240 



INTRODUCTION TO CYTOLOGY 



Some of the above named investigators, notably Miss Digby (1910, 

 1912, 1914, 1919), Miss Fraser (1914), and Miss Nothnagel (1916), 

 have laid emphasis upon the view that the split seen in the early hetero- 

 typic prophase has its origin in the telophase of the last premeiotic divi- 

 sion, each chromosome persisting through the intervening resting stage 

 in the double condition. It is consequently held, as fully stated by Miss 

 Digby (1919) in her account of the archesporial and meiotic phases of 

 Osmunda (see Fig. 92), that the lateral pairing of thin threads in the 



Fib. 92. Diagram showing behavior of chromosomes in premeiotic and 



meiotic phases in Osmunda, according to Digby (1919). 



a, split which originates in telophase of premeiotic mitosis, persists (though obscured at 

 times) through heterotypic prophases, reappears in heterotypic anaphase, and becomes 

 effective in homceotypic mitosis, b, split which originates in heterotypic telophase, 

 persists obscured through homceotypic prophases, reappears in homceotypic anaphase, 

 and becomes effective in post-homceotypic division, x, plane of conjugation. 



heterotypic prophase which the advocates of Scheme A have regarded 

 as a conjugation of entire chromosomes is in reality only the reassocia- 

 tion of the two halves of one chromosome which had been split in the 

 preceding telophase. Such a reassociation is thought to occur in every 

 prophase, somatic and heterotypic, since these workers regard chromo- 

 some splitting as regularly a th'lophasic phenomenon. The split which 

 forms in the last premeiotic telophase functions in the homceotypic 

 mitosis: the homceotypic division is therefore looked upon as the continua- 

 tion of the premeiotic division, the heterotypic mitosis being an inter- 

 polated process bringing about numerical reduction. Not only does this 

 premeiotic split reappear in the anaphase of the heterotypic mitosis to 

 function in the homoeotypic, but a new split develops in the heterotypic 



