No. I.] THE GERM-BANDS OF LUMBRTCUS. 189 



the excretory system. As we have seen, this system first ap- 

 pears as a continuous longitudinal cord of cells ( "the nephridial 

 row " ) lying in the somatopleure, and my observations on this 

 point are in accord with those of Whitman on Clepsine,^ of Hats- 

 chek on Criodriliis^ and of Edouard Meyer on Polymnia nebu- 

 losa.^ Although this cord never acquires a lumen in Lumbricus 

 there can be no doubt from Meyer's observations and my own 

 that it is homologous with the longitudinal excretory canal of 

 Polygordius, Lanice, and Polymnia, which is likewise solid at first 

 (Meyer), and in Polymnia consists of a single cell-row. Several 

 morphologists have compared this canal directly with the seg- 

 mental duct of vertebrates ; but the homology has thus far 

 remained an open question on account of the lack of decisive 

 embryological evidence. This evidence, I venture to believe, is 

 afforded by my observations on the origin of the nephridia in 

 Lumbticns, taken in connection with recent studies on the seg- 

 mental duct. It is impossible to doubt that the nephroblasts of 

 Z«;«^r/«/5, and, therefore, the nephridial rows and the nephridia 

 (excepting the funnels), are derivatives of the outer germ-layer, 

 and, in view of this conclusion, the likeness between the devel- 

 opment of the nephridial row and that of the segmental duct, 

 as described in the recent papers of Spec,"* Flemming,^ and 

 van Wijhe,^ is very significant. In the rabbit (Hensen, Flem- 

 ming), guinea-pig (Spee), and in Raja (van VVijhe) the seg- 

 mental duct arises as a solid cord of cells that is split off from 

 the outer layer and grows at its hinder end by the prolifera- 

 tion of a limited area of the ectoblast. This area is continu- 

 ally carried backwards as the embryo elongates, and, barring 

 certain unimportant differences in the number and arrange- 

 ment of the cells, Flemming's figures of cross-sections, near the 

 growing end of the segmental duct, agree closely with my own 

 sections through the hinder part of the nephridial row in 

 Lnmbricns. An essentially similar account of the origin of the 



1 Zoologischer Anzeiger, No. 218, 1886. 



2Studien iiber Entw. d. Anneliden. Arb. ans. d. zool. Inst. Wien, I., 1878. 

 » Communicated by Lang, Fauna und Flora d. Golfes von Neapl; XL Die Poly- 

 claden, p. 678. 



* Arch. f. Anat. und Phys., 1884. 

 »Arch. f. Anat. und Phys., 18S6. 



* Zoologischer Anzeiger, No. 236, 1886. 



