No. I.] GERM-LAYERS IN CLEPSUSTE. jgi 



The series of sections shown in Figs. 15-19, beginning near 

 the middle of a posterior somite, and running forwards to the 

 middle of the next in advance, shows that the cells of the nephric 

 plates multiply in depth as well as breadth. Fig. 18 represents 

 a section on the boundary-line of two somites, in which not 

 a single nephridial cell could be found. It is in this region that 

 we meet with a pair of large cells {sex) lodged in the meso- 

 derm. A single pair of these cells occurs in each somite, and 

 their position in the walls of the septa suggests that they may 

 be the mother-cells of the testicular organs. Nusbaum claims 

 to have traced the development of these cells into the sexual 

 organs; but he has evidently confounded the cells of the neuro- 

 nephric stratum with the sexual cells. 



Returning to the nephridia, the points of chief interest in 

 their development appear to be the following : — 



1. Derivation from the ectoderm. 



2. Earliest appearance in the form of simple, longitudinal 

 cell-strings. 



3. Each nephridial cell-string is a product of a single ter- 

 minal cell, — the nephroblast. 



As soon as I became aware of the precise origin of the ne- 

 phridia, I began to question the validity of the opinion that the 

 nephroblasts were ectoblastic. It was almost universally believed 

 that the nephridia take their origin in mesoblastic elements. 

 In view of this, I did not venture to discuss the question in my 

 preliminary paper (No. 16). 



Wilson's paper on Lumbricus settles the point, leaving little 

 room for a reasonable doubt as to the ectoblastic nature of all 

 the teloblasts concerned in the production of the neuro-nephric 

 stratum. The establishment of this fact, taken in connection 

 with recent discoveries pointing to the ectoblastic origin of 

 the vertebrate segmental ducts, paves the way to a better under- 

 standing of the phylogenetic derivation of these organs. 



In this connection Bergh's discovery that the larval nephridia 

 of the Gnathobdellida3 arise as lateral outgrowths from the 

 germ-bands is especially important. Adding this to the dis- 

 covery of distinct nephridial cell-strings, we have a remarkably 

 perfect picture of the more important steps in the development 

 of the pronephros of Petromyzon. 



