CLEAVAGE IN ANNELIDS. 



105 



Fig. 12.- 



Podarke, forty-two hours; prob., 

 problematic bodies. 



From their staining reactions I infer that in Podarke, as in 

 Nereis, these organs have a glandular function. A ventral 

 band of cilia is 

 present, but I 

 have not been 

 able to discover 

 any paratroch. 



The enteron 

 has grown and 

 expanded so as 

 to completely ob- 

 literate the body 

 cavity. Owing to 

 the slow develop- 

 ment of muscular 

 tissue on the dor- 

 sal surface of the 

 larva, the body 

 wall is composed 

 of two very thin layers, ectoderm and endoderm, while the 

 body wall on the ventral side is much thicker, and the amount 

 of mesodermal tissue is evidently much greater. At present I 

 am unable to state just what are the relations of the meso- 

 dermal portion of the larva of this age, but an investigation 

 now in progress will undoubtedly clear them up. Certain 

 it is that the amount of definite mesoderm is very small when 

 compared with such a form as Amphitrite, for in an earlier stage 

 of approximately twenty hours, when Amphitrite has already 

 begun to elongate and the germ bands are composed of "a great 

 many " cells (Mead does not state how many), Podarke has no 

 more than two mesoblast cells on a side. The young larva is 

 very muscular, undergoing all sorts of distortions when irritated ; 

 but this activity is produced by the larval mesoblast cells de- 

 rived from ^d, Jc, and Ja. These cells wander apart from one 

 another, elongate, and form the larval musculature. As said 

 before, I am not yet sure about the relations between these two 

 sorts of mesoderm and the other body layers during the later 

 larval stages ; but I believe that it will be found that by far the 



