. OOGENESIS AND EARLY EMBRYOLOGY ASCARIS 561 



male pronucleus. These chromatic bodies develop directly into 

 the ellipsoidal chromosomes (fig. 61), which later (figs. 62 and 

 63) assume the characteristic dyad-like form. 



3. Fertilization 



The pronuclei, as is the case in A. megalocephala (Vejdovsky 

 '12; Zacharias, '12; et al.), do not ordinarily fuse before the first 

 segmentation division, although occasionally they may do so 

 (fig. 63). Ordinarily the walls of the two pronuclei break down 

 first on their adjacent faces (fig. 61), and afterwards on the 

 sides directed toward the poles of the spindle (fig. 62), the chro- 

 mosomes meanwhile grouping themselves into the equatorial 

 plate and the spindle fibers making their appearance (fig. 64); 

 but in the cases where fusion takes place, only the adjacent 

 portions of the nuclear membranes break down, the two nuclei 

 thus uniting into one 'fusion' nucleus, which contains either 

 thirty-six (fig. 63), or thirty dyad-like chromosomes, depending 

 upon the character of the sperm which entered the egg. 



The process described above is essentially the same as that 

 observed in A. megalocephala by Zacharias ('12) and the ma- 

 jority of other workers. According to Zacharias, however, the 

 formation of either a 'fusion nucleus' or a 'fusion spindle,' by 

 the two pronuclei does not constitute fertilization. If one de- 

 fines the term fertilization as meaning the intermingling of the 

 parental constituents of the nucleus by a process of actual fusion 

 of the chromosomes, then the conditions observed in A. canis 

 do not warrant the statement that fertilization has taken place 

 before the first segmentation division occurs. If this is the cor- 

 rect definition of fertilization, — as Zacharias ('12) and others 

 maintain, — then in A. canis, as well as in A. megalocephala, true 

 fertilization occurs only in the 'resting nuclei' of the daughter 

 cells of the first cleavage. It is at this time that the individu- 

 ality of the chromosome is first lost, and it is during the stages 

 of the subsequent reorganization of each daughter nucleus that 

 the opportunity^ is afforded for a complete interchange of such 

 hereditary characters as are borne by the chromosomes. In 

 case qualities are carried by the non-chromatic portions of the 



JOURNAL OF MORPHOLOGY. VOL. 30, N'O. 2 



