MUSEUM OF COMPARATIVE ZOOLOGY. 147 
The mesenchymatous growth tissue of olis resembles the sexual cells, 
however, in 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, 6, and the Anlagen 
of other new cerata. Thus, as in any young individual we may distin- 
guisbh 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, 
asa 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 Aolis, 
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. 
