74 BULLETIN: MUSEUM OF COMPARATIVE ZOOLOGY. 
micromeres are separated from the yolk-bearing macromere. These three 
cells contain all the ectoblast, and by their repeated division form the 
blastoderm. Certain cells of the blastoderm, which are derived from the 
first two micromeres, give rise to a portion of the mesoblast, hence these 
two micromeres are not purely ectoblastic. The third contains only 
ectoblast. In the fourth cleavage a mesoblast cell is separated from the 
yolk-macromere, which now represents entoblast alone. 
The sixteen-cell stage, therefore, is composed of fourteen derivatives 
of the three micromeres, one mesoblast cell, and one entoblast cell (yolk- 
macromere). The entoblastic yolk-macromere is nearly enveloped by 
the fourteen smaller cells composing the blastoderm, only a small part 
of the entoblast cell being exposed at the blastopore. The single meso- 
blast cell lies at the posterior edge of the blastopore, and were its history 
not known would certainly be regarded as a cell of the blastoderm. At 
the fifth cleavage each of the sixteen cells divides, the two resulting 
mesoblastic cells still remaining at the surface. At the sixth cleavage 
all the cells except the two entoblast cells divide, thus producing a sixty- 
two-cell stage. During the sixth cleavage the two mesoblastic cells, 
before dividing, sink beneath the blastoderm, as this closes over the ento- 
blast and obliterates the blastopore. At the same time four cells of the 
blastoderm, lying at the anterior and lateral edges of the blastopore, 
divide parallel to the surface. The four deep cells thus formed beneath 
the blastoderm constitute a part of the mesoblast. The mesoblast, then, 
is derived in part from one cell which is separated from the entoblast in 
the fourth cleavage (sixteen-cell stage) and in part from four other cells 
which are detached from the blastoderm during the sixth cleavage. 
Gastrulation is of the epibolic type, and is the result of the extension 
of the blastoderm over the entoblastic yolk-macromere. During the 
sixth cleavage, which leads to the formation of a sixty-two-cell stage, the 
blastoderm usually closes over the blastopore, which marks the ventral 
and posterior part of the future embryo. 
In the general features of the late development of the embryo the 
results of this investigation confirm those of some earlier workers. 
VI. Nomenclature of Cleavage. 
For convenience in describing the cell-lineage of Lepas and in making 
comparisons with the development of other forms, it is desirable that 
some system of cell-nomenclature should be applied. 
The common systems, which have been developed with special refer- 
