208 GEORGE W. TANNREUTHER 
meet the highest need of the developing animal? It has been 
demonstrated, again and again, in annelids, molluses and even 
in polyclads, that homologous cells of like generations give rise 
to like parts in the developing embryo and the adult. The 
occurrence of these conditions in such widely separated forms 
furnishes a very interesting and important phase in the study of 
cell lineage. The tendency has been rather to emphasize these 
resemblances, than to give special stress to the exact conditions 
which occur in any one species in its different stages of develop- 
ment. It is true, however, that the general form of cleavage 
may be inherited from a long series of ancestors, probably from 
some of the Turbellarian worms. But the problem of more 
direct importance in any one group is, why such variation in 
the size, form, direction and rate of cleavage? 
1. IN THE CLEAVAGE OF BDELLODRILUS 
In Bdellodrilus we have a determinate type of cleavage, i.e., 
the fetal as well as the adult structures can be shown to have a 
definite or direct cell lineage, and can be traced back to the 
unsegmented egg. The structure of the ovum is quite homo- 
geneous, and at the time of maturation, the egg can be definitely 
oriented as to the future axis of the body. Before the first cleav- 
age is complete, the parts of the ovum which give rise to the differ- 
ent germ layers can be traced or ascertained with a fair degree 
of accuracy, 1.e., definitely localized parts which’ give rise to 
definite organs or structures. 
“Adaptation in cleavage can manifest itself only in three pos- 
sible ways or modes of cleavage variation, which are, as has been 
pointed out by Lillie, Mead, Conklin and others, the following: 
first differences in the rate of cleavage; second differences in the 
size; and third, differences in the direction of cleavage.”’ 
The general plan of cleavage in Bdellodrilus, is similar to that 
of other forms. The ectoderm is derived from the four basal 
cells, by three successive horizontally formed cleavages. The 
mesoderm from a fourth cleavage of the posterior macromere D 
and the entoderm from the remaining cells. The first cleavage 
