136 PROFESSOR E. VAN BENEDEN. 



transparent material, stained by hsematoxylin and picrocar- 

 mine, escape. A small nucleolus, generally spherical, is 

 found in the nucleus. The nuclei themselves are invisible in 

 living Dicyemidse, but are brought into view by the action 

 of acetic acid. 



The locomotion of the Dicyemidse is effected by the 

 vibratile cilia, which cover the whole surface of the body. 



The axial or endodermic cell. — This cell extends through- 

 out the whole length of the body, and is in contact with all 

 the cells of the endoderm ; it is cylindrical and is pointed at 

 each end ; it is difficult to decide whether or not a limiting 

 membrane is present. The substance of the cell is traversed 

 by a protoplasmic network whose meshes are filled with a 

 transparent homogeneous and colourless substance of gelati- 

 nous consistency (fig. 2). The lamellae of protoplasm com- 

 posing this network are very delicate, and may be seen as 

 very fine lines presenting granulations here and there ; the 

 form of the meshes thus produced is extremely variable, and 

 the whole network is perceived only with difficulty in the living 

 organism, except when the embryos disturb it as they pro- 

 gress slowly through the axial cell \ it is readily brought into 

 view, however, by treatment with osmic or acetic acid or with 

 logwood. The vacuoles or spaces in the network are filled 

 with a hyaline fluid which does not mix with water, and 

 forms a large spherical drop at the extremity of an endodermic 

 cell which has been cut across. This cell invariably presents 

 an enormous egg-shaped nucleus situated near its centre (figs, 

 land 9, w), which is generally of regular form, and is bounded 

 by a thick membrane with a double contour line, which can 

 be broken by pressure. The interior of the nucleus is 

 traversed by a network of exceedingly fine filaments, called 

 the nucleoplasm, but it is only seen in old nuclei; here 

 and there also one or two large and highly refracting cor- 

 puscles,the pseudo-nucleoli, are seen. The nucleolus is always 

 single, small, and generally spherical in shape, whilst its posi- 

 tion within the nucleus varies much. 



Reproduction and embryonic development. — The axial cell 

 is at the same time the germ-gland and the uterus. Since 

 the publication of KoUiker's work it has been known that 

 the Dicyemidai produce two kinds of embryos, one vern)iform, 

 the other infusoriform that the two are never found in the 

 same Dicyema ; and that the external characteristics of the 

 individuals which produce embryos of the first kind are 

 often different from those producing larvce of the second de- 

 scription. The species of Dicyemidse which produce vermi- 

 form young Mr. Van Beneden calls *' Nematogenous," 



