Page:Popular Science Monthly Volume 85.djvu/133

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129
THE CELLULAR BASIS OF HEREDITY

Chiefly because these two kinds of spermatozoa occur in equal numbers McClung in 1902 concluded that this accessory chromosome was a sex determinant. In 1905 Wilson discovered in a number of bugs that while there were two types of spermatozoa, one of which contained, and

PSM V85 D133 Diagrams of sex differentiation in the bug protenor.png

Fig. 36. Diagrams of Sex Differentiation in the Bug, Protenor. The oocyte contains 6 chromosomes and the spermatocyte 5 chromosomes which are not yet united into synaptic pairs; the "sex" chromosomes are shown in black and white, two are present in the oocyte, but one is present in the spermatocyte. In the reduction division the synaptic pairs separate, giving rise to two types of spermatozoa, one of which has the sex chromosome and the other lacks it; all ova are alike in this regard. If an egg is fertilized by a sperm without the sex chromosome a male results; if fertilized by a sperm containing the sex chromosome a female results. (After Wilson with modifications.)

 

PSM V85 D133 Diagrams of sex differentiation in the beetle tenebrio.png

Fig. 37. Diagrams of Sex Differentiation in the Beetle, Tenebrio, showing 5 synaptic pairs of chromosomes (there are actually 10 pairs); in the oocyte all pairs are equal in size; in the spermatocyte one pair is unequal. These pairs separate in the reduction division giving rise to two types of spermatozoa and one type of ova; eggs fertilized by one type of sperm give rise to females, those fertilized by the other type give rise to males. (After Stevens with modifications.)

 

the other lacked, the accessory chromosome, there was only one type of egg, since every egg contained the accessory chromosome, and he pointed out that if an egg were fertilized by a sperm containing an accessory, two accessories would be present in the zygote, this being the condition