MEIOTIC PHASE IN MARSUPIALS 201 



EXPLANATION OP PLATES 12, 13, AND 14. 



The scale given on PI. 12 applies to all the figures except 

 1 9 and 38, which are on a smaller scale. 



Lettering. 

 p, pre-pachytene plasmosoine. Px, plasmosome which appears at the 

 time that the sex chromosomes are uniting, x, Y, the sex chromosomes. 



Figs. 1-22, Macropus ualabatus; Figs. 23-38, Petauroides 



V o 1 a n s . 



Plate 12. 

 Fig. 1. — Resting spermatogonium. 



Fig. 2. — Metaphase, spermatogonia! mitosis, eleven chromosomes. 

 Fig. 3. — ^Metaphase, spermatogonia! mitosis, twelve chromosomes. 

 Figs. 4, 5. — Metaphase, ovarian follicle cells, ten chromosomes. 

 Fig. 6. — Late leptotene nucleus. 

 Fig. 7. — Synizesis. 



Fig. 8. — Early pachytene nucleus showing condensation of X. 

 Fig. 9. — Later stage, showing pairing of X and Y. 

 Fig. 10. — Late pachytene nucleus. 

 Fig. 11. — Early diplotene nucleus. 



Fig. 12. — The chromosomes of an early diplotene nucleus shown 

 separately. 



Fig. 13. — Contracting bivalents in two adjacent cells. 



Plate 13. 



Figs. 14, 15, — Two metaphases of the first meiotic division, to show 

 modes of attachment of XY to an autosome. 



Figs. 16 a, b. — Autosomes, with attached XY bivalent, from more 

 advanced metaphases, to show separation of X and Y. 



Fig. 17. — Anaphase of first division. 



Fig. 18. — Pair of young secondary spermatocytes, still connected by the 

 spindle remains, one with large compact chromatic body, the other without. 



Fig. 19.— A group of secondary spermatocyte nuclei to show the 

 dimorphism. Half with large and half with small chromatic body (pre- 

 sumably X and Y). 



Fig. 20. — Early prophase of a second division with the X-chromosome. 



Fig. 21. — Anaphase of a second division with the Y-chromosome. A 

 small cyphlasmic inclusion is shown at the bottom right-hand corner. 



Fig. 22. — Anaphase of a second division with the X-chromosorae 

 (indistinguishably fused with an autosome). To avoid overlapping the 

 two groups^have^been slightly shifted laterally m drawing. 



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