EMBRYOLOGY OF BDELLODRILUS 149 
in most of the cells and the teloblasts of the right and left halves 
of the embryo are formed. Bilateral symmetry now becomes 
definitely established and the animal increases in length very 
rapidly. 
CLEAVAGE 
1. DESIGNATION OF CLEAVAGE CELLS 
In the designation of the cleavage cells, for the sake of uni- 
formity and convenience, I have for the most part adopted the 
system followed by Wilson in his ‘‘Cell lineage of Nereis,’ and 
Lillie in his study on ‘“‘The embryology of the Unionidae.” The 
first four cells (macromeres) are designated by the capital letters 
A, B, C and D. The generations of micromeres (ectomeres) 
by the small letters a, b, c, and d. The first index number 
indicates the generation to which the ectomere belongs. Thus 
al, b!.2 or c!1-2 or d! 4 all belong to the first generation; c?, b?,d?.3 
belong to the second generation, etc. A, B, C and D corre- 
spond to the vegetal pole; a, b, c and d to the apical pole. When 
a cell divides the products receive the designation of the parent 
2,1 
cell with the addition of a further index number; thus b? i és 
Exceptions to this rule are made only in the case of special 
cells, which, for convenience, receive special designations: thus 
d? of the second generation of ectomeres becomes the ‘first soma- 
toblast’ and is designated by (X), and its small derivatives by x', 
x?, x3, etc.; d‘ the ‘second somatoblast’ is designated by (M). 
2. TYPES OF CLEAVAGE 
a. The oblique period of cleavage: one io twenty-four cells 
First cleavage: The first cleavage occurs about five to ten 
hours after the deposition of the egg. The time varies somewhat 
with external conditions. The plane of division passes through 
the area where the polar bodies are formed (fig. 1) and divides 
the egg into two very unequal parts, AB and CD (text fig. 1 
and fig. 2). The smaller of the two cells AB is anterior, and 
