308 LANGENBECK. [VoL. XIV. 
apart. The larger micromeres stand just above the larger 
macromeres, and the same holds true for the smaller micro- 
meres and macromeres. In the figures the macromeres and 
their descendants will be designated by the large letters, and 
the micromeres and their descendants by the small letters. 
The macromeres AB, CD, and EF and their descendants are 
drawn in red ink, while GH and the micromeres are in 
black. 
From this time the macromeres always divide before the 
micromeres, and the larger macromeres before the smaller ones. 
The subsequent cleavage planes which divide the two larger 
macromeres are alternately meridional and equatorial, while 
the planes dividing HF and the micromeres are always meridi- 
onal. This may be due to mechanical causes; since these 
blastomeres, lying upon the larger ones, are somewhat flat- 
tened, as shown in Fig. 25, the spindle would find more room 
in the horizontal than in the vertical plane. 
16-cell stage. — After the sixth hour two vertical cleavages 
atmeht aneles to each other @ive rise tow, 2 .C, Dawa, 
Ff, a, 6; \6, die; J, & 2k (Pigs. 3,\4)) > When! the’ egs comes to 
rest A and C flatten against each other, but B and D are forced 
apart by G and A (Fig. 3). It is to be noticed that the zone of 
small cells lies obliquely over the oval egg. Mlle. Wagner ('91) 
finds the same oblique zone of small cells in Melita, but the 
obliquity was, in that form, brought about by a rotation of 
the cells in the 4-cell stage. Her account agrees in part with 
the results of Van Beneden and Bessels (69), whose Figs. 10, 
11, and 12 show exactly the same oblique arrangement. In 
the text Van Beneden and Bessels make no mention of this 
oblique zone of cells, but in their Fig. 9, which represents the 
8-cell stage, the cells are arranged symmetrically with refer- 
ence to the long axis of the egg, while in their Fig. 10, where 
the macromeres are just beginning to pass into the 16-cell 
stage, the oblique position of the micromeres is manifest; 
therefore, it would seem that a rotation occurred just at this 
time. In Microdeutopus the obliquity is occasioned by the 
angle which the second cleavage plane makes with the first 
(Fig. 19). 
