192 GERM-CELL CYCLE IN ANIMALS 



as the tapeworm, to be able to form both male and 

 female germ cells, since it may at any time become the 

 only one of its species to occupy the alimentary canal 

 of a host. Hermaphroditism in such a case, however, 

 is of no benefit if self-fertilization is not possible. 

 Although there are thousands of hermaphroditic 

 species of animals there are comparatively few whose 

 eggs are known to be fertilized by spermatozoa from 

 the same individual. We must therefore distinguish 

 between morphological and physiological hermaphro- 

 ditism and recognize the fact that the former condi- 

 tion is much more prevalent than the latter. Among 

 the species in which self-fertilization normally occurs 

 are certain rhabdoccels, digenetic trematodes, ces- 

 todes, ascidians, and mollusks. Van Baer, in 1835, 

 claims to have observed self -copulation in the snail, 

 Lymncea auricularia; that is, an individual with its 

 penis inserted in its own female opening. That 

 species of this genus fertilize their own eggs has 

 frequently been stated by investigators. Frequently 

 the spermatozoa of an hermaphrodite are capable 

 of fertilizing the eggs of the same individual, 

 but penetrate more readily the eggs of other individ- 

 uals. Such is the case in the ascidian, dona in- 

 testinalis (Castle, 1896; Morgan, 1905). 



Both sorts of germ cells are seldom produced at 

 the same time by hermaphrodites. Those species 

 in which spermatozoa mature first are called protan- 

 dric ; this is the usual condition. In a few cases 

 eggs are formed first and later spermatozoa; in- 

 dividuals in which this occurs are called protogynic. 



