PE BULLETIN: MUSEUM OF COMPARATIVE ZOOLOGY. 
formed at the expense of the yolk-cell until the blastopore closes, is 
completely disproved by the facts of cell-lineage. 
b. Closing of Blastopore. — Groom did not see the closing of the blas- 
topore in L. anatifera, but he (’94, p. 141) described it for other species 
as follows: “The end of the yolk projects out at one point as a small 
rounded elevation. . . . A merocyte appears in the centre of this, and 
fills the gap between the surrounding cells, and finally emerges from 
the yolk as the blastomere.”’ 
This description is far from being in harmony with the facts in the 
case of L. anatifera. The closing of the blastopore has been shown in 
this paper to be due to the repeated divisions of the ectoblastic deriva- 
tives of the three micromeres (ad, c®, d*:?) which are separated from 
the yolk-macromere in the first three cleavages. The ‘merocyte”’ 
which Groom saw in the blastopore (see his Fig. 127) is represented by 
the protoplasmic mass concentrated around the nucleus of the entoblast 
cell, which is situated as shown in my Figure 54 (Plate 6). I have 
shown by tracing the cell-lineage that this cell divides (Fig. 52, fifth 
cleavage), usually before the closing of the blastopore, sometimes during 
the sixth cleavage of the ectoblastic cells, and that the resulting ento- 
blast nuclei are later found deeper in the yolk. Nussbaum observed in 
Pollicipes a division of the yolk before the blastopore closed. Groom 
(94, p. 147) states that this may rarely occur, a condition which is 
completely at variance with his account of the closing of the blastopore. 
The evidence presented in the present, account of the cell-lineage leads 
to the conclusion that no cell is cut off directly from the yolk to fill the 
blastopore. It has been shown that at the time of closing there are two 
nuclei in the yolk, not as Groom stated, a single one. Hence Groom’s 
conclusion, that the ‘ merocyte ” which fills that blastopore “ before be- 
coming shut off as a blastomere, gives off a single nucleus into the yolk” 
(94, p. 198), cannot be accepted. The evidence is completely opposed 
to such a view. It appears that in Groom’s account of the closing of - 
the blastopore, his view of “emerging merocytes” has led, as in the 
early stages, to an erroneous interpretation. 
c. Differentiation of the Germ-Layers.—Groom’s account of the 
‘“‘meso-hypoblast ”’ agrees in general with the descriptions of all the 
earlier authors, who regarded this as represented by the yolk-cell, or 
cells, after the closing of the blastopore. Groom (’94, p. 146) writes : 
“The closing of the blastopore is almost immediately followed by the. 
division of the yolk into two pyramids or segments ; the formation of 
the mesoblast immediately commences by the successive cutting off and 
