PARASYNDESIS AND TELOSYNDESIS 



47 



achromatic band, has indeed been cited in evidence (Montgomery, 1903 ; 

 Schellenberg, 1911). As, however, this break is frequently not in the 

 middle of the loop (Schellenberg), it cannot be taken as the point of 

 junction of the conjugating chromosomes, since the limbs of the diakinetic 

 and metaphase rings, loops, etc., are equal. There can indeed be little 

 doubt that these breaks are the transverse constrictions which develop 

 across the contracting chromosomes in Lepidosiren and elsewhere. It is 



FIG. 19. 



A-C, parasyndesis in Planaria gonocephala. (After Schleip, Zool. Jahrb. Anat., 1907.) D-F, parasyndesis 

 in Dytiscus marginalis (after Henderson, Z.w.Z., 1907) ; A, D, leptotene ; B, E, zygotene; C, F, diplotene 

 stages. 



characteristic that these transverse constrictions are often by no means 

 in the middle of the chromosomes (Fig. 65). 



This very short and incomplete summary of the arguments for and 

 against the only two schemes of meiosis which can lay any claim to 

 generality must suffice for the present. It is clear that parasyndesis 

 is the hypothesis accepted in this book, and we shall find, especially in 

 Chapters V. and VI., that we continually meet with observations as well 

 as experimental results which are readily intelligible on the assumption 

 that conjugation of the chromosomes takes place by parasyndesis, but 

 are quite inexplicable on the theory of telosyndesis. 



