54 bulletin: museum of compaeative zoology. 



ferin<» fundamentally iu kind from those already sliowu in the earlier 

 stages. 



We must now turn to a consideration of 



The Estodersi. 



The cells which I have called the entoderm — following in this Zelinka 

 ('91) and Tessin {'86) — are those derived from the single large cell of 

 the quadrant D, which passes within the egg in the manner already de- 

 scribed. I have already given an account of the first cleavages of this 

 cell after it has become partially covered by the other blastomeres. As 

 will be recalled, at the fifth and sixth cleavages the spindles occupied 

 twice in succession the same position, one end lying in the anterior 

 median line, between the ventral cells of quadrants A and B (Plate 5, 

 Fit'. 35, and Plate 6, Fig, 48). Here was given off at each of these 

 cleavages a minute vesicle, the entire process being comparable in ex- 

 ternal featui-es to the successive formation of two polar cells at a given 

 spot on the surface of the egg. The two vesicles thus foi-med maintain 

 their position for some time (Plate 7, Fig. 55), but as the surrounding 

 cells become invaginated, I have found it impossible to follow their 

 later fate. 



We will follow the cleavage of the large cell d''-^ (Plate 6, Fig. 50), 

 which forms the greater bulk of the entoderm. 



After the sixth cleavage (Fig. 49), the asters in d''-^ at once separate 

 nearly in the dorso-ventral axis of the egg, as shown in Figure 50. The 

 line joining them is at first a little oblique, the ventral aster being a 

 little to the right. This obliquity soon corrects itself, and the asters 

 come to lie in the sagittal plane. As the spindle is formed, its dorsal 

 end moves to the posterior side, so that the spindle is no longer in the 

 dorso-ventral axis of the egg. This stage is shown in Figure 64 (Plate 

 8) ; as may be observed in this figure, the spindle is neither in the 

 longer axis of the cell nor at right angles to it, but oblique. The cloud 

 of granules, which soon after the last division occupied a region on the 

 anterior side of the cell, underneath the two vesicles d^-^ and d''--, now 

 surrounds the aster at the dorso-posterior end of the spindle. 



The cleavage is unequal, sepai-ating (Fig. 65) a smaller dorso-posterior 

 cell, d^-^, from a larger anterior one, d^-^. The cloud of granules 

 remains in the smaller, dorsal cell, forming a band about its periphery, 

 80 as to leave a free space surrounding the nucleus. The position of 

 the animal pole of the egg with reference to this cell should be carefully 

 noted, as the relation remains constaiit, at least for a time, during the 



