276 



INTRODUCTION TO CYTOLOGY 



germinal vesicle undergoes no change unless the spermatozoon has 

 entered the egg. 



Fig. 106. — Maturation ' and fertilization in Ascaris. 

 A, spermatozoon about to enter egg. B, spermatozoon inside; first maturation mitosis 

 in progress. C, first maturation mitosis completed; first polar body budded off. D, 

 second maturation mitosis, forming second polar body; sperm nucleus below. E, male and 

 female pronuclei, each with 2 chromosomes, meeting. F, first cleavage mitosis, showing 2 

 paternal and 2 maternal chromosomes. (After Hertwig.) 



Fig. 107. — Fertilization in Physa (snail.) Sperm head and amphiaster at right, with 

 long fiagellum extending toward left. Second maturation mitosis in progress. (After 

 Kostanecki and Wierzyski, 1896.) 



The Fusion of the Gametes. — In most cases the whole spermatozoon 

 enters the egg (Fig. 107). In some sea urchins only the head and middle 

 piece enter, while in Nereis the head alone passes in, the middle piece 



