ZOOLOGY AND BOTANY, MICROSCOPY, ETC. 195 



and in them neuroglia-nuclei are more or less sparingly developed. 

 The contents of the tubes consist of primitive tubes which are extremely- 

 slender or cylindrical, separated from each other by membranes of a 

 firm supporting spongioplasm, which very much resembles the neuroglia 

 substance ; these slender tubes contain a hyaline, viscous substance — 

 hyaloplasm, which is the real nervous substance, and is very often 

 exuded from fresh isolated nerve-tubes in the form of small hyaline 

 pearls. The fibrillae and fibres described by most writers, do not. in 

 Mr. Nansen's opinion, exist. 



In many of the largish nerve-tubes of Nephrons and Homarus there 

 is a concentration towards a kind of axis ; this axis may be more or less 

 narrow, and consists of a bundle of central primitive tubes which have 

 stouter spongioplasmic sheaths and smaller diameters than the otber 

 primitive tubes. In the other animals examined this concentration 

 cannot, as a rule, be observed ; but there is a slight indication of it in 

 some nerve-tubes of Nereis. 



The ganglion-cells of all bilateral invertebrates consist of a nucleus 

 with distinct membrane and a varying internal structure, and also of a 

 protoplasm with various constituents ; the cells are inclosed in a mem- 

 brane of neuroglia substance. The principal constituents of the proto- 

 plasm are primitive tubes of the same structure as those in the nerve- 

 tubes ; some of them very frequently circulate concentrically round the 

 nucleus, and so give a concentrically striated appearance to the ganglion- 

 cells. In some, especially those of Homarus and Neplirops, the primi- 

 tive tubes are partly united in bundles or in smaller or larger masses, 

 which stain much more lightly than the rest of the protoplasm, not- 

 withstanding the presence in it of a number of primitive tubes. 



In very many, possibly in all, ganglion-cells, there is a spongio- 

 plasmic reticulation, extending from the inclosing neuroglia membrane 

 into the protoplasm, between the primitive tubes and intimately con- 

 nected with their spongioplasmic sheaths. In addition to this reticula- 

 tion there is. probably, a special fatty (? myeloid) substance ; it is not 

 improbable that it is the same substance as that which in a number of 

 animals is connected with a pigment (? haemoglobin). The cells give off 

 nervous processes and protoplasmic processes ; of the former there is 

 always one and never more ; it is generally directed centrally, towards 

 the dotted substance. The unipolar cell is the most common in 

 bilateral invertebrates, but, when the cells are multipolar, the other 

 processes are protoplasmic ; these are generally short, and directed 

 peripherally, and seem to have a nutritive function, being connected 

 with the neuroglia. In structure and appearance they are quite similar 

 to the protoplasm of ganglion-cells. Their contents are primitive tubes 

 which spring from the protoplasm of the cells, and generally in such a 

 manner as to converge uniformly from the whole protoplasm towards 

 the pole where the nervous process issues ; here they unite and form its 

 contents. 



The clotted substance is found to consist chiefly of nerve-tubes and 

 primitive tubes (and nerve-fibrillse, which are only small primitive 

 tubes) ; these tubes consist of a neuroglia sheath, and semi-fluid con- 

 tents (hyaloplasm), their structure only differing from that of the 

 primitive tubes in the greater strength of their sheaths ; these tubes 

 and fibrils do not anastomose, but only form a more or less intricate 

 web ; the reticu ation seen in sections is not a true reticulation, but is 



