436 TREADWELL. [Vol. XVII. 



opening, as I believe, moves backward to the position described 

 above, and I think other openings break through it, but of this 

 I am not certain. When first formed, the partition is ciliated 

 above and below. I do not know how long that condition lasts. 

 (See PI. XL, Fig. 60.) The section passed a little to one side 

 of the central opening, which therefore does not show in the 

 drawing. 



The suggestion has been made that this partition represents 

 the first septum, but I do not believe that is so. So far as 

 I can discover, only entoderm cells enter into it. It seems to 

 me rather to lie at the boundary line between " stomach " and 

 " intestine " of the larva, and to be merely an exaggeration of 

 the constriction found at that point in other annelid larvae. 

 (See No. 7, PI. XXXVIII, Fig. 26.) 



As already stated on p. 435, the wall of the archenteron swells 

 out so as to lie close under the ectoderm. Since dorsally both 

 layers are very thin and transparent, it becomes difficult, on 

 surface views, to make out the outlines of the cells. Indeed, 

 it frequently is impossible, except by rolling the specimen so 

 as to get an optical section, to determine to which layer a 

 given nucleus belongs. Ventrally (PI. XL, Fig. 60), the ecto- 

 derm is much thicker. 



At this point the cell lineage naturally ends, and the study 

 of the trochophore begins. Since the problems of the latter 

 subject are so different from those of the former, it has seemed 

 best to reserve the metamorphosis of the trochophore for 

 another paper. 



Comparison ivith other Annelids having '^ egnaT' Cleavage. 

 — The most complete account of the cleavage of an equally 

 segmenting annelid previously published is that of Mead 

 (No. 22) on Lepidonotus. To Dr. Mead's generosity I have 

 been indebted for preparations of the later stages. A few 

 observations have also been made on the cleavage of Hydroides 

 dianthns and Stheiiolais picta, both of which resemble Podarke 

 in their mode of cleavage. I have not as yet carried the 

 development of these forms far enough to justify a detailed 

 account of the process, but enough has been seen to show 

 as I pointed out in a preliminary paper (No. 29, a) that bilateral 



