GENERAL EMBRYOLOGY. 147 



characteristic feature of the ripe egg capable of fertilization. In 

 other words, by a double division there have been formed from the 

 immature egg four (sometimes three) cells, of which one has 

 received by far the greatest part of the original mass of the cell 

 and constitutes the ripe egg, while the others are small bodies like- 

 rudimentary eggs. The name directive corpuscles was given to. 

 them because in very many cases their position renders possible & 

 definite orientation of the egg; i.e., a diameter, the long axis, can 

 be passed through the egg, one end of which is marked by the 

 directive corpuscles. With reference to later processes of develop- 

 ment this end is called the animal pole of the egg, the opposite 

 end the vegetative pole. 



Relation between Maturation and Fertilization. In many cases the- 

 maturation takes place before the entrance of the sperm, either in the- 

 ovary or at the beginning of the oviduct ; in many animals, on the con- 

 trary, there ensues a pause after the first polar body has been formed ; the- 

 egg then requires the penetration of a spermatozoon in order to complete 

 the further changes, i.e., the formation of the second polar body and 

 reconstruction of the egg-nucleus. This dependence of the last phenomena 

 of maturation upon the beginning of fertilization led for a long time to- 

 the error that the formation of the polar bodies was a part of the fertiliza- 

 tion process itself. 



2. Fertilization. 



Copulation and Artificial Fecundation. The term ' fertiliza- 

 tion ' in the scientific sense refers to the internal processes which, 

 after the meeting of the egg and spermatozoon, go on in the 

 interior of the former and end with a complete fusion of the two 

 sexual cells; on the other hand, special expressions are necessary 

 for those preparatory processes whose purpose is to render fertiliza- 

 tion possible. Very often, but not in all cases, there is necessary 

 an active transfer of the sperm from the male to the female, a 

 copulation. In case of many marine animals, particularly most 

 fishes, echinoderms, ccelenterates, the eggs and the spermatozoa 

 are discharged into the water, and the union of these (impregna- 

 tion or fecundation) depends upon chance. One can bring about 

 then artificially what is accomplished by nature, by obtaining from 

 the sexual organs the ripe products and bringing them together. 

 For example, by suitable pressure upon the body of sexually ripe 

 fishes the eggs may be collected in one dish, the sperm in another, 

 and the contents of the latter poured over the former, and thus 

 in many cases an entirely normal development may be obtained. 

 Such a proceeding is called artificial impregnation. 



