BIGELOW: EARLY DEVELOPMENT OF LEPAS. 119 
in the yolk-cell, in the sixteen-cell stage, and the separation of a 
mesobiast cell is now known to have been an erroneous interpretation. 
The series of stages is now so complete as to leave no doubt that the 
mesoblast cell is separated from the yolk-entoblast in the fourth and 
not in the fifth cleavage. 
In the sixty-two-cell stage the origin and position of cells is certainly 
the same asin L. anatifera. The “secondary mesoblasts” were observed 
and figured during my earlier studies, but were interpreted as deriva- 
tives of the primary mesoblast, which seemed to divide more rapidly 
than did the other cells. It now appears from a study of all phases of 
the sixth cleavage that there are eight mesoblast cells in the sixty-two- 
cell stage, only four of which are derived directly from the ectoblast. 
Up to this stage the divisions of the primary mesoblast are the same 
as have been described in detail in the case of L. anatifera. In living 
eggs recently studied, and also in preparations of favorably preserved 
material, I have observed the cell-wall between the two entoblast 
nuclei of this stage, and it follows that — contrary to my former sup- 
position — there is no exception to the rule that every nuclear division 
during the cleavage is associated with total cell division. 
VIII. Extension of the Mesoblast and Entoblast. Later 
Development of the Germ-Layers. 
The mesoblast in the 122-cell stage consists of a mass of cells at the 
posterior end of the embryo, near the former position of the blastopore 
(Plate 9, Figs. 78-86). The arrangement of the cells leaves no doubt 
about the position of the blastopore, but orientation of the succeeding 
Stage is more difficult and uncertain. During the next division the 
embryo begins to elongate posteriorly. A comparison of the blastoderm 
cells on the ventral surface of the 122-cell and 250-cell (estimated num- 
bers) stages leads to the suggestion that the elongation is due to flat- 
tening of the ventral blastoderm cells, while those on the dorsal surface 
remain columnar in form. At any rate, this elongation appears to be 
confined mostly to the ventral region of the blastoderm, anterior to the 
former position of the blastopore. The result is that the cells which 
closed the blastopore and the adjoining mesoblast cells are moved from 
the ventral surface towards the extreme posterior end, where for a time 
the mesoblast consists of a conical mass of cells (compare Plate 9, Fig. 
80 with Plate 10, Fig. 87). The rapid division of the mesoblast cells 
produces a plate, which grows forward on the dorsal side of the embryo 
