BIGELOW: EARLY DEVELOPMENT OF LEPAS. 75 
ence to the conditions in the developing eggs of annelids and mollusks, 
are dominated by the conception of cells cleaving in sets of fours or quar- 
tets. The system of Blochmann (’81) and its successors have, with few 
exceptions, been applied to eggs in which a quartet of macromeres (in a 
morphological sense) is formed by the first two cleavages, and by later 
cleavages these give rise to successive quartets of micromeres. In all 
the annelids and mollusks in which the cell-lineage has been determined 
with certainty, the cells of the four quadrants (a, 6, c, d) formed by the 
first two cleavages are equivalent, in that each cell contains a portion of 
the two primary germ-layers, ecteblast and entoblast. The mesoblast 
is not so distributed with reference to the quadrants. It will be shown 
in this paper that the four-cell stage of Lepas is not a quartet of equiva- 
lent cells so far as the two primary germ-layers are concerned. Whereas 
in the annelidan and molluskan eggs each cell of the four-cell stage con- 
tains both ectoblast and entoblast, in Lepas three of these cells (a, 8, ¢) 
contain ectoblast but no entoblast ; and the fourth cell (d) contains both 
ectoblast and al/ the entoblast. In the annelids and mollusks the cells 
of the first quartet of micromeres (eight-cell stage) contain the ectoblast 
which is first separated from the entoblastic macromeres ; but in Lepas 
one of the cells of the two-cell stage is the first ectoblast to be separated 
from entoblast. 
Enough has been said, in anticipation of the account of the cleavage, 
to make it evident that the well-known quartet systems of nomenclature 
would not have their usual significance as indexes of homologies, if 
applied to the cleavage of Lepas, for the cells of the four-cell stage in 
annelids and mollusks are apparently not comparable with the cells of 
the same stage of Lepas, which would be given the same designations. 
However, a quartet system has been employed for the purposes of this 
paper, for the reason that it is convenient and familiar. The above 
statements will show that the system has not been used here with a view 
to indicating by it homologies with which it has become associated in its 
application to the spiral cleavage of annelids and mollusks. As far as 
regards the cirripede egg, the known facts do not seem to me to warrant 
the interpretation that cleavage occurs in cells grouped as quartets in the 
sense in which the term is applied to spiral cleavage ; and while the 
notation of a quartet system has been adapted to the purposes of this 
paper, the term “quartet ” has not been applied in description as desig- 
nating groups of cells in the cleaving egg of Lepas.* 
1 See Addendum by E. L. M. and W. E. C. (p. 136) following the General 
Summary. 
