GENERAL EMBRYOLOGY. 175 



among the echinodcrms and the higher animals, it does not 

 occur at all. On the contrary, sexual reproduction appears 

 among the Protozoa only in its first beginnings; it grad- 

 ually supplants the asexual mode among the coclcntcrates 

 and worms, and predominates among the echinodcrms and 

 higher animals. Parthenogenesis occurs, we may say, in- 

 terpolated into the region of sexual reproduction chiefly 

 among the arthropods, more rarely among the worms. The 

 manner in which this takes place, the parthenogenetic mode 

 of reproduction in addition to the strongly sexual mode, is 

 a more certain proof that here originally in all cases a nor- 

 mal fertilization prevailed, and only special conditions of 

 life have brought it about that the eggs have acquired 

 the capacity to develop without spermatozoa. For many 

 species it has been conclusively shown that the purpose of 

 parthenogenesis is to render possible a rapid increase of the 

 species because of the scarcity of males. So long as par- 

 thenogenesis prevails, bark-lice and ivatcr-fleas distribute 

 themselves with quite extraordinary rapidity over the area 

 accessible to them, while the appearance of males occasions 

 a slower increase. 



GENERAL PHENOMENA OF SEXUAL REPRODUCTION. 



In sexual reproduction, a series of developmental pro- 

 cesses is observed which is repeated in an essentially similar 

 manner in all multicellular animals, and hence these should 

 be spoken of here together. They are: (i) the maturation 

 of the egg; (2) the process of fertilization; (3) the process 

 of cleavage ; (4) the formation of the three germ-layers. 



i. Maturation. 



The egg with the large vesicular nucleus, the germinal 

 vesicle, as we have learned to call it in histology, cannot 

 yet be fertilized ; to render it capable of fertilization it must 

 undergo a series of changes the process of maturation 

 (ripening), which consists in the replacement of the germinal 

 vesicle by a very much smaller egg-nucleus, and simulta- 



