112 BULLETIN: MUSEUM OF COMPARATIVE ZOOLOGY. 
transverse sections of the same stage, of which 81 is the most posterior. 
In the blastoderm at this stage there is nothing worthy of note except 
the indentation which marks the former position of the blastopore. The 
cells in this region are rarely as closely arranged as in the other parts of 
the blastoderm. 
The mesoblast cells are crowded together, and it is impossible to dis- 
tinguish in all cases between those derived from the primary mesoblast 
and those from the ‘‘ secondary mesoblast.” As used in the description 
of later stages, the term mesoblast includes both the primary and 
“secondary mesoblast.” 
The possibility of origin of mesoblast cells from the blastoderm after 
the sixth cleavage has been kept in mind during the observations, but 
there is no evidence of such an origin. The cleavage spindles in all parts 
of the embryo have been seen, but not one perpendicular to the surface 
has been detected. Moreover, the mesoblast cells have been repeatedly 
counted in sections and their nuclei have also been counted in transpa- 
rent preparations of the entire egg, and there have never been seen more 
cells than could be accounted for by the division of the eight mesoblast 
cells described in the sixty-two-cell stage. 
It should be mentioned that by rapid decolorization of specimens 
stained in borax carmine it has often been found possible to draw the 
color from the nuclei of the blastoderm cells and stop the reaction while 
the mesoblast nuclei were still brilliantly stained. With such prepara- 
tions it is easy to count the nuclei of the mesoblast cells in the entire 
egg. This method has been employed in all the stages with mesoblast. 
The entoblast nuclei are stained brightly by this carmine method, and 
are easily identified in transparent preparations of entire eggs, as well as 
in sections. In all stages between that of thirty-two cells and that with 
about one hundred and twenty cells there is no evidence of division of 
these nuclei. In these stages only two “ resting ” nuclei are to be found 
in the yolk, as shown in Figures 78-80 and 81-86 (Plate 9). Usually 
in the 120-cell stage the two nuclei are enlarged, while the chromosomes 
are distinct. Evidently the nuclei are preparing for division, but the 
spindles are rarely seen until after the blastoderm cells have divided 
again. In the resulting stage, with about two hundred and fifty cells, 
four entoblast nuclei are often seen. It does not seem possible that 
there can have been an overlooked division of these nuclei. Moreover, 
the origin of the mesoblast cells has been determined to be independent 
of the two entoblast cells, which are seen in this and in the preceding 
stage. 
