No. 3.] THE CELL-LINEAGE OF NEREIS. 403 



elongates (Fig. "jG), the vacuoles coalesce so as to form a sinu- 

 ous canal in the protoplasm. As the elongation proceeds, the 

 canal becomes narrower and more distinct, and the head-kidney 

 is thus converted into a slender tube (Figs. Jj, 81, 82). Up to 

 the stage shown in Fig. ^6 the head-kidney is certainly unicellu- 

 lar ; whether the nucleus divides in later stages I cannot say. 



When fully formed, the head-kidneys entirely surround the 

 body and again lie partly inside the prototroch, as in Fig. 82. 

 Near their anterior ends (which appear always to overlap, as 

 shown in Fig. 79), they expand somewhat and then taper 

 to a blunt point. Their posterior ends (Figs. 6j to 70) are 

 pointed and terminate near the gap in the prototroch. In the 

 latest stages in which I have observed it (Fig. 89) the head- 

 kidney lies completely inside the prototroch, considerably flat- 

 tened against the cells of the latter. Its ultimate fate remains 

 undetermined. 1 



1 1 have termed this organ the head-kidney only with a certain reservation, since 

 I have not succeeded in observing any evidence of cilia in the cavity, or any sign of 

 an external opening. It is, however, very difficult to explore the interior of the organ 

 in the living embryo on account of the confusion produced by the active vibrations 

 of the prototrochal cilia; and the external opening of the head-kidney in other forms 

 is notoriously difficult to see. The only other structures with which it might be con- 

 founded are the ring-nerve and the ring-muscle, with the appearance of which in 

 other annelid trochophores I am familiar. Neither the effect of methyl-blue nor of 

 osmic acid gives decisive results. The protoplasm shows no longitudinal striation, and 

 the canal always distinctly appears. 



It should be added that I have not been able to see either ring-nerve or ring- 

 muscle in the Nei'eis trochophore. That the muscle is really absent is indicated by 

 the fact that the prototrochal region is never contracted either in preserved speci- 

 mens or in living larvae, however the latter may be stimulated. 



In view of the peculiar mode of origin of the " head-kidneys," I may be 

 permitted a few words of explanation. I had observed various stages of the enclosure 

 and migration of the nephroblasts in scores of specimens before realizing the true 

 nature of the process, for such a migration seems from an a priori point of view so 

 improbable that the possibility never occurred to me, especially as I had looked for 

 a wholly different origin for the head-kidney. In the earlier stages I had often 

 observed the nephroblasts projecting below the ectoblast, and in later stages the 

 developing head-kidneys lying below the prototroch. The disappearance of the 

 nephroblasts from their original position I supposed to be caused simply by their 

 withdrawal into the ectoblast. I was led to the discovery of what actually occurs 

 first, by my failure to account for the nephroblasts in the surface-views, and second, 

 by finding stages like Fig. 74, where the nephroblast hes directly inside of the proto- 

 troch. A careful examination of the preceding stages soon showed every transition, 

 and I believe that whatever doubt may exist as to the nature of these organs, there 

 can be none as to their mode of origin. 



