8 THE EMBRYOLOGY OF INSECTS AND ARACHNIDS- 



was probably mistaken: for the elongated cells in question divide in subsequent 

 stages, and thus give rise to cells corresponding in all particulars to migratory 

 mesoderm cells (I' Figs. XII & XV). All the stages in the division of these cells 

 and their conversion into migratory mesoderm were not traced, but after a study of 

 the sections drawn, and from many others not drawn, there can be little doubt that 

 the cell mass at the bottom of the blastopore takes no part in the formation of 

 nervous tissue. The non-participation of these cells in the formation of nervous 

 tissue is also shown from a study of the development of the nerve commissures. 



In Figure XV, which represents a transverse section of a well advanced embryo, 

 it will be seen that a granular substance, probably formed by the breaking down of 

 cells, has appeared on the dorsal surface of the ganglia (NS in Fig. XV). The 

 cells at the bottom of the blastopore take 110 part in the formation of this granular 

 substance. Still later in embryonic development the commissures, both longitudinal 

 and transverse, can be recognized as extensions of the granular material formed near 

 the dorsal surface of the nerve ganglia. In these later stages there is no trace of 

 elongated cells at the bottom of the grove, but between the nerve ganglia and form- 

 ing the peritoneal coat or neurilemma of the nervous system are migratory mesoderm 

 cells. 



Figure XXIII represents two thoracic and the first two abdominal ganglia in 

 median longitudinal section. Between the ganglia will be seen the migratory 

 mesoderm cells (I') which probably arise in the manner just described, forming the 

 neurilemma or peritoneal coat of the nervous system, but evidently taking no part 

 in the formation of commissures. Only a portion of the migratory mesoderm cells 

 arise from cells lying between the nerve cords : the greater portion of it is probably 

 derived from cells of the inner layer. These migratory mesoderm cells have a round 

 cell body which does not stain, containing a nucleus which stains deeply. They 

 are similar in appearance, though apparently not in origin, to migratory mesoderm 

 cells described by Reichenbach for the Crustacea. 



The supra-oesophageal ganglion arises differently from the other nerve ganglia. 

 It appears first as a thickening of the lateral portions of the procephalic lobes. 

 Figure XIII is a drawing of a section through the procephalic lobes ; the lateral 

 portions of which (I in Fig.) are thickened considerably. On the median portion 



