76 GERM-CELL CYCLE IN Al^TMALS 



parthenogenesis. If, on the other hand, the re- 

 productive cells of the gemmule are of multiple 

 origin, they may either be looked upon as true germ 

 cells which form a group physiologically equivalent 

 to the morula stage in the development of an egg, 

 or as a collection of regenerative cells capable of 

 producing a new individual. 



In this connection should be mentioned the bud- 

 ding of Tethya (Deso, 1879-1880) which develops 

 from a group of amebocytes (Maas, 1910) and the 

 gemmules of Tedania and Esperella (Wilson, 1902) 

 and of hexactinellids (Ijima) which become ciliated 

 larvae. Wilson has shown " that silicious sponges, 

 when kept in confinement under proper conditions 

 degenerate in such a manner that while the bulk 

 of the sponge dies, the cells in certain regions become 

 aggregated to form lumps of undifferentiated tissue. 

 Such lumps or plasmodial masses, which may be 

 exceedingly abundant, are often of a rounded shape 

 resembling gemmules, more especially the simpler 

 gemmules of marine sponges (Chalina, e.g.), and 

 were shown to possess in at least one form (Stylo- 

 tella) full regenerative power. When isolated they 

 grow and differentiate, producing perfect sponges " 

 (1907, p. 295). These "lumps of undifferentiated 

 tissue" have also been noted by F. E. Schulze 

 (1904) and recognized as probably reproductive; 

 they have been named by this author, " sorites," and 

 have been called by several authors "artificial 

 gemmules." The process involved in their forma- 

 tion is termed "regresjsive differentiation." The 



