114 bulletin: museum of comparative zoology. 



formed at tho expense of the yolk-cell until the hlastopore closes, is 

 completely disproved by the facts of cell-lineage. 



h. 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 fiicts 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 {a¥, 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 cnto- 

 Ijlast nuclei are later found deeper in the yolk. Nussbauiu 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 tho 

 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 tho " 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 



