FERTILIZATION 



113 



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

 experimentation. Since that time successful experiments have been 

 carried out with invertebrates other than echinoderms, especially annelids 

 and mollusks, and with fishes and frogs, none of which develop par- 

 thenogenetically 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 an animal as high as 

 the frog, however, tadpoles were reared which metamorphosed into adults. 

 139. 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 

 enters only a nucleus the centrioles, and a very little cytoplasm. The 



B 



Fig. 44. — Diagrams showing the successive steps in the fertilization of the egg cell of a 

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

 "The Cell," by the courtesy of The Macmillan Coinpany.) 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 Ijody ; 

 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. 43iJ. 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. 43). On the other 

 hand (Fig. 44), 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 



