THE EMJiRYOLOGY OF A SCORPION. ' 125 



surface view, has by this time made considerable progress in 

 several important points. The thoracic appendages are slightly- 

 segmented (PI. XVIIj fig. 39, «j».), though this is not apparent 

 in a surface view. The chelicerse have moved in towards the 

 middle line, and the mouth is now concealed between their 

 bases. The chelae are very large, and have their pincers well deve- 

 loped. The coxal gland, which opens at the base of the fifth 

 pair of appendages, is no longer a straight tube, but has become 

 bent on itself, so that a section through it (PI. XVII, fig. 39, 

 cox.) shows the tube cut in three places. It can still, however, 

 be traced through a series of sections as a simple tube opening 

 into the coelom. The abdominal appendages have undergone 

 great changes. The genital opercula are still simple thickenings 

 of the epi- and meso-blast, but the pectines (PI. XVII, fig. 

 40) have become folded in a direction parallel to the long axis 

 of the body, i.e. transverse to their own axis. The most im- 

 portant change is, however, that of the four following abdo- 

 minal appendages. These (PI. XVII, fig. 41) are pushed in 

 so as to form shallow cup-shaped cavities. The inpushiug is on 

 the posterior part of the appendage, and is directed slightly 

 forwards. This is the commencement of the formation of the 

 lung-book. 



The cephalic segment, which is shown in PL XVII, fig. 38, 

 extended in the same plane as the ventral surface of the 

 embryo, is no longer so distinctly bilobed as in the last stage. 

 The cerebral invaginations (PI. XVII, figs. 42, a and h, and 43) 

 are much shallower, and have entirely joined together, so that 

 there is now only a single inpushing. This lies just in front of 

 the chelicerse (PI. XVII, fig. 38). The brain is being formed 

 from the sides of the inpushing, and shows a very characteristic 

 structure. The mass of cells is more or less grouped round 

 small circular clear spaces (fig. 43), which give to this part of 

 the brain the appearance of being composed of a number of 

 small vesicles. I have not succeeded in tracing the development 

 of the nerve-fibres, which occupy the centre of the cerebral 

 ganglion (fig. 43). This central portion appears at this stage 

 perfectly transparent and empty. 



