OOGENESIS AND EARLY EMBRYOLOGY ASCARIS 545 



twelve being smaller than the remaining twenty-four. Study of 

 the corresponding stages of the primary spermatocytic chromo- 

 somes showed twenty-four large and six smaller tetrad-like 

 bodies. As the six small chromosomes were there (Walton, 

 '16 a) shown to be the heterochromosome group of the 'X' type, 

 and as the female is of the ' 2X' type, it does not seem illogical to 

 conclude that these twelve small primary oocytic, tetrad-like 

 chromosomes are the members of the heterochromosome group. 



The next step in the formation of the mature chromosomes is 

 one of pseudo-reduction, by which the number of chromosomes 

 is reduced to one-half the original number, that is to eighteen. 

 This is the haploid number, and appears before the maturation 

 divisions; for this reason the term pseudo-reduction, in the sense 

 of Gregoire ('10), is applied here to this process. The true re- 

 duction comes later, at the time of the maturation divisions, so 

 that the reduction of the prophase chromosomes, that is the 

 pseudo-reduction, is a reduction in number only. 



The tetrad chromosotnes begin to pair, possibly according to 

 their sizes, although no differences in size, except for the general 

 difference between the autosomes and the heterochromosomes 

 mentioned above, can be seen, and therefore no members of de- 

 finite pairs can be recognized before their union. The two 

 members of a pair (fig. 23) come to lie with their broad faces in 

 contact, that is with the planes of the two tetrads parallel to 

 each other. An achromatic bridge of connecting material is 

 built up between them. The union is thus one by parasyndesis, 

 and places A. canis in the class with A. megalocephala. 



Sabaschnikoff ('97) was the first to trace, in A. megalocephala, 

 the formation of the tetrad as the result of the conjugation of 

 two longitudinally split chromatin threads, though this had 

 previously been predicted by Boveri ('87) and O. Hertwig ('90). 

 Tretjakoff ('04 b) definitely proved their formation in A. megalo- 

 cephala, and contended that parasyndesis was the method of 

 their formation. In the later works of Boveri ('99, '09 a, '09 b) 

 and in those of Brauer ('03), Bonnevie ('08, '12), Vedjovsky 

 ('07, '12) and others, this view has been adopted. Griggs ('06) 

 also supports this explanation of the formation of the tetrads, 



JOURNAL OF MORPHOLOOy, VOL. 30, NO. 2 



