70 RICHARD EVANS. 



VI. The Development of the Nervous System and 

 Ventral Organs. 



The development of these two systems has been traced up 

 to a certain stage in describing the four embryos considered 

 in an earlier part of tliis paper. The common rudiment 

 of the nervous system and ventral organs was found to 

 consist of a thickening of the ectoderm wliich^ develop- 

 ing from before backwai-ds, became continuous from the 

 anterior to the posterior end. The internal layer of the 

 ectodermal thickening becomes separated off as the rudi- 

 ment of the nervous system, while the outer layer, at first 

 continuous, gives rise to the ventral organs. It is not until 

 the embryo has nearly reached the stage of development 

 shown in fig. 5 on PI. 5 that any signs of breaking up 

 of the rudiment in question appear. The nervous system 

 does not become divided into separate ganglia, even in the 

 adnlt, except in so far as the sh'ght swellings occurring 

 between each pair of appendages indicate such a division. 

 This undifferentiated condition of the nervous system renders 

 metameric comparison with the mesoblastic somites a matter 

 of no small difficulty. The rudiment of the brain and first 

 pair of ventral organs is, at first, an undivided mass; but in 

 an embryo which has reached the state of development 

 shown in fig. 5 on PL 5 the lobes of the brain are 

 making their appearance, and the first pair of ventral organs 

 have been invaginated, though on the inner side they are 

 continuous with the brain ganglia lying" above them. Fig. 

 23 a on Pi. 9 shows a section of the brain which passes near 

 the mid-dorsal line and above both the base of the antennae 

 and the eyes. The upper side of the figure corresponds to 

 the dorsal aspect, and passes through the archicerebral lobes 

 of the brain {a. lo.), while the ventral side shows the lateral 

 lobes [lo.''-, lo.^, and lo.^). Fig. 24 on PI. 9 represents a some- 

 what oblique section of the frontal part of the brain, and it 

 will be noticed that, on the antero-frontal aspect, the archi- 

 cerebral lobes project forward as small prominences, which 



