Nove LAE VWEGG OR SAMIA, AND TRS VOGEAVAGE. aay 
This circular groove looks externally as if it might actually 
cut off polar segments, but as an examination of sections (Cuts 
6, 7, 13, and 15) will show, the groove descends more or less 
obliquely, or centripetally, so that when completed its inner 
deeper boundary would be lost in confluent vacuolar spaces. 
As the plane of the groove probably never passes below the 
vacuoles, one might say that it describes an axial mass in the 
upper hemisphere approximating in shape an inverted truncated 
cone. Cut 61s taken in the plane of the dotted line in Diagr. E. 
The circular groove appears to cut off two cells; this appear- 
ance is due to the section passing near the margin of the polar 
field. In Cut 7, which passes parallel with the second cleavage 
(plane of dotted line, Diagr. F.), these cells are still continuous 
with the yolk below. Cuts 13 and 15, taken along the dotted 
lines of Cut 7, further illustrate this point. It will be seen 
that at their bases the segments lying within the fourth cleav- 
age grooves (IV) are likewise continuous with the yolk. Cut 
8, taken at the level of the dotted line in Cut 7, shows the 
central grouping of the vacuolar spaces and the depth of the 
cleavage grooves at this time. 
Dean (No. 3, p. 427, Fig. 24) figures and describes the 
“central blastomeres”’ of this stage as ‘separate from the 
underlying germ-yolk.” In describing the next cleavage we 
shall explain how the central cells are cut off. 
Fifth Cleavage. — The fifth cleavage comprises two distinct 
sets of grooves. One set appear on the surfaces of the eight 
primary segments in the form of meridionals. In Fig. 26 they 
are just beginning, and are more advanced in Fig. 27. The 
other set extend horizontally in the eight cells of the polar 
field, and of course are not visible from the surface. This 
cleavage thus brings about the 32-cell stage, and makes the 
central portion of the calotte two cells deep. The origin of 
yolk cells is thus not limited to the margin of the calotte, as in 
pelagic teleost eggs, but extends to all the cells beneath those 
of the polar field. These cells go on budding off cells to the 
germ-disc. It is rather interesting to see this horizontal cleav- 
age appearing with the passage from the 16 to the 32-cell stage, 
because in the corresponding stage in the pelagic fish egg it 
