118 BULLETIN : MUSEUM OF COMPARATIVE ZOOLOGY. 
Figures 95-121 (Plates 11, 12) show how close is the resemblance 
to the cleavage of L. anatifera. Except in size and some unimportant 
details, the various stages of the two species are indistinguishable, and 
the description of the figures of L. anatifera may be applied to those of 
L. fascicularis. 
A renewed study of the few old preparations, supplemented by many 
new ones, shows that I (’96) was wrong in the conclusion that the 
ectoblast is detached from the yolk-macromere by means of four succes- 
sive divisions (96, ectomeres A, 5, C, and D). The supposed fourth ecto- 
mere (96, Figs. 6 and 7 D) is the primary mesoblast cell. In origin and 
position it corresponds exactly with the mesoblast cell (d***) seen in the 
sixteen-cell stage of L. anatifera. I now interpret the spindle seen in 
the yolk during the fifth cleavage (’96, Fig. 7), which was then supposed 
to represent the separation of the mesoblast and the entoblast, as a 
rare case of precocious division of the entoblast. Study of the complete 
series, with all mitotic phases represented, shows that in L. fascicularis, 
as in L. anatifera, the first, second, and third cleavages form micro- 
meres containing the ectoblast and “ secondary mesoblast,” while the 
fourth cleavage separates mesoblast and entoblast from each other. 
With regard to the planes of cleavage and orientation, I find no 
important disagreement with L. anatifera. The descriptions of the 
first and second cleavages in the preliminary note were similar to those 
of L. anatifera given in this paper. The rotation during the first 
cleavage was not then known. The equatorial nature of the third 
cleavage was not clearly shown by the figure of a four-cell stage with 
inclined spindles in the preliminary note; Figures 100-103 (Plate 
11) in this paper better represent the four-cell stage and the third 
cleavage. The figure of the eight-cell stage (96, Fig. 6) was drawn 
from an egg which is now known to have been incorrectly oriented. ° 
Eggs which give exactly such camera tracings will, when properly 
oriented by moving the cover glass, always show the same arrangement 
of cells as that seen in Figures 104-106 in this paper. 
Figure 6 of the preliminary paper represented a separation of 
mesoblast and entoblast (fourth cleavage), and not as was incorrectly 
assumed, the formation of a “fourth ectomere.” Figures 108-110 are 
the corresponding figures in this paper. 
The primary mesoblast cell, shown in Figure 8 of the preliminary 
paper as filling the blastopore, represented the delayed fifth cleavage, 
which was in progress. The single entoblast nucleus was not yet under- 
going the fifth cleavage. The inferred connection between the spindle 
