THE NERVE-ELEMENTS OF THE EMBRYONIC LOBSTER. 71 



them in essential details. Some of these motor elements send their 

 fibres out of the cord through the anterior nerve roots, others through 

 the posterior. 



The motor elements previously described are all characterised by 

 the fact that the fibre leaves the central nervous system through one 

 of the roots of that ganglion in which the cell attached to it is situated. 

 The portion of the element which lies within the central nervous 

 system is therefore entirely confined to one ganglion. In a number 

 of elements, which have since stained, whilst the cell lies in one 

 ganglion, the fibre passes out of the cord by the nerve-root of some 

 other ganglion. In one such element, the cell lies in the anterior 

 portion of the central mass of ganglion cells of Th. VII., and gives 

 ofi' a fibre which runs outwards and then upwards, to Th. VI., where 

 it passes out by the posterior root of the ganglion. The fibre gives 

 off a stout arborescent branch in Th. VII., and a straight transverse 

 branch in Th. VI., which passes across to the opposite side of that 

 ganglion. 



Of elements belonging to new types, perhaps the most interesting are 

 those which, taking origin in a single cell, have two or more branches, 

 which pass out of the central nervous system by the nerve-roots of 

 different ganglia. For example, a cell lying in the anterior portion of 

 the lateral mass of ganglian cells of Thorax VI IL, gives off a moder- 

 ately fine fibre, which very soon bifurcates, one branch passing 

 immediately out of the ganglion through the anterior nerve-root, 

 whilst the other runs forwards along the ganglionic cord. The forward 

 branch pursues a perfectly straight course until it reaches Th. III., 

 where it gives off a branch, which passes out through the posterior root 

 of that ganglion. After giving off this branch the fibre continues to 

 Th. II., where it turns and leaves the ganglion through the posterior 

 root. Hence this element, the cell of which lies in Th. VIII., sup- 

 plies fibres to at least three nerve-roots of different ganglia, namely, 

 the anterior nerve-root of Th. VIII,, the posterior root of Th. III., 

 and the posterior root of Th. IL, and all these fibres have their origin 

 in a single cell. 



In the Abdominal Ganglia, staining of nerve elements can be 

 obtained in two ways. In the case of embryos in the early or medium 

 stage, fibres which have taken up the methylene blue in the thorax, 

 often continue to absorb the colouring matter in the abdomen, and the 

 cells with which they are connected are thus brought to light. The 

 best results for the abdominal ganglia can, however, be obtained by 

 special preparation of embryos, which are very near the hatching point. 

 In such embryos the abdominal ganglia may be dissected out from the 

 surrounding tissue by careful manipulation with needles. Special care 



