BIGELOW: EARLY DEVELOPMENT OF LEPAS. 115 
sub-division of nucleated segments from the two yolk segments.” Ac- 
cording to Groom these yolk-segments after separation of the mesoblast 
divide and form endoderm cells. 
In opposition to this it has been shown in the present paper that the 
mesoblast clearly does not originate directly from the yolk-cells after the 
closing of the blastopore; but from certain cells which have been desig- 
nated in this account as primary and secondary mesoblasts. The origin 
of all these cells has been definitely traced. Moreover, evidence has 
been presented to show that the two yolk-entoblasts do not begin to 
divide after the thirty-two-cell stage until at least one hundred and 
twenty cells are present, of which more than a dozen are mesoblastic. 
Since the entoblast cells do not divide during these stages, they cannot 
be the direct progenitors of any of the mesoblast cells. All the evi- 
dence given seems conclusive and opposed to Groom’s interpretation. | 
The figures of Groom fail to establish his conclusions regarding the 
origin of mesoblasts from yolk-entoblasts, for in no case are nuclear spin- 
dles, the only unimpeachable evidence of such origin, shown. His inter- 
pretation of the origin of mesoblast cells seems to be based upon their 
position. In numerous preparations I have seen all the conditions 
which Groom figures, but I have found no evidence opposed to my in- 
terpretation of the origin of the mesoblast. Groom did not have trans- 
parent preparations of entire eggs, and his account of the mesoblast is 
based entirely upon sections. His figures represent isolated sections, 
when in many cases only complete series of sections would be convinc- 
ing. His erroneous conclusion, that the mesoblast is cut off in a series 
of divisions occurring in a pair of yolk-cells (“ meso-hypoblast ”), may 
have resulted from certain conditions which I have frequently noted. 
Sometimes in stained sections the cell-boundaries of the mesoblast cells 
are invisible, they appearing to be continuous with the yolk. Under such 
conditions the mitotic spindles of the mesoblast cells might easily be mis- 
taken for division of the yolk-cells to form new mesoblast cells. I have 
seen many such cases which exactly simulated some of Groom’s figures, 
but after removal of the cover glass and restaining, the cell-boundaries 
of the mesoblast cells and the nuclei of the yolk-entoblasts appeared as 
usual. 
Nussbaum (’90) described the mesoblast in Pollicipes as formed by the 
division of blastoderm cells surrounding the blastopore before it closes. 
The mesoblast was said to grow inwards and anteriorly over the yolk. 
The account of the origin of mesoblast given in the present paper makes it 
probable that Nussbaum’s description is in a general way correct. Had 
