424 TREADWELL. [Vol. XVII. 



comes from these cells than from the corresponding cells in 

 Amphitrite. In fact, a very limited portion of the subiimbrella 

 is formed from the 'K-cells, as is shown in PI. XXXIX, Fig. 48 ; 

 PI. XL, Figs, 58 and 59. To this point, which is mainly of 

 importance in a discussion of homologies, I shall return. (See 



p. 467.) 



As a result of this migration and expansion of the dorsal 

 ectoderm cells, the X-group are forced ventrally and finally 

 concresce on the ventral surface (PI. XXXIX, Figs, 46-48). 

 I am not absolutely certain that some of the cells figured as 

 lying just outside the X-group may not really belong to the 

 latter. The descendants of the third group of micromeres lie 

 close to the X-group on either side, and so many shiftings 

 occur that it is impossible to follow every cell. For our present 

 purpose the precise history of every cell is not absolutely neces- 

 sary. The point of importance is that the X-cells surround 

 the proctodaeum and, if it be safe to rely on analogy, make up 

 the budding zone of the larva. I have thus far been unable to 

 rear larvae, though they will live in confinement for as many as 

 ten days. Probably connected with the lack of proper food 

 (for the small amount of yolk in the Qgg must be rapidly used 

 up) is an exceedingly slow rate of development during the time 

 that they will live in captivity. Similar difficulties with other 

 annelids have been described by Hatschek (No. 11) in Eupo- 

 matus, von Drasche (No. 7) in Pomatoceros, and Mead (No. 22) 

 in Lepidonotus. Beyond a slight increase in length, practi- 

 cally no growth takes place after the second day. In Amphi- 

 trite, Mead has proved that all of the body behind the first 

 segment arises from descendants of 2d. For reasons above 

 mentioned, I am unable to make any positive statements about 

 their fate in Podarke, but in view of their position I think the 

 presumption is strong that here also the body ectoderm arises 

 from 2d. 



Of interest in this history of the X-cells in Podarke is the 

 fact that their divisions are not bilateral, so that until at least 

 a very late stage the cells are not symmetrically placed with 

 regard to the sagittal plane. PI. XXXIX, Fig. 46, for example, 

 shows a marked asymmetry. This asymmetry is partly due to 



