GENERAL EMBRYOLOGY. 147 



characteristic feature of the ripe egg capable of fertihzation. 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 lilce 

 rudimentary eggs. The name directive corpuscles was given to 

 them because in very many cases their position renders possible a 

 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 develojj- 

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

 end the vegetaiive pole. 



Relation between Maturation and Fertilization. — In many cases the 

 maturation takes place before tlie 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 tlie 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 Ijeginning of fertilization led for a long time to 

 tlie 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 tiie 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 liand, special expressions are necessary 

 fur 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 

 cojntlation. In case of many marine animals, particularly most 

 lisbes, echinoderms, co^lenterates, the eggs and the spermatozoa 

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

 tii.in i<r fecundation) depends upon chaiu'e. Oite 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 ri}ie 

 fishes the eggs may bo collected in one dish, the s])crm in anotlier, 

 ami the contents of the latter jioured f'Vcr tlie former, and thus 

 in numy cases an entirely normal develo2)iiient may be (ibtained. 

 Such a proceeding is called artificial impregnation. 



