GENERAL EMBRYOLOGY. 165 



Heteronomy and Homonomy. The distinction be- 

 tween heteronomy and homonomy is of extreme physio- 

 logical interest. The more different the segments of an 

 animal become the more dependent they are upon one an- 

 other in order to be able to function normally ; so much has 

 the whole become unified that the single parts can live only 

 while the continuity is maintained. On the contrary, if the 

 connection between the parts is less intimate, they are 

 more similar, and the more able to exist after separation 

 from one another. This is most beautifully shown in 

 instances of mutilation. It has been observed that when 

 many species of Luinbricidce are cut in two each part not 

 only lives on by itself, but it even regenerates the part 

 which is lacking; if, on the other hand, the same thing is 

 done to a heteronomously-segmented animal, either death 

 immediately ensues, as in the case of the higher vertebrates, 

 or the parts live for a short time a hopeless existence, as 

 can be seen in the case of frogs, snakes, insects, etc. In 

 metamerism a phenomenon is repeated which obtains widely 

 in the animal kingdom, and contributes towards its higher 

 development ; first, there is a reduplication of parts, then, 

 a division of labor, so that the final result is a whole 

 composed of many parts, yet uniformly organized. 



II. GENERAL EMBRYOLOGY. 



Origin of Organisms. Since every development begins 

 with an act of generation, the ways by which new organisms 

 may arise should be mentioned first in this chapter. If we 

 wish to limit ourselves to that which has been actually ob- 

 served, we must still cling to the old saying of the re- 

 nowned Englishman Harvey, " Omne vivum ex ovo," and 

 modifying it somewhat say, Omne vivum e vivo : that every 

 living organism is derived from another living organism. 

 We must limit ourselves to the mode of origin which has 



o 



been termed tocogony, or generation by parents. The 

 great importance which the question of generation without 

 parents, or spontaneous generation, has obtained through 



