No. 3-] PODARKE OBSCURA VERRILL. 405 



time, but owing doubtless to temperature conditions, different 

 lots vary considerably in rapidity of development. (See Mead, 

 No. 22, p. 269.) The 4-cell stage arises from the 2, by the 

 equal division of both cells, there being no large D, as in 

 other annelids. (See PI. XXXVI, Fig. 3.) The spindles 

 are not quite parallel with one another, so that two cells rotate 

 upward and two downward. As a result, the familiar " cross 

 furrow " appears. This is found at both poles, that at the 

 upper being at right angles with the lower, and considerably 

 shorter than it. The origin and significance of this furrow 

 have been discussed by Conklin (No. 5, a), and I can add noth- 

 ing to what he has said. It is of considerable practical impor- 

 tance in Podarke for purposes of orientation. The direction of 

 this furrow at the lower pole is positively the only means of 

 orientation before the completion of the 56-cell stage. I have 

 taken a good deal of pains to ascertain if it remains constant in 

 direction up to that time. There is no doubt that it does, 

 although soon after the completion of the 64-cell stage this 

 direction may be lost, owing to movements of the entomeres 

 whose position determines the existence of the furrow. 



The relation of the two first cleavage planes to the body 

 axes is a point on which most investigators have laid consider- 

 able stress. In the cytogeny of annelids and mollusks two 

 apparently distinct relations are found. In Nereis (No. 34, d), 

 Crepidula (No. 5, a), and Limax (No. 17), for example, the sec- 

 ond cleavage furrow is said to coincide with the future median 

 plane, while in Amphitrite (No. 22) and Arenicola (No. 4) the 

 quadrants are anterior, median, right, and left. This apparent 

 discrepancy has been explained by Lillie (No. 21) and Conklin 

 (No. 5, a) as arising from the fact that only a small part of the 

 furrow, that between the entomeres, has been considered in 

 determining the orientation. Conklin has shown that in anne- 

 lids and mollusks there is no exception known to the rule that 

 the second and fourth quartettes lie in the median and trans- 

 verse planes, while the first and third lie between them. An 

 examination also of Wilson's figures of Nereis will show that 

 the second cleavage furrows, if traced between the ectomeres, 

 lie at some distance from the median plane of the embryo. 



