104 BULLETIN: MUSEUM OF COMPARATIVE ZOOLOGY. 
the cells could not have been harmonized with the general plan indi- 
cated by the direction of the spindles of the fifth cleavage as represented 
in Figures 44-47. 
Summary of Fifth Cleavage. 
The blastoderm cells of the sixteen-cell stage divide synchronously. 
The primary mesoblast (d°”) and yolk-entoblast (d®*') are greatly 
delayed in cleavage. 
The blastoderm has extended far over the yolk-entoblast. 
Regular arrangement of cells of definite origin is as characteristic of 
this as of preceding stages. 
9. Srxta CLEAVAGE. Sixty-two CELLS. CLOSING OF THE BLASTOPORE. 
THe GERM-LAYERS. 
The twenty-eight cells of the blastoderm of the thirty-two-cell stage 
are the first ones to undergo the sixth cleavage. Cases are often seen 
in which all of the blastoderm cells have spindles arranged approxi- 
mately perpendicular to those of the preceding cleavage. About the 
time that the resulting fifty-six cells pass into the “ resting ” phase the 
two daughter cells of the primary mesoblast (d**, d®*) are found to be 
in division. The two entoblast nuclei (d*!, d**) remain undivided 
until a much later stage. The sixth cleavage, therefore, results in the 
formation of a sixty-two-cell stage. 
A preliminary description of the sixty-two-cell stage resulting from 
the sixth cleavage will aid in the discussion of the details of that 
cleavage. Figure 56 (Plate 7) represents an optical sagittal section of 
an egg with closed blastopore. All of the twenty-eight blastoderm 
cells of the preceding stage have divided. The two yolk-entoblasts 
(d°", d®°-*) have not divided. The two mesoblast cells (d®?, d**) are in 
the sixth cleavage. Two cells (6° and c’*) are represented between 
these mesoblasts and the blastoderm in the region of the closed blastopore. 
These two cells contribute to the mesoblast of the embryo, and for pur- 
poses of description they may be called the “secondary mesoblasts,” to 
distinguish them from the mesoblasts, d°* and d°*, which are derived from 
the primary mesoblast d*? (Plate 5, Figs. 44, 45), which was separated 
from the yolk-entoblast in the fourth cleavage. Referring to Figures 72 
and 73 (Plate 8), which represent transverse sections, it will be seen that 
there are two pairs of “secondary mesoblasts” (ms’b/’.), an anterior pair, 
6" and 67:7 (compare Plate 7, Fig. 62), and a posterior pair, a7*® and c™®. 
The series of sections represented by Figures 74—77 (Plate 9) shows con- 
