cells of invertebrates, especially insects, and 

 of some vertebrates. According to Winiwarter (Arch, de Biol., T. 27, 1912) the nuclei 

 of human spermatogonia contain 47 chromosomes, while those of the oogonia contain 48. 

 When the reduction of chromosomes takes place in the male cells, one unpaired chromo- 

 some fails to divide and passes intact to one or the other daughter cells; hence half of the 

 spermatids contain 24 chromosomes, the other half only 23. All the oocytes and polocytes, 

 on the contrary, contain 24. There is thus one extra chromosome in each mature ovum 

 and in each of half the spermatozoa. This chromosome, because of peculiarities of size 

 or shape, can be identified easily in many animals, and is termed the accessory X,or sex 

 chromosome. McClung was the first tc assume that the X chromosome is a sex detriminant. 

 It has since been shown by Wilson and others that the sex chromosome carries the female 

 sexual characters. When, in the case under consideration, a spermatozoon with 24 

 chromosomes fertilizes an ovum, the resulting embryo is a female, its somatic nuclei 

 containing 48 chromosomes. An ovum fertilized by a sperm cell containing only 23 

 chromosomes (without the sex chromosome) produces a male with somatic nuclei con- 

 taining but 47 chromosomes. These observations of Winiwarter on man have not yet 

 been confirmed by other investigators. There is no reason to doubt, however, that sex 

 is determined in man essentially in the manner described, which agrees with the easily 

 observed phenomena in insects. 



In certain moths and birds the sex chromosome system is the exact reverse of the 

 common scheme just explained, but the operation of the mechanism is otherwise similar. 

 The spermatozoa of these forms are all alike in chromosome constitution while the eggs 

 are of two sorts. 



