DEVELOPMENT OF NBPHKIDIA OF PHERETIMA 65 



laterally. Secondary pharyngeal nephridia are formed distal 

 to the primary ones as buds on the ducts of the primary pairs 

 of nephridia (fig. 19 and Text-fig. 3). 



5. Development of the Primary Integumentary 



Nephridia. 



Although I observed short intracellular canals in certain 

 ectodermal cells of very young embryos (gastrulae) in the 

 living condition, and believed these cells to be of the nature 

 of larval excretory cells, I could not definitely locate them 

 in preserved and stained embryos, nor could I examine and 

 arrive at any definite result about these cells in sections. 

 I shall, therefore, confine myself to the development of per- 

 manent nephridia alone. 



As already indicated, the first set of nephridia to make their 

 appearance in Pheretima are the ectodermal primary 

 nephridia, a pair in each segment. At a stage of develop- 

 ment when the embryo has fifty to fifty-five clearly-defined 

 segments and is about 4 mm. long, we can easily see some 

 of the anterior segments (seventh and eighth, for *iample) 

 possessing a pair of fully-formed nephridia opening on the 

 surface of the body-wall. Each of these segments, at this 

 stage, resembles in this respect a segment of the adult L u m - 

 b r i c u s , and we may even call this stage of development of the 

 nephridial system of Pheretima the ' meganephric ' stage. 



The early history of these nephridia is very similar to that 

 described in Lumbricus, Criodrilus, and other worms 

 by previous writers. In an advanced gastrula in which the 

 mesodermal bands are well formed and in which cavities are 

 beginning to appear, we can recognize the earliest beginnings 

 of nephridia. On examining such an embryo, when it is still 

 a rounded sphere and has not begun to elongate, as for example 

 the one shown in fig. 1, which is about 140 /x in diameter, 

 we see the mesoblastic bands diverging from the two large 

 mesodermal pole-cells lying at the future posterior end. These 

 bands lie along the two sides of the ventral surface of the 

 embryo, and, on careful focusing, we can also see the begin- 



