MARSUPIAL SPEBMATOGENESIS 205 



spermatogonia! metaphase plates (PI. 15, figs, 2 and 3). In 

 the female there are the comparatively large autosomes (14) 

 and two much smaller chromosomes. These two smaller 

 chromosomes, similar in size and shape, are the sex chromo- 

 somes. The chromosome complex of the female is therefore 

 14 + XX. 



In the male counts were obtained from equatorial plates of 

 the dividing spermatogonia, and from first and second meiotic 

 division figures. The number of chromosomes obtained from 

 these stages was sixteen. In the spermatogonia! plates the 

 chromosomes are arranged in a circle around a central clear 

 space. Inside the circle of chromosomes a chromosome much 

 smaller than the others is seen ; also near this small chromo- 

 some a small chromatic dot is to be seen (PI. 15, figs. 2 and 3). 

 From their subsequent behaviour these are identified as the 

 X- and Y-chromosome respectively. The chromosome formula 

 of the male is therefore 14 + XY. 



Structure of Testes. 



The testes have the typical mammalian structure of numerous 

 convoluted tubules. Close to the wall of the tubules are 

 situated the Sertoli cells, spermatogonia, and the early meiotic 

 prophases. Passing in towards the lumen of the tubule, the 

 later stages of the maturation divisions occur, leading up to 

 the formation of the spermatozoa. These are found nearest 

 the lumen of the tubule. There appears to be a definite layering 

 of the cells of the different stages, although some overlapping 

 of the inner layers occurs. 



Spermatogonia. 



The nuclei of the early spermatogonia are oval in shape, and 

 are frequently lobed. The nuclei of the later generations of 

 spermatogonia are much smaller and are approximately circular 

 in shape. In the resting condition the spermatogonial nucleus 

 contains a well-defined nucleolus which stains very densely with 

 the iron haematoxylin. This is evidently of the nature of 

 a plasmosome impregnated with chromatin. The chromatin 



