DEVELOPMENT OF NEPHRIDIA OF PHERETIMA 59 



Vejdovsky and Beddard speak of two perfectly distinct 

 membranes forming the shell of the cocoon. I have not been 

 able to see these two membranes in the case of Pheretima 

 cocoons, the shell of which seems to me to be single-layered. 



4. General Outline of the Development of 

 Nephridia in Pheretima. 



The three sets of nephridia of Pheretima, namely, the 

 integumentary, the septal, and the pharyngeal arise in the 

 embryo at successive stages of its development. In order to 

 elucidate, therefore, the development of the whole nephridial 

 system consisting of these three distinct series of nephridia 

 and their ducts, it is necessary to examine a large number 

 of embryos of widely different ages. The work is rendered 

 laborious and difficult on account of three facts : firstly, that 

 each type of nephridium develops independently of the other — 

 these several types are not derived one from the other ; 

 secondly, that the nephridia of the three series develop at 

 different ages and in different positions in the embryo ; and 

 thirdly, that each series consists of numerous nephridia that 

 go on developing for a long time even after the embryo has 

 left the cocoon. But before going into the details of each 

 stage of nephridial development, I shall provide here an out- 

 line sketch of the development of the elaborate excretory 

 system of this worm. 



Leaving aside the transitory excretory cells the earliest 

 beginnings of permanent nephridia appear in this worm, as 

 in Lumbricus (18), Khynchelmis (16), and Crio- 

 drilus (15), as teloblasts lying on the surface of the embryo, 

 ventral to the mesoblastic bands and in front of the meso- 

 dermal pole-cells. While these teloblasts form part of the 

 surface epiblast in very young embryos (300 ij- long), they soon 

 sink below the surface and come to lie between the definitive 

 ectoderm and the mesoderm. Strings of cells are budded off 

 from and in front of these ectodermal teloblasts, and it is these 

 cell-rows (nephroblasts) that form the material foundation 

 (' Anlage ') from w^hich are derived all the future nephridia. 



NO. 261 F 



