254 MEAD. [Vol. XIII. 



Formation of metameres. — By the time the blastopore has 

 closed, the paratroch has become conspicuous, and has nearly 

 attained its ring-like form (Figs. 58, 62). (This is not per- 

 fected, however, until after the invagination of the proctodoeal 

 cells — about Fig. 64.) 



The subumbrella now begins to elongate rapidly owing to 

 the further cleavage of the cells just in front of the paratroch 

 — the budding zone. 



The constant parallelism of the prototroch and paratroch 

 indicates clearly the amount of elongation of the trunk region, 

 and shows that it grows on all sides, dorsal, ventral, and lateral, 

 with equal rapidity. The consequent changes of contour are 

 illustrated in the text Figs. X-XVIII. The ectoderm of the 

 trunk becomes thinner and thinner as the latter elongates. 

 The first indication of metamerism is a groove which appears 

 a little behind the prototroch (Fig. 64), and separates the head 

 segment from that of the trunk. I believe that all the ecto- 

 derm posterior to this groove arises from the somatic plate, or, 

 in other words, from the descendants of cell d^ of the i6-cell 

 stage; it is possible that a part of the region in front of the 

 groove is also occupied by somatic-plate cells. 



After further elongation, the trunk divides into two segments. 

 The posterior elongates and divides, thus giving rise to three trunk 

 segments. The subsequent elongation of the body is accompanied 

 by the repeated division of the ultimate metamere. In text Fig. 

 XVII, four setigerous segments are outlined, though the proto- 

 troch and paratroch still persist at opposite ends of the larva. 



No ectodermal teloblasts are distinguishable at any time 

 during the formation and elongation of the trunk. 



Although I have not made a complete study of the structures 

 of the late trochophore and the development of the larval organs, 

 I will record the following observations. 



Alimentary tract. — The cells of the entoderm plate sink 

 into the segmentation cavity in the manner described above, 

 without producing any marked depression on the surface of the 

 egg (Figs. 61, 62, text Fig. VI). Here they commence to 

 divide and form a solid mass of cells, which, with the mesoderm, 

 completely fills the segmentation cavity. 



