52 GEORGE W. TANNREUTHER 
material. This makes it possible, not only to recognize the posi- 
tion of the nucleus, but to be able to make out the exact position 
of the cleavage spindle in the living egg. 
Second cleavage: The second cleavage is meridional and takes 
place at an angle of forty-five degrees to the median plane of 
the future adult. The two cells divide at different times (occa- 
sionally both cells divide simultaneously). These two cleavages 
taken together represent the second cleavage in otherannelids. 
CD divides first into two very unequal parts (text fig. 2 and 
figs. 3, 78). The division of AB is nearly equal (figs. 5 and 6). 
The largest cell, D, is posterior. B is anterior, inclined a little 
to the right. C is right (text figs. 3-4) and A is left with refer- 
ence to the median axis of the future worm. The large cell 
D has a tendency always to divide first. The exact formation 
of the four macromeres must be carefully worked out, and 
correctly understood, since their position largely determines 
the orientation of the future organs. 
For descriptive conveniences the region of the first generation 
of ectomeres will be considered as the upper or apical pole and 
the point directly opposite, as the lower or vegetal pole. The 
centers of the upper and lower poles of the dividing ovum, coin- 
cide with the median longitudinal plane of the adult worm. 
The poles however may be shifted somewhat anteriorly or pos- 
teriorly, with reference to the macromeres, more especially to 
D in the formation of d?. When viewed from the upper pole 
A and C are in contact, while B and D are separated. But when 
viewed from the ventral pole A and C are separated and B and D 
are in contact (figs. 6-7). This extensive cross furrow found at 
the vegetal pole is also present in forms like Nereis, Clepsine and 
Crepidula; while in those forms like Unio, in which the greatest 
mass of the four macromeres is concerned in the formation of 
ectoderm instead of endoderm, the cross furrow is greatest at 
the animal pole. These cross furrows (‘Brechungslinie’ of 
Rauber) have no special significance in the development of 
Bdellodrilus, as the cleavage of the macromeres is carried to 
the end, immediately after the three generations of ectomeres 
are formed. In those forms like Nereis, where the cross furrow 
