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A. Normal Cleavage. 
The first two (meridional) cleavages divide the ovum into four equal 
parts. The third (first equatorial) gives rise, as HATSCHEK first 
showed, to four larger “macromeres” below, and four smaller “micro- 
meres” above (fig. 2, A), the latter being often at first rotated tow- 
ards the right (as in Annelids, Mollusks and Turbellaria). 
It is at the fourth cleavage that my account diverges from that 
of HarscneK. According to this author the fourth cleavage is strictly 
meridional and radial, there being “nur eine Hauptachse, die vom 
animalen zum vegetativen Pole geht”. I find three leading types in 
the fourth cleavage (with many transitional varieties) all of which are, 
I believe, capable of normal and complete development. The first 
and rarest form is of the true spiral type found in Annelids and 
Mollusks, and agrees very closely with the cleavage of Hydroides 
or Polygordius. The second type, more frequent than the first 
but not common, approaches nearly to the radial form, as described 
by Hatrscuex. The third and much the commonest type (fig. 2, B, ©, D) 
is bilaterally symmetrical with respect to the first cleavage-plane, and 
in its pure form shows a remarkable similarity to that of the Asci- 
dians. In this type the macromeres divide unequally in a somewhat 
oblique vertical plane, thus giving rise to four larger cells (1, 10, 7, 
16, fig. 2, C) grouped about the lower pole, and to two symmetrically 
placed pairs of smaller cells (2—9; 8—15). The micromeres show 
many variations (see fig. 2, B) but in typical cases divide vertically 
in a plane nearly parallel to that of the first cleavage (fig. 2, C). 
Beyond this point it is not at present necessary to follow the 
cleavage. 
B. Double Cleavage. 
The cleavage of a completely isolated blastomere 
of the 2-celled (or 4-celled) stage is not a half-clea- 
vage but agrees essentially with that of a complete 
normal ovum. Two types of cleavage exist, in both the half-sized 
and quarter-sized embryos. The first type (fig. 2, F') is exactly like 
the normal entire cleavage, the 8-celled stage consisting of four equal 
macromeres and four smaller micromeres. The second type (fig. 2, G) 
differs from the first in the fact that two of the macromeres are some- 
what smaller than the others. In later stages the two forms agree 
closely, except in size, with the normal entire embryos. 
At first sight it may seem that the second type is an approxi- 
mation to a true half-cleavage — i.e., to one half of the normal 
cleavage. This however is not really the case. A comparison of the 
at ioe 
