54 CONK LIN. [Vol. XIII. 



embryo, and therefore when first formed divided the ovum 

 into an anterior and a posterior half ; while the second furrow, 

 the one in which the polar furrow bends to the left, coincides 

 with the median plane of the embryo, and hence divided the 

 first two blastomeres into two right and two left macromeres.^ 

 While it is thus easy to determine from the earliest appear- 

 ance of the first cleavage what the antero-posterior axis of the 

 future embryo is to be, it is not possible to distinguish the 

 anterior end from the posterior until the stage with twenty 

 ectoderm cells. Fig. 22, when the mesentoblast is formed. A 

 similar relation of the first cleavage plane to the embryonic 

 axes is also found in Teredo (Hatschek, '80), Umbrella (Hey- 

 mons, '93), and Nereis (Wilson, '92). In Crepidula I believe it 

 has no causal relation to the bilateral symmetry of the embryo. 

 The egg itself is not bilateral with respect to the first or 

 second cleavage plane, as has been pointed out (p. ), but is 

 from the first up to the time when the mesentoblast is formed 

 radially symmetrical. So far as the entoderm cells are con- 

 cerned, the second furrow lies nearly in the median plane of 

 the bilateral embryo, and the first furrow nearly at right 

 angles to this ; but among the ectoderm and mesoderm cells 

 such shiftings of position occur that the final plane of bilateral 

 symmetry in no way corresponds with either of the first two 

 cleavage planes. This conclusion will be treated more fully 

 after the facts upon which it is based have been taken up in 

 their regular order. 



II. The Segregation of the Ectoblast. 



I . Fonnation of the First Quartette of Micromeres. Figs. 12, 

 7J, Diagram 3 {p. 60). 



The third cleavage separates four protoplasmic micromeres 

 from the four yolk-containing macromeres. The karyokinetic 



1 It will be seen that in all the figures except those of the first plate the first 

 furrow runs from right to left on the plate ; for the sake of appearance merely, 

 the figures of the first plate are arranged so that the first furrow runs up and 

 down. In the first plate, therefore, the antero-posterior axis runs from right to 

 left as the figures are arranged on the page, while in all the other plates it runs 

 up and down. 



