126 MALCOLM LAURIE. 



The retina of the central eyes is still a thickening of the 

 dorsal layer of the cerebral invagination (PI. XVII^ fig. 43, 

 rtn.). It is visible in surface view (fig. 38, oc.) as a white spot 

 on the margin of the invagination. The hypodermis imme- 

 diately outside it is somewhat thickened, and will in this 

 region form the vitreous layer (fig. 43, vit.). 



The ventral nervous system is now completely separated from 

 the hypodermis (figs. 40 and 41, n, c). The cells are beginning 

 to congregate together to form the ganglia, though the nerve- 

 cord between the ganglia is still largely cellular. Nerves are 

 seen growing out from the ganglia as thick cords of cells 

 (fig. 40). The ganglia contain a clear space in their centre 

 which later is occupied by a mass of fibres. 



The tail (PL XVII, fig. 38) has now attained its full 

 number of segments but the sting is not yet formed. The 

 gut extends up almost the whole length of the tail. There is 

 no sign yet of the formation of the proctodasum. The hypo- 

 blast in the rest of the body remains as a scattered layer of 

 cells. 



The mesoblast has now grown round the body as a double 

 layer, with the coelomic space between. In the middle line of 

 the back, where the right and left folds of mesoblast meet, 

 there is a somewhat irregular thickening in which both soraa- 

 topleure and splanchnopleure seem to be involved. From this 

 thickened band, which extends from close behind the brain to 

 the beginning of the tail, the heart is formed. On the ventral 

 side in the thoracic region the mesoblast of the outer layer is 

 broken up into long strings of cells — the muscles — so that the 

 coelomic space can no longer be very definitely made out. 



The stomodaeum reaches as far as the back of the cerebral 

 ganglion. This is the limit of its growth, and it remains a 

 closed tube until, at a much later stage, the gut has grown 

 forward and united with it. 



Stage M. — The embryo (PI. XVII, figs. 44 and 45) does 

 not show very much change in surface view. The thoracic 

 appendages are longer and distinctly segmented. They overlap 

 across the middle line and conceal the pcctines. The chelicerae 



