142 PROFESSOR E. VAN BENEDEN. 



dermic cell, which is fusiform or cylindrical, and which 

 extends the whole length of the body; and of a layer of 

 cell-plates lying on the surface of this cell, like simple pave- 

 ment epithelium. There never exists any trace of an inter- 

 mediate cell-layer, or of a general space between the axial 

 cell and the ectoderm. 



III. — The cells which form the anterior extremity of the 

 body form also the head of Dicyemidse, and they are 

 peculiar in their form and composition. There are eight or 

 nine of these polar cells, and .they form the polar velum, 

 being disposed in two rows concentrically round a central 

 point called the oral pole of the Dicyema. Cells called the 

 parapolar may also contribute to form the cephalic dilatation. 

 The Dicyemidse are bilaterally symmetrical, and this sym- 

 metry is particularly well marked in the head of all species 

 and in the infusoriform embryos. 



IV. — The ectoderm is made up of cell-plates which form 

 half canals, and are applied by their concave faces to the 

 axial cell. In these cells refracting globules appear, which 

 accumulate at certain points to form protuberances, to which 

 Prof. Van Beneden has applied the term " warts." 



V. — The endodermic cell has the structure of a vegetable 

 cell, or the endodermic cells of a Hydromedusa or the body 

 of a Noctiluca. It is traversed by a protoplasmic network, 

 whose meshes are filled with a hyaline fluid of gelatinous 

 consistency. 



VI. — It is in this cell of the endoderm that the germs are 

 formed, and it is here that they develope the embryos. 



VII. — Each species of the Dicyemidae includes two kinds 

 of individuals, the Nematogenous and the Rhombogenous, 

 which differ in their external characteristics, in their organi- 

 zation, and in the character of the germs which they pro- 

 duce, in the mode of formation of these germs, and in the 

 development and constitution of the embryos. The Nema- 

 togenous produce vermiform embryos, and the Rhombogenous 

 infusoriform embryos. 



VIII. — The Nematogenous germs are produced endogen- 

 ously in the protoplasmic filaments of the axial cell. An 

 i>u epibolic gastrula, whose ectoderm consists of a single cell, 

 is formed with mathematical precision after cleavage has 

 taken place. The cells of the ectoderm multiply, the blasto- 

 pore closes, the body elongates, and two germs appear in the 

 axial cell ; finally, the gastrula becomes a vermiform embryo, 

 which passes through the body-wall of the mother into the 

 external world. From this period the transformation of the 

 Dicyema into an adult takes place by the progressive increase 



