THE DEVELOPMENT OF THE NERVOUS SYSTEM. 487 



which is consistent with the developmental history and final 

 condition of the Arthropod brain, and renders a great number 

 of facts intelligible. I advance it with some diffidence, as I am 

 aware of the difficulties which beset any investigation of the 

 development of the nerve-centres, and the ease with which 

 the significance of a layer of cells may be misinterpreted, but 

 I present it as a working hypothesis which may serve as a 

 guide to future investigators. The evidence in its favour 

 depends on but few preparations, and is the result of the 

 study of a single insect ; the difficulties of obtaining satis- 

 factory evidence is very great owing to the extreme rarity of 

 good preparations in the earlier stages of the development of 

 the larva in the proper planes for the investigation of the 

 question, as suitable sections are by no means easy to prepare. 

 In examining the nymphoid state of the embryo, I discovered 



FIG. 64.- A sagittal section of the neuroblast of a newly-hatched larva. </, head 

 discs ; v, ventricular cavity of the hemisphere ; ; central stroma of the neuro- 

 blast. 



a condition, which is figured in PI. XV., Fig. i, in which the 

 preoral ganglia are apparently vesicular, and subsequently I 

 found the observation borne out by the condition of the nerve- 

 centres in the newly-hatched larva (Fig. 64). From these 

 observations, I am strongly urged to the view that the remark- 

 able mantle layer in the adult larva is the roof of a cavity, the 

 remains of which form the ventricular cavities of the brain of 

 the imago. 



It is true that the early vesicular stage, which appears to be 

 the probable interpretation of my sections, has not been hitherto 

 suspected in the Arthropod brain, and although Kowalevski [97] 

 discovered a central cavity in the brain of Sagitta, he observed 



