4l6 TREADWELL. [Vol. XVII. 



that the reversion is always to the radial from the bilateral 

 (from the type of the posterior arm to that of the anterior) 

 and not in the opposite direction is suggestive. The facts are 

 perhaps too few in number to warrant wide inferences, but so 

 far as they go they seem important. 



The next division of the cross cells emphasizes still more the 

 bilateral symmetry. The division of the basal cells in A and 

 B quadrants, begun in PI. XXXVII, Fig. 17, is completed in 

 PI. XXXVII, Fig. 18, and there result three subequal cells 

 in a row in each arm. In C and D quadrants the upper cells 

 divide next, but divide bilaterally, the cells sloping inward 

 (PI. XXXVII, Fig. 18). In C quadrant the division is equal, 

 in D quadrant unequal. (See PI. XXXVII, Fig. 19.) This 

 result was so unexpected that I suspected it might be an 

 abnormality, but in examining a large number of prepara- 

 tions I found always the same result. 



While this last division is in progress in the basal cells of 

 the posterior arms the terminal cells divide, ici. 1.2.2 usually 

 a little in advance of idi. 1.2.2. The division is bilateral, and 

 there result two small cells, lying a little farther from the 

 median plane than the center of the large terminal cell. These 

 small cells are very little larger than their nuclei, which stain 

 very deeply, and they lie on top of the cross, between it and 

 the basal cells of the arm. They are excellent landmarks for 

 later study of the embryo, as it is possible at a glance to recog- 

 nize the C and D quadrants by means of these small deeply 

 staining cells (PI. XXXVII, Fig. 20). 



In Nereis, Wilson (No. 34, d) described cells similar to these 

 in origin and position, which he called, provisionally, " head 

 kidneys." In Amphitrite corresponding cells were found by 

 Mead (No. 22), from which arise the huge mucous glands of 

 the trochophore, and Wilson (No. 34, /) has since suggested 

 that possibly they are slime glands in Nereis also. In Podarke 

 the next division of the dorsal arms of the cross (PI. XXXVII, 

 Figs. 20 and 21) buds off one other cell on either side. This 

 cell is very little larger than the first one, and the two pairs 

 remain in this position until very late in the development. 

 The last stage in which I was able to recognize them with 



