110 bulletin: museum of COMrARATIVE ZOOLOGY. 



of the mesoblast cells, and iu 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. Gl), 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 tliat 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 Cleavage. 



All derivatives of the three micromeres (aP, c^ and d*-'^ and of the 

 two primary mesoblasts {d'^'^, d^*) undergo division. The two entoblast 



