152 CYTOLOGY chap, v 



Lepidoptera (Phragmatohia — Seller, 1913); Hymenoptera {Osmia cor- 

 nuta — Armbruster, 1913). 



The chromosome numbers in prophase are as follows in these cases : 



Spermatogonia. Spermatocyte I. Spermatocyte II. 



The only cases in which we have any further information as to the 

 behaviour of these secondarily united chromosomes are Phragmatohia 

 and Gallus. In these the secondary spermatocyte pairs are resolved into 

 their elements (though only exceptionally in Gallus) before the second 

 meiotic division takes place. It is to be presumed that in the other 

 cases mentioned they also separate before syngamy. 



Another instance of pairing between chromosomes in a haploid 

 nucleus is found in the spermatogonia of the bee (see p. 91, footnote). 

 In the female bee, according to Nachtsheim (1913) the 32 chromosomes 

 unite into 16 bivalents in the oogonia. In oocyte I. they pair a second 

 time to form 8 tetravalents. The mature e^g has therefore 8 bivalents, 

 which, however, fall apart into univalents about the time of fertiUzation. 



At present the above cases cannot be said to fit without forcing into 

 any general scheme of chromosome behaviour. Probably, however, 

 fusion of the chromosomes should be regarded, not as a pairing strictly 

 speaking, but as a general tendency to fusion which usually stops short 

 at fusion in couples. Cutler (19 18) found that in the pheasant, where 

 2n is probably 20-22, the number of chromosomes in the first meiotic 

 division is lo-ii, while in the secondary spermatocytes 1-8 masses of 

 chromatin appear, indicating a tendency to fusion carried to various 

 degrees. Even in Gallus, though the fusion in couples is usually complete, 

 giving 4 chromosome masses in secondary spermatocytes which lack the 

 X-chromosome, and 5 chromosome masses in those which contain it, 

 yet metaphases II. with six or seven such masses are not unusual. 



^ Two are sex chromosomes. Other workers, however, have given the number of chromo- 

 somes in man, in whom they are very difficult to count, in varying figures up to 40. 



2 One is a sex chromosome. 



^ One is a sex chromosome. The exact number of chromosomes was, however, impossible 

 to determine beyond doubt. In view of Guyer's more recent conclusions as regards Gallus 

 it seems probable that there are two sex chromosomes, with a total of 18 chromosomes. 



^ Two are sex chromosomes. 



^ Spermatogonia! number inferred from the number of bivalents in meiosis. 



® This is the haploid number, the case being parallel with that of the bee (p. 91). 



