Development 43 



in such a way as to cause the discharge of both sexual 

 products simultaneously. In nature, as they lie close to- 

 gether, such a stimulus as a rise in temperature probably 

 brings about the same result, and it is quite possible that 

 few ova fail to be found by spermatozoa. 



An ovum unites with a single spermatozoon only. 

 When the two cells finally meet, the activity of the flagel- 

 lum soon forces the nucleus or head into the body of the 

 ovum. The flagellum itself does not enter. Its function 

 being completed, it is cast off. 



In the American oyster a curious phenomenon appears 

 just at this time. The nucleus of the ovum divides and 

 a small cell containing half of it, separates from the 

 ovum. The process is soon repeated, and a second small 

 cell appears beside the first. These polar cells, as they 

 are called, are shown in Figure n, II to VI, and are des- 

 ignated by the reference-letter p. They have no part in 

 forming the body of the embryo, and though they remain 

 attached to it for some time, they finally are lost. The- 

 oretically their nature can be very satisfactorily ac- 

 counted for. The formation of polar cells has been wit- 

 nessed in most animal eggs. It precedes fertilization, 

 and is referred to as the maturation of the ovum. 



The male and female nuclei now lying within the 

 ovum, move toward each other, touch, and finally fuse 

 to form a single new nucleus. Once more we have an 

 ovum with its nucleus, but it is now a complex of male 

 as well as of female elements. This wonderful process 

 is still called fertilization, a term given to it when it was 

 supposed that the male cell simply stimulated or excited 

 the ovum to produce a new individual. We now know 

 that it has an equal part with the ovum in forming the 

 body of the young. 



