JENNINGS: DEVELOPMENT OF ASPLANCHNA HERRICKII. 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 sliown 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 mai'gin of the blastopore ; the 

 larger cell is posterior, its products forming the posterior lip of the 

 blastopore. Tiie second cleavage plane, though modified in the pos- 

 terior part of the egg, is approximately longitudinal. In the four-cell 

 stage (Plate 2, Fig. 8) the two cells A^ 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, 

 CD , are respectively 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 macromere 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 fundamei. ally 

 from the orientation used by Zelinka ('91) for the developing e<Tg of 

 the rotifer Callidina russeola. In that species the eg^ is of the t ime 

 form as in Asplanchna. After extensive shifting during developme-^t, 

 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 the e^s,, therefore, the anterior end, the oppo- 

 site (my micromei'e 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 the 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 "macromere end" in tlie 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. 



