The Bearing of Cytological Research on Heredity. 337 



of chromosomes, to say nothing of paternal and maternal homologies, has 

 been obstinately contested ; it must be admitted that the proof is still far 

 from complete for the chromosomes generally. Nevertheless, in spite of all 

 scepticism, the drift of later research has been, I think, steadily in its 

 favour. Both in plants and in animals the diploid nature of the chromo- 

 some groups in the somatic cells is often clearly visible to the eye, owing to 

 conspicuous size-differences among the chromosomes. In such cases, as was 

 first urged especially by Montgomery and by Sutton, the chromosomes may 

 be sorted out into pairs according to their size. In a few cases, of which the 

 Diptera offer the most striking examples, the sorting out is performed by 

 nature, all the chromosomes being actually grouped side by side in pairs 

 according to their size (fig. 1) * The conclusion here becomes highly probable 

 that each pair includes a maternal and a paternal member, and that these are 

 destined to conjugate in synapsis. In the case of the sex-chromosomes, to 

 which I shall return, the probability becomes a certainty. 



d e f 



Fig. 1. — Exact drawings of the diploid chromosome groups in various Diptera, showing 

 the chromosomes grouped in pairs ; a, b, c, e, from Stevens ; d, f, from Metz. 



a, Calliphora vo?nitaria, <? ; b, the same, ? ; c, Sarcophaga sarrcccina, $ : d, Droso- 

 phila amcena, <$ ; e, D. ampelopkila, ? ; /, D. funebris, ? . 



* These facts were illustrated by photographs of the chromosomes in Drosophila and 

 Musca, from preparations by C. W. Metz, who has for some time been engaged with this 

 problem in my laboratory. 



