GENERAL EMBRYOLOGY. 



the first may have again divided (Fig. 89, df}. The part 

 of the directive spindle still remaining after the second 

 division becomes a resting vesicle-shaped nucleus, which is 

 called the egg-nucleus, the characteristic feature of the ripe 

 egg capable of fertilization. In other words, by a double 

 division there have been formed from the unripe 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 rudi- 

 mentary eggs. The name "directive corpuscles" was 

 given to them from the fact that 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 cor- 

 puscles. With reference to later processes of develop- 

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

 opposite end the vegetal pole. 



Relation between Maturation and Fertilization. In 

 most cases the maturation takes place either in the ovary 

 or at the beginning of the oviduct, previously to the en- 

 trance of the spermatozoon into the egg; in many animals, 

 on the contrary, there ensues a pause after the first polar 

 body has been formed ; the egg then requires the penetra- 

 tion of a spermatozoon in order to complete the further 

 changes, i.e., the cutting off of the polar bodies and recon- 

 struction of the egg-nucleus. This dependence of the last 

 phenomena of maturation upon the beginning of fertiliza- 

 tion led for a long time to the error that the formation of 

 the polar bodies formed a part of the fertilization process 

 itself. 



2 . Pert iliza t io n . 



Copulation and Artificial Fecundation. -- If one 

 wishes to use the term "fertilization" in the scientific 

 sense, he must limit himself to the intimate occurrences 

 which, after the meeting of the egg and spermatozoon, go 

 on in the interior of the former and result in a complete 

 fusion of the two sexual cells; on the other hand, he must 



