FERTILIZATION 



115 



stimulation, and echinoderms have since been the favored types in such 

 experimentation. Since that time successful experiments have been 

 carried out with annelids and mollusks, and also with fishes, frogs and 

 rabbits, none of which develop parthenogenetically in nature. Several 

 types of stimuli — mechanical, thermal, and chemical — have been found 

 to be effective. The adult condition has been attained in but few cases. 

 In animals as high as frogs and rabbits, however, the young have devel- 

 oped into adults. 



141. Steps in Fertilization. — Usually the whole sperm cell enters the 

 egg cell, but in some cases more or less of the tail is left outside and there 

 enter only a nucleus, the centrioles, and a very httle cytoplasm. The 



Fig. 47. — Diagrams showing the successive steps in the fertilization of the egg cell of a 

 round worm, Ascaris, which matures after the entrance of the sperm cell. {From Wilson, 

 ''The Cell," by the courtesy of The Macmillan Company.) A, the entrance of the sperm cell; 

 the egg cell is in the condition of a primary oocyte. B, the formation of the first polar body ; 

 development of a sperm aster. C, the matured egg cell, with the polar bodies; the male 

 pronucleus has increased in size; from the one centriole has developed two, each with an 

 aster, and a spindle lies between them. D, the two pronuclei, now about equal in size and 

 each containing chromosomes, meet on the spindle. E, a pause corresponding to that in 

 Fig. 46£. F, first cleavage division. 



nuclei of the two cells, which are now called, respectively, the male and 

 female 'pronuclei, may, if both are mature, at once approach and fuse. 

 In this case cell division follows after a time (Fig. 46). On the other 

 hand (Fig. 47), the entrance of the sperm cell may take place before the 

 egg cell has attained the necessary maturity, in which case the male 

 pronucleus remains at one side until the maturation of the egg cell is 

 complete and undergoes a slow growth in size by absorbing the fluid 



