GENERAL ZOOLOGY 



Perivitelline space 



Fig. 5.11. Amphimixis and early cleavage in Parascans equorum (Ascaris megalocephala bi- 

 valens), a nematode. A, the male and female pronuclei have approached one another as the 

 mitotic spindle for the first cleavage is formed; each contains two chromosomes, the haploid 

 number. B, the four chromosomes (diploid number) at the equatorial plate of the first 

 mitotic spindle, seen from the side. C, the four chromosomes at the equatorial plate of the 

 first mitotic spindle, seen from one end of the spindle. Z), a two-cell stage in which both 

 nuclei are in the late prophase of mitosis (the second cleavage). All figures x640. 



Fertilization. The union ol an egg and spermatozoon is known as 

 fertilization, or syngamy, and the resulting cell, which has the capacity to 

 develop into a new individual, is called a zygote. Two separable and very 

 important phenomena, which were recognized almost immediately, are ob- 

 served during fertilization. In 1875 Hertwig appreciated the fact that it is 

 the union of spermatozoon and egg that stimulates the egg to begin its 

 development; this aspect of fertilization is known as activation. Not until 

 1883, when Van Beneden studied fertilization in Parascans, was the additional 

 significance of the fusion of cells from two parents recognized. Fol had 

 observed meiosis in 1875, but Van Beneden had much more favorable ma- 

 terial and could see that the nuclei of spermatozoon and ovum contributed 



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