vi] THE INTERPRETATION OF MEIOSIS 89 



A different explanation of the occurrence of two successive 

 maturations, due to JANSSENS, is of such great theoretical 

 importance that it must be mentioned here, though it will 

 be referred to again later. JANSSENS (1909) maintains that 



FIG. 12. Diagram of chromosome division in somatic and in 

 maturation mitoses. 



a. Two chromosomes in metaphase, and b. in anaphase, of a somatic or 

 spermatogonial mitosis, c. Transverse section illustrating the doubling 

 in thickness and splitting of a chromosome before division. 



d. Bivalent chromosome of first maturation division, consisting of two uni- 

 valents, each longitudinally split. 



e. Anaphase of first maturation division, separating complete, split univalents 

 from each other. 



f. g. Metaphase and anaphase of second maturation division, separating longi- 



tudinal halves of the univalent. 



when the chromosomes twist round one another, as they 

 often do in strepsinema, they may fuse at the points of 

 contact, and when they separate parts of the two homo- 

 logous chromosomes may exchange places. He also gives 

 evidence that when the heterotype chromosomes split 



