SPECIAL PROBLEMS OF MEIOSIS 79 



consequently a univalent at meiosis). The X in the 

 male shows intense positive heteropycnosis in its distal 

 half (i.e. that farthest away from the spindle attach- 

 ment which is almost terminal) throughout zygotene 

 and pachytene. This heteropycnosis extends to the 

 proximal half during diakinesis and prometaphase, 

 after which de-condensation usually begins to set in. 

 The X- chromosome associates itself with the spindle 

 of the first meiotic division somewhere between the 

 equator and one of the poles. Its spindle attach- 

 ment does not divide at the first division and conse- 

 quently both the chromatid halves of the X go to 

 the same pole. Thus two kinds of secondary sperma- 

 tocytes are formed, one with, the other without an 

 X-chromosome. At the second division the X divides 

 and goes to both poles in those secondary sperma- 

 tocytes in which it is present. Thus in this case the 

 univalent X shows the first type of behaviour of the 

 four mentioned above. The opposite condition where 

 a single X divides in the first meiotic division and 

 goes undivided to one pole in the second division 

 is met with in some other insects (the Beetle Pho- 

 tinus and the Bugs Anasa tristis, Alydus and 

 Protenor)}^^^ ^^^' ^^^' ^^^ The Locust /SW^/^toc^rca and 

 the Bug Archimerus also illustrate the truth of the 

 third principle (that extreme positive heteropycnosis 

 may prevent either pairing or chiasma-formation or 

 both) since even in polyploid spermatocytes contain- 

 ing two or three X-chromosomes these behave as 

 univalents, although they lie very close to one 

 another, as a result of an attraction analogous to 

 ' somatic pairing ' I's, iss (jpjg jg)^ 



A more complicated type of sex-determining 

 mechanism is found in mammals, where both an 

 X and a Y are present in the male. In the Rat ^* 

 the two sex -chromosomes both have submedian 

 spindle attachments. There is a region in the Y 

 which is homologous to a corresponding region in the 

 X. This region includes the spindle attachments in 



