MUSEUM OF COMPARATIVE ZOOLOGY. 147 



The mesenchymatous growth tissue of ^olis resembles the sexual cells, 

 however, iu this, that while it goes to produce the mesenchyme of any 

 ceras, a, not all of it is used up in forming the mesenchyme of ceras a, 

 but some of it remains behind to form a new ceras, b, and the Anlagen 

 of other new cerata. Thus, as in any young individual we may distin- 

 guish between the differentiated tissue and the germ tissue from which 

 new individuals will arise, so in any ceras we may distinguish between 

 the differentiated tissue and the embryonic tissue from which new cerata 

 will arise. 



While, however, the sexual cells have the capacity of reproducing new 

 individuals indefinitely, the mesenchyme at the base of the cerata does, 

 as a matter of fact, produce only a limited number of cerata. Of this 

 limitation there are, however, all degrees. In some cases, as in Doto, 

 only one ceras is produced in a transverse row ; in some species of ^olis, 

 on the other hand, young cerata are produced, even in adult individuals, 

 at the ventral end of the long transverse rows, so that here the growth is 

 apparently limited only by the duration of life of the individual. In all 

 cases the limitation in the reproduction of cerata must be considered as 

 resulting, not from the limited capacity of reproduction of the embryonic 

 tissue, but from the needs of the species. 



Cambridge, December 20, 1892. 



C, 



