570 A. C. WALTON 



is apparently dyad in nature, since it is constricted by the ' Quer- 

 kerbe' of Haecker. The present work on the oogenesis has 

 shown the method by which this type of chromosome is formed 

 in A. canis, and has, perhaps, also thrown some light upon the 

 nature of the 'Querkerbe.' 



Beginning with the last generation of the oogonia (fig. A, 1), 

 we find the chromosomes emerging from the clumped condition 

 of the chromatin at first in the form of ovoid bodies, which 

 almost immediately show indications of a transverse constric- 

 tion (fig. A, 2), the 'Querkerbe' of Haecker ('95, p. 586). In 

 the prophase of the first oocytes and spermatocytes, this dyad 

 chromosome exhibits a longitudinal split (fig. A, 3 a), thus be- 

 coming a tetrad in structure. A little later in the prophase of 

 the first oocytic division, the tetrads unite into pairs — the di- 

 tetrads — by a process of parasyndesis (fig. A, 4). The results 

 in the male are so similar to those in the female that there seems 

 no reason to doubt that the method of di-tetrad formation there, 

 also, was a case of parasyndesis. 



Careful study shows that in the di-tetrad chromosomes the 

 original plane of parasyndesis is clearly distinguishable from the 

 'Querkerbe.' It is the more apparent of the two constrictions 

 (fig. A, 5 b), and coincides with the long axis of the tetrad (fig. 

 A, 6 b; 7 a) resulting from the first oocytic division. At the 

 first division (fig. A, 6 a) the chromosomes separate along the 

 plane of the longitudinal constriction previously noted (fig. A, 

 3 a) . This division is therefore equational amd simply reduces 

 the quantity, not the qualities, of the chromatic material. In 

 the male there were found twenty-four tetrad autosomes and six 

 tetrad idiosomes before the pseudo-reduction — by parasynde- 

 sis — took place. After syndesis, there were twelve di-tetrad 

 autosomes and six tetrad idiosomes; the chromosomes of the 'X' 

 group (idiosomes) not uniting in pairs, as do the autosomes. 

 in the female, however, the twelve idiosomes ('2X' group), 

 as well as the twenty-four autosomes, unite in pairs, the result 

 being eighteen di-tetrad chromosomes. In the first spermato- 

 cytic division the 'X' group goes undivided to one pole, so that 

 for the autosomes the division is equational, whereas for the 



