No. r.] THE GERM-BANDS OF LUMBRICUS. 19 j 



Clepsine have apparently become reduced to two in Lumbricus 

 (see postscript), and the "lateral teloblasts" of the two animals 

 appear to be homologous. It is possible, however, that the 

 cells so-called in Lumbricus are homologous with the outer 

 nephroblasts in Clepsine, the outer rows having disappeared. 



The development of the six anterior teloblasts in Lumbricus 

 seems to give the key to an explanation of the phylogenetic 

 origin of their homologues in Clepsine. It is easy to understand 

 the origin of these cells in Lumbricus from the ectoblast, through 

 greater and greater concentration of development at the posterior 

 ends of the germ-bands until first a small group and ultimately 

 single cells, or pairs of cells, became differentiated into neuro- 

 blasts, or nephroblasts. In Lumbricus these cells are at first 

 ordinary ectoblast cells, which afterwards sink below the surface, 

 though still remaining imbedded in the ectoblast. In Clepsine, 

 by acceleration of development, they are covered by the ectoblast 

 at a very early stage, but it is a significant fact that they arise 

 from a blastomere (see Whitman's well-known paper) the remain- 

 ing portion of which breaks up into ordinary ectoblastic cells. 

 Bryn Mawr, Pa., Feb., 1887. 



Postscript, July, 1887. — Since the foregoing article was sent to the printer I 

 have examined the development of L. communis (= trapezoides) , and of Z. agricola. 

 The former is the species studied by Kleinenberg, and the latter has been examined 

 especially by Kowalevsky. Both agree closely with Z. olidus in the structure and 

 development of the germ-bands, and the teloblasts are especially well seen in Z. 



communis. 



L. agricola agrees very closely with L. olidus, but Z. communis differs from both 

 these species in one very interesting respect, namely, that there are five instead of 

 four teloblasts in each germ-band. The mesoblasts and neuroblast are precisely as in 

 the other species, but there are two nephroblasts in addition to the so-called lateral 

 teloblast, which is situated a little farther forwards. Z. communis therefore agrees in 

 every respect with Clepsine. 



A careful comparison of these embryos with those of the other two species has 

 convinced me that the cell I have called the "lateral teloblast" in Z. olidus (X.), 

 is, in reality, a second nephroblast, and that the lateral teloblast of Z. com7nunis and 

 Clepsineho.s disappeared in Z. olidus and Z. agricola, as suggested on p. 121. I 

 have not always been able to find the lateral teloblast in L. communis, but the cor- 

 responding cell-row is always present. This row is Hkewise present in the single 

 embryo of Z. agricola of the proper stage that I have been able to examine, though 

 the lateral teloblast is absent. This embryo (which has been identified beyond 

 question) has a pair of very large primary mesoblasts, although Kowalevsky states 

 that he was unable to find them in embryos of Z. agricola. 



[Methods. —The best results have been obtained by treatment of the embryos with 

 Perenyi's fluid (10-15 minutes) and subsequent staining with borax-carmine, alum- 

 carmme, or picro-carmine. Very clear temporary preparations of early stages may be 



