1904.] NATURAL SCIENCES OF PHILADELPHIA. 725 



pore. Both of these questions have received much attention from 

 embryologists, and have been so thoroughly discussed by the writers 

 on cell-Hneage that it would be superfluous for me to attempt here to 

 treat the subject at length, and so I shall confine m3^self to stating what 

 these relations are in the Dinophilns embryo and to comparing them 

 with a few other forms. 



As already mentioned, the cell 2d is of very great value as a landmark. 

 From its origin up to the time when the last traces of it are seen, it 

 marks the posterior end of the embryo. Of course it may be said that 

 since the first three cleavages are of the spiral type, and not of the 

 bilateral type, and since x^ and x^ are not exactly eqvial in size, that the 

 centre of 2d before these cleavages could not well be situated at the 

 same point as that of 2d after them. This, however, is not a point of 

 practical importance. To all intents and purposes the centre of the 

 cell 2d lies in the future median plane of the embryo. The animal and 

 vegetal poles lie in this plane, so that its relation to the cleaving egg 

 can now be determined. 



2d, after its formation, lies in the furrow between Ic and ID. A 

 glance at the figures from fig. 10 to fig. 33 shows that the second cleav- 

 age plane between the macromcres, up to the 72-ccll stage (fig. 33), very 

 nearly coincides with the median plane of the embryo, since both 2d 

 and the vegetal pole lie in this furrow. At the 72-cell stage, however, 

 these relations are beginning to undergo a change. As described under 

 the history of the entomeres, the macromeres undergo a rotation 

 through 45 degrees, bringing 5D and 5B into the former plane of the 

 second cleavage. This point will be made clear by reference to figs. 

 50 and 52. The median plane of the embryo and adult then passes 

 through 5B and 5D, and forms an angle of 45 degrees with the plane of 

 the second cleavage between the original macromeres. In conclusion, 

 it follows, since 2d marks the posterior region of the embryo, and since 

 during the early cleavage stages it lies in the furrow between the pos- 

 terior macromeres, that the second cleavage plane does coincide in a 

 general way with the future sagittal plane of the embryo, although it 

 is evident that at the 4-cell stage the cleavage plane between C and D 

 must pass to the right of the sagittal plane of the embryo. This 

 result is at variance with the results obtained among the Annelida by 

 most writers, though Wilson (1892) found that the second cleavage 

 coincides with the sagittal plane of the future embryo. Much import- 

 ance, however, cannot be attached to these relations of the entomeres 

 with the embryo, since they are the result of shiftings between the 

 macromeres and micromeres, which may occur at an early period. In 



