38 SUMMARY OF CURRENT RESEARCHES RELATING TO 



ill tlio form of simple, longitudinal cell-strings, and that each nephridial 

 cell-string is a product of a single terminal cell — the nephrohlast. It 

 is suggested as an explanation of the divergent accounts which have 

 been given as to the origin of the nephridia that both mesoblasts and 

 nephroblasts arose primarily from a common ectodermic basis ; the 

 genetic relations of the two cells have remained essentially the same, 

 but the time of their differentiation as distinct cells varies. If the 

 division takes place within the ectoderm, then each makes its exit from 

 the original scat separately and independently of the other ; if, on the 

 other hand, division is delayed until after the separation from the 

 ectoderm is accomplished, then the nephrohlast appears to arise from 

 the same source as the mesoblastic bands, and thus to form a part of 

 them. 



There is a special note on the significance of the teloblasts or blasto- 

 meres derived from the posterior macromere of the dividing ovum ; they 

 are one of the most remarkable features of annelid development, and 

 represent specialized centres of proliferation, with most marvellous 

 powers of assimilation and reproduction. The author regards them as 

 constituting the trunk-bud, and as thus being the primary seat of all the 

 truly metameric elements of the animal. Primarily they represented 

 the basis of non-metameric organs, in which the regenerative power was, 

 or became, pre-eminent. He refuses to recognize the tenability of the 

 theories which regard the somites of segmented animals as derivatives 

 of gut-pouches, and declares that metamerism does not first exhibit itself 

 either in the archenteron or the mesenteron. 



Salivary Glands of Leech.* — Sig. D. Bertelli has investigated the 

 structure of the salivary glands in Hirudo medicinalis. These glands 

 are situated at the so-called roots of the jaws. They are unicellular, 

 nucleated, pyriform, and very numerous. Each has an efferent duct, 

 and contains a granular substance which is also observed to occur in 

 the ducts. These proceed upwards, penetrate the jaw beside the elements 

 forming the root, and open on the free margin. By setting the animal 

 to work, and then rapidly examining the jaws in a 1/2 per cent, salt 

 solution, the author was able to observe the granular substance flowing 

 from the free margin. 



Germ-bands of Lumbricus.'j" — Prof. E. B. Wilson has a preliminary 

 notice of his study of the development of Lumbricus olidus ( = L.fcetidus). 

 As in the species examined by Kowalevsky and Eleinenberg, the germ- 

 bands end behind in a pair of large "mesoblasts" at the expense of 

 which the bands increase in length throughout the whole course of 

 development. As development proceeds six other large cells are added, 

 and these eight may, in the language of Whitman, be spoken of as telo- 

 blasts. Each of the eight gives rise to a row of cells, at first single, 

 which extends forwards between the ectoblast and endoblast ; the rows 

 proceeding from the "mesoblasts" soon widen into a pair of broad 

 plates which ultimately give rise to the septa, muscles, vessels, and 

 possibly setigerous glands. The six remaining rows are intimately 

 related to the mesoblast. The two inner rows give rise to the halves of 

 the nerve-cord, and their large cells are, therefore, neuroblasts precisely 

 as in Clepsine ; the adjoining rows will give rise to the nephridia, and 



* Proc. Verb. Soc. Toscana Sci. Nat., v. (1887) pp. 284-5. 

 t Journ. of Morphology, i. (1887) pp. 183-92 (1 pi.). 



