CURTIS: LIFE HISTORY OF PLANARIA MACULATA. 533 
planarian after the stage in which the embryo consists of seventy 
or eighty similar blastomeres, as follows : from the blastomeres arise 
(1) the primary ectoderm, (2) the primary endoderm, forming a 
kind of blastocoele in which lie a few wandering cells. From these 
last there arise one after another the secondary ectoderm, the sec¬ 
ondary endoderm, the remainder of the parenchyma, the nervous 
system, gland cells, and muscle fibers, the definitive pharynx consid¬ 
erably later, the reproductive organs, and in case of mutilation the* 
parts which are made good. lie further says that from his exami¬ 
nation of a number of planarians he finds that those species having 
the power of regeneration most marked, possess the greatest num¬ 
ber of these cells. In view of their supposed primitive character, 
Keller prefers the term Stammzellen” to von Wagner’s “Bildungs- 
zellen.” 
IIow far our knowledge of the cell lineage of Triclads admits of 
such a definite interpretation, I am not ])repared to say. The theory 
is a suggestive one, though from the nature of the case, impossible 
of actual demonstration. Spindle-shaped and oval cells with OA^al 
nuclei and the characteristic nucleoli of the Stammzellen ” do 
exist in jP. maculata embryos at the time when the permanent 
pharynx is forming. I should myself be inclined to the more con¬ 
servative statement which is merely a resume of the facts, that 
the cells left between the primary ectoderm and endoderm of the 
embryo produce pretty much everything else and that when the 
syncytial parenchyma can be first discovered there are also individ¬ 
ual cells scattered through it, which, though the larger ones may be 
easily distinguished as such, are connected with the syncytial mass 
of the parenchyma by every possible intermediate stage, and this 
series of intermediate stages may be interpreted as “ Stammzellen ” 
becoming parenchyma or as parenchyma becoming “ Stammzellen.’’ 
If the latter be the interpretation, these cells would be continually 
arising from the parenchyma, we should lose the continuity of the 
■“ Stammzellen ” which Keller assumes have come on down unchanged, 
giving off the others as from an undiffei'entiated stock, and we should 
then consider the cells of the parenchyma as the final ones to which 
we can trace the power of regeneration. 
The facts which do come out clearly from this review are, that we 
can distinguish (1) the parenchyma proper and (2) certain other 
individual cells with intervening stages between the two and that 
