110 BULLETIN: MUSEUM OF COMPARATIVE ZOOLOGY. 
of the mesoblast cells, and in good transparent preparations of the entire 
egg are easily recognizable. The cleavage plane separating the yolk- 
entoblast cells may occupy various positions at this stage. If the pri- 
mary mesoblasts are symmetrically placed with reference to the median 
plane (Plate 7, Fig. 64), the cleavage plane in the yolk coincides ap- 
proximately with the sagittal plane of the embryo ; but when one of the 
primary mesoblasts is anterior or dorsal to its sister cell, the plane of 
separation between the entoblasts is inclined towards the horizontal, or, 
if vertical, is oblique to the long axis, as in Figure 63. _ In all cases it 
appears to extend from near the plane separating the right and left pri- 
mary mesoblasts towards the antero-dorsal side of the embryo (Figs. 63, 
64, 65, 73). This relation suggests that the horizontal and oblique 
positions are secondary and due to movement of the yolk when the pri- 
mary mesoblast cells are forced beneath the blastoderm and adjusted to 
unsymmetrical positions. The fact that when the primary mesoblasts 
retain their original symmetrical relation, the cleavage plane in the yolk 
is found apparently coinciding with the sagittal plane, lends support to 
this view. 
It may be of interest to notice that the cleavages involved in the seg- 
regation of the germ-layers are always the same, no matter whether the 
blastoderm is completed in the sixth or seventh cleavages. The cleav- 
ages separating from the yolk-cell the micromeres which form the blas- 
toderm are not variable in number, but definite (three) ; and there is no 
variation in regard to the number of micromeres which produce the 
variable numbers of blastoderm cells required to cover the yolk. This 
conclusion is opposed to that of Groom (94, p. 141). (See review of 
literature on late cleavage.) This relation is exactly what has been found 
in the case of the eggs of gasteropods and annelids, in which it has been 
shown (Conklin, ’97, pp. 61-63) that the number of micromeres (ecto- 
blasts) separated from the macromeres (mes-entoblast) is constant for all 
species which have been studied, although the macromeres in some cases 
are very large and require a large number of ectoblastic cells to complete 
the blastoderm ; in such cases — precisely as in Lepas anatifera and L, 
fascicularis — there is more subdivision of the micromeres before the 
blastoderm is completed. It appears that the same relation exists in 
the case of the other species of Lepas. 
Summary of Sixth Oleavage. 
All derivatives of the three micromeres (ab?, c? and d*:”) and of the 
two primary mesoblasts (d°*, d°*) undergo division. The two entoblast 
