Structure and Development of the Nephridia. 393 



(Piate 25, Figs. 44, 46 — 49) as in the adult. The slip of transversa 

 muscle which binds the nephrostome down to the body-wall has already 

 appeared at the stage of Fig. 44, in a manner not satisfactorily 

 imderstood. The slip however is associated with the segmentai 

 blood-vessel and is attached to the anterior border of the nephro- 

 stome near the point where the nephrostomial blood-vessel takes it 

 rise. The fact of its attachment to the dorsal lip at the anterior 

 angle of the fimnel becomes thus to some degree intelligible. 



The origiu of the germ-cells, which arise in connection with 

 the early nephridial vessels, may be briefly referred to bere. Mere 

 mention of the place and manner of origin of these cells will be 

 sufficient, since the spermatogenesis and ovigenesis of Arenicola bave 

 been the subject of special investigation at Woods Hole by other 

 authors an account of whose work will it is hoped appear before long. 



The nephridial vessels in somites VII — X (2""^ to 5*^ nephridia) 

 are associated with the formation of the gonads. The early germ- 

 cells in connection with each nephridium become distinguishable 

 soon after the appearance of the blood-vessel of the latter, and arise 

 as a proliferation of the peritoneal cells of its wall. They appear 

 first on the anterior and first-formed portion of the vessel, i. e. in 

 the region immediately adjoiuing the posterior angle of the funnel, 

 and at their first appearance (Piate '23, Fig. 27) bave the form of 

 somewhat enlarged vesicular nuclei surrounded by a small quantity 

 of protoplasm and otherwise differing but slighty from the unaltered 

 peritoneal cells. As growth proceeds the germ-cells increase in 

 number and in size and acquire a very characteristic appearance 

 Piate 25 , Fig. 56). The nucleus becomes greatly enlarged and is. 

 (surrounded by a well-defined protoplasm of homogeneous appearance 

 Fig. 56 represents the appearance of these cells (oö-or spermatogonia) 

 on the second nephridium of a larva of a well-advanced stage. 

 The large vesicular nuclei, whose chromatin is arranged in scattered 

 clumps closely applied to the nuclear membrane, already bear a 

 dose resemblance to the nuclei of the large oögonia or spermato- 

 gonia of the adult gonad. The germ-cells usually appear on their 

 respective nephridia in the order of the formation of these organs, 

 i. e. in order from before back. I bave never found them in con- 

 nection with the first or last pair of nephridia, which typically do 

 not bear gonads in the adult. 



The later development of the nephrostome consists essentially in 

 the extension of its epithelial surface, and in the completion of the 



26* 



