No. 3.] PODARKE OBSCURA VERRILL. 423 



later stage I have seen, occupying the position of these 

 cells on either side, a row of two or three small cells, which 

 apparently have come from a division of X3.2.1+ and X3.2.2+, but 

 I am unable to give any positive statements about their 

 origin or destiny. They have an appearance which strongly 

 suggests teloblasts, but I do not know if they have that 

 character. Indeed, the cells X3.2.1 and X3.2.2 (PI. XL, Fig. 

 59) look very like teloblasts, but if they are their teloblastic 

 growth must begin much later, for none of the ectoderm 

 anterior to them has, in the stage figured, come from their 

 division. Their later position also resembles very much that 

 of the paratroch cells figured by Mead (No. 22) in Amphi- 

 trite, but in the latest stages I have seen no cilia are found 

 on them. 



I have described these divisions of the X-cells at some length 

 because of their value for purposes of comparison with other 

 annelids. Amphitrite and Arenicola (No. 4) are the only annelids 

 where the exact origin of the paratroch is known. Podarke, 

 as said before, probably has no paratroch, but so important an 

 organ might, on one theory of cleavage, be supposed to be 

 present as an ancestral rudiment ; and I was interested to 

 follow the cleavages in the three genera to see if any similari- 

 ties appear. The divisions of the X-cells in Podarke beyond 

 the stage of PI. XXXVIII, Fig. 30, have no similarities what- 

 ever to those of Amphitrite (No. 22) nor to those of Arenicola, 

 as I learn from a comparison with figures kindly given me 

 by Dr. Child. 



I have already spoken of the way the dorsal ectoderm cells 

 flatten out and spread apart. It is as if the cells from the 

 upper hemisphere spread out like a fan, with their center near 

 the upper edge of X3.2, and carried the X-cells with them around 

 towards the ventral side. The direction in which the forces 

 which produced this shifting must have acted is shown in 

 PI. XL, Fig. 59, by the elongated cells X3.2.1 and X3.2.2. 



The break in the prototroch remains open much later than 

 in Amphitrite, and is represented as just closing in PI. XL, 

 Fig. 58. Whether or not this be regarded as the cause, the' 

 result is that a much larger portion of subumbrellar ectoderm 



