No. I.] GERM-LAYERS FN CLEPSIIVE. 139 



III. THE ECTODERM AND ITS PRODUCTS. 

 I. The Ectoderm. 



The origin of the first four ectodermic cells (micromeres) has 

 been described under the head of cleavage and axial relations. 

 By the addition of numerous other micromeres, arising, mainly, 

 from the anterior and the lateral macromeres, a sort of blasto- 

 disc is gradually formed, centered at the upper pole of the egg. 

 This blastodisc is not wholly ectodermic, for a few of its deeper 

 cells, as we have seen, represent the earlier entoderm cells, as 

 was first suggested by Bergh. The superficial, ectodermic por- 

 tion of the blastodisc gives rise to the epidermal layer and its 

 derivatives, the stomodseum, sense-organs, etc. 



The ectoderm includes, in addition to the superficial portion 

 of the blastodisc, all the teloblasts, except the two larger and 

 and deeper ones, which represent mesoblasts. The grounds for 

 regarding the eight smaller teloblasts as part of the ectoderm 

 are the following: i. They have at the outset a superficial 

 position at the hind edge of the blastodisc. 2. Two of them 

 give rise to the ventral nerve-cord. 3. In Lumbricus (vide 

 Wilson) they lie in, and plainly form a part of, the general ecto- 

 derm. 



2. The Ventral Nerve-chain. 



In a preliminary paper (No. 16) I have already stated that 

 the nerve-chain of Clepsine first appears in the form of two sim- 

 ple, unsegmented rows of cells ; and, further, that each row 

 is the product of a single cell, the neuroblast. At the time 

 this fact was announced nothing of the kind was known in any 

 other animal ; and Nusbaum, the latest authority on Clepsine, 

 had just arrived at an entirely different conclusion, and one 

 altogether more in harmony with traditional views. A similar 

 discovery has since been made by Wilson in Lumbricus, and, 

 fortunately, the evidences in both cases can now be presented 

 side by side. The subject is one which has received a good 

 deal of attention, and given rise to considerable discussion. 

 Before reviewing the opinions of other writers, or giving my 



(16.) Whitman, C. O. The Germ-Layers of Clepsine. Zool. Anz. No. 

 218. 1886. 



