OOGENESIS AND EARLY EMBRYOLOGY ASCARIS 557 



('89) all deny the presence of a micropyle for the entrance of 

 the sperm into the egg in these ascarids. They all state that 

 the male and female pronuclei unite to form a fusion nucleus 

 before segmentation can take place. According to these authors 

 the male pronucleus is formed by a rearrangement of the ele- 

 ments of the sperm nucleus, and the female pronucleus is made 

 up of what remains of the egg nucleus after the two polar bodies 

 are given off. 



Marcus ('06) confirms for A. canis (?) the opinion of some of 

 the earlier writers that the sperm is amoeboid, and may enter the 

 egg at any point of its surface. Once within the egg, the refrac- 

 tive body, or what is left of it, breaks down and becomes lost in 

 the egg cytoplasm. The compact nucleus of the sperm in form- 

 ing the male pronucleus breaks up into the characteristic haploid 

 number (in A. canis (?), eleven) of dyad chromosomes. The 

 centrosome brought in by the sperm sets up an astral system in 

 the cytoplasm of the egg, and functions at the first segmentation 

 division. According to Marcus the two pronuclei, each with 

 eleven dyad chromosomes, fuse before the first cleavage of the 

 egg. At division each of the dyads so divides that each daugh- 

 ter nucleus acquires twenty-two dyads (eleven from each pro- 

 nucleus), which unite in pairs, making eleven tetrads. This 

 union of the dyads to form tetrads completes, in the opinion of 

 Marcus, the act of fertilization. The second cleavage shows the 

 process of 'diminution' in the soma cell, but not in the 'stem' 

 cell. The stem cells continue to show eleven tetrad chromosomes, 

 whereas the soma cells show twenty-two dyads. 



In the following account a careful study has been made of the 

 formation of the male pronucleus from the spermatozoon, and 

 of the female pronucleus from the inner half of the second mat- 

 uration figure. 



1. The formation of the male pronucleus 



The sperm in this species (A. canis) is mature at the time of 

 copulation, and nourishes itself up to the time of its entrance 

 into the egg by feeding on the nutriment accumulated in the 

 refractive body (Walton, '16 a). Depending upon the interval 



