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Heredity and Environment 



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. 51 B). 

 Later, in the reduction division, when the synaptic pairs sep- 

 arated, 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. 51 C and D). 



Chiefly because these two kinds of spermatozoa occur in equal 

 numbers McClung in 1902 concluded that this accessory chromo- 

 some 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 the other lacked the accessory 

 chromosome, there was only one type of egg, since every egg con- 



Qocyte Reduction 



Division 



Protenor Type 

 Ova 



Spermatocyte Reduction Spermatids 



Di vision 



Fig. 53. Diagrams of Sex Determination 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, two are present in the oocyte, but only one in the sperma- 

 tocyte. In the reduction division the synaptic pairs separate, giving rise to 

 two types of spermatids, 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.) 



