JENNINGS: DEVELOPMENT OF ASPLANCHNA HERRIOKII. 15 
the region where the blastopore is found at a late stage. The orien- 
tation is thus based upon the structure of the gastrula. The chief axis 
of the gastrula is the line connecting the animal and vegetative poles. 
The first cleavage plane, though coinciding with the chief axis of the 
gastrula, is, as shown by the later development, transverse to the long 
axis of the embryo. The smaller cell of the two-cell stage is anterior, 
since its products occupy the anterior margin of the blastopore ; the 
larger cell is posterior, its products forming the posterior lip of the 
blastopore. The second cleavage plane, though modified in the pos- 
terior part of the egg, is approximately longitudinal. In the four-oell 
stage (Plate 2, Fig. 8) the two cells 4? and B°’, resulting from the 
division of the smaller cell, AB”, are respectively left anterior and right 
anterior, while C? and D’, produced by the division of the larger cell, 
OD’, are respeotively right and left posterior. 
A section taken transversely to the chief axis of the gastrula will be 
spoken of as a transverse section. A section at right angles to this, 
passing from anterior to posterior and including the animal and vegeta- 
tive poles, is a sagittal section. A section at right angles to both of 
these, cutting both the animal and the vegetative pole and passing 
through the right and left sides, is a frontal section. 
As will be seen from the above, in the two-cell and four-cell stages, 
the micromere end coincides with the anterior, the maeromere end with 
the posterior end.! 
The orientation given above is based upon the relation of the egg to 
the axes of the gastrula; the same is true of the orientation used in 
most of the recent works upon cell lineage. It differs fundamentally 
from the orientation used by Zelinka (791) for the developing egg of 
the rotifer Callidina russeola. In that species the egg is of the same 
form as in Asplanchna, After extensive shifting during development, 
the anterior end (in both Asplanchna and Callidina) comes to lie in the 
region of that end of the egg which I have called the macromere end. 
Zelinka calls this end of tho egg, therefore, the anterior end, the oppo- 
site (my micromere end) the posterior end. Anterior and posterior in 
Zelinka’s orientation of course remain constant with regard to the form 
of the egg, but not with relation to tho parts of the embryo. Thus, if 
1 It is of the greatest importance to observe that I do not use the terms “ micro- 
mere end” and * maeromere end” in the same sense in which“ micromere pole“ 
and “macromere pole" are sometimes used, as-synonymous with “animal pole“ 
and “vegetative pole.” The two terms are used only as a convenient way of 
indicating a peculiarity of the rotifer egg. 
