)4] of the Nervous System of the Embryonic Lobster. 409 



[I. Elements which consist of a ganglion cell in the cord and a 

 fibre which runs out at a lateral nerve root. Some, at least, 

 of these elements, possibly all, are connected with muscles, 

 and are motor elements. 



[I. Elements which consist of a cell lying outside the central 

 ganglionic chain, and a fibre running from it to a ganglion. 

 These must be regarded as sensory elements. 



, Elements of the first group, co-ordinating elements, are of four 

 kinds. 



A. Elements made up of a cell in the brain or one of the ganglia, 

 and a fibre which runs posteriorly to the end of the cord, giving off 

 collateral branches to the nenropile in each ganglion through which 

 it passes. For reasons to be explained, these elements must be 

 regarded as placed temporarily in the groups of co-ordinating 

 elements. It may be necessary to place them in a new group by 

 themselves. 



Two kinds of A elements may be distinguished : 



(a.) Those wh:"oh decussate with the corresponding element of the 



opposite side. 

 (6.) Those which pass down on the same side of the cord as that 



on which the cell lies. 



The elements A (a) inserted in the ganglion of Antenna II and 

 Thorax II (fig. I) may be taken as typical of the fibres which decus- 

 sate. The cell lies in the lateral mass of ganglion cells, and the fibre 

 gives off lateral branches to the neuropile before crossing to the other 

 side. After the decussation has taken place the fibre turns back- 

 wards, and runs down the ganglionic chain, giving off collateral 

 branches to the neuropile of each ganglion through which it passes. 

 The fibre A (a) Thorax II, together with a corresponding fibre in 

 Thorax I, have been traced as far as the last abdominal ganglion, but 

 no definite ending has been made out. A (a), Antenna II, was only 

 actually traced to Abdomen 5, but it showed no sign of ending in that 

 ganglion, and probably continues to Abdomen 6. It will be observed 

 that the decussation of A (a), Antenna II, takes place through the 

 transverse bridge, behind the oasophagus. Similar elements have 

 stained in Thorax III and Thorax V.* 



The element A(a)Br (fig. 1) may be best considered here. It con- 

 sists of a large cell on the ventral surface of the brain, from which a 

 moderately thick fibre runs at first forwards and upwards to the 

 dorsal surface. After turning outwards, the fibre runs backwards to 

 a point immediately in front of the oesophagus, where it passes across 



* In the present communication tlie thoracic ganglia are numbered consecutiTely 

 I XI. Of these I VI form the anterior thoracic ganglion of the adult, whilst 

 VII XI form the five posterior ganglia. 



