No. 3.] THE CELL-LINEAGE OF NEREIS. 387 



3. The Third Cleavage. 



In the third cleavage, which takes place about one and three- 

 quarters hours after fertilization, the spiral character first 

 comes clearly into view. Each of the blastomeres divides 

 somewhat obliquely (Fig. 7^) into unequal parts, a smaller upper 

 micromere, which contains none of the large oil-drops, and a 

 larger macromere below. The four micromeres thus formed 

 {a}, b^, c^, d^), I shall call the first group of micromeres. From 

 them arise the entire upper hemisphere of the trochophore, the 

 head-kidneys, and the cells of the prototroch.^ 



Side views at the moment of division (Fig. 7^) show that each 

 micromere is displaced somewhat towards the left. This dis- 

 placement increases during the division, and reaches a maximum 

 during the succeeding resting stage (Figs. 10, 11, 12), when the 

 four micromeres alternate with the macromeres. [This is only 

 approximately true, since the left posterior micromere {d^) only 

 slightly overlaps the anterior macromere, A.'\ The displacement 

 may be described as a rotation of the four micromeres through 

 an angle of 45° to the right [the embryo being viewed from 

 above], and the cleavage as a whole may be characterized as a 

 right-handed spiral — i.e. following the hands of a watch. 



I have examined the transition from the four-celled to the 

 eight-celled stage with especial care, in order to determine if 

 possible whether the rotation of the micromeres (which is a very 

 common phenomenon in other animals) is a purely mechanical 

 process caused by pressure, etc., or is rather a result of the 

 internal phenomena of division. The result leaves little doubt 

 that external mechanical causes cannot be regarded as the only 

 cause of the rotation, since the nuclear spindles show the spiral 

 arrangement before there is any external sign of division. Fig. 7 

 (from a hardened and stained specimen) shows a four-celled 

 stage from the upper pole, immediately before the division. The 

 upper poles of the spindles are represented by the centres of 

 the stars, the lower by the small circular outlines. Of the four 



1 Von Wistingbausen asserts that they give rise only to the cerebral ganglia with 

 their appendages, and hence terms them " encephaloblasts." If this be true, the 

 development of N. Dumerilii must differ to an extraordinary degree from that of 

 the American species; but it appears to me that v. Wistingbausen gives very little 

 evidence in support of his conclusion. (See p. 435.) 



