DEVELOPMENT OP PHRTGANIDS. 595 



end of the germinal band, and extends towards the posterior 

 extremity, the only difference being that in insects the medul- 

 lary or neural furrow is formed in nearly the same place where 

 the gastrula occurred, while in birds the medullary furrow is 

 located somewhat in front of the primitive streak.^ 



Still another point of comparison between the nervous 

 system of Vertebrates and insects is shown by the fact that 

 in both groups a differentiation into two kinds of nerve- 

 cells occurs — an inner layer with granular nuclei, and an outer 

 layer, composed of faintly-marked nuclei. The former layer 

 corresponds to the grey nerve-matter of Vertebrates, and the 

 latter to the white substance. This differentiation in structure 

 indicates the same physiological differentiation of the nervous 

 system in both groups. 



Blatta Germanica. 



Since the completion of the paper on Neophalax, a few 

 observations have been made upon the development of Blatta 

 Germanica, some of which may be mentioned here pre- 

 liminarily, reserving a more detailed and complete account 

 until the observations on this form have been completed. 



In the embryos of Blatta as well as in those of most if not 

 all other insects, there appears in each of the segments at a 

 certain time a great number of clear, highly refractive par- 

 ticles that at first might be taken for oil globules, and which 

 have always been regarded as such. On more careful exami- 



' The gastrula-groove in insects disappears from the anterior end of the 

 germinal band, backwards; while the medullary or neural furrow, which 

 commences somewhat in front of the anterior extremity of the gastrula-groove, 

 is forming in the same direction ; before, however, the gastrula-groove has 

 entirely disappeared from the posterior extremity, the medullary furrow has 

 extended to the posterior end, where it opens into the gastrula, both 

 becoming continuous and forming a necjrentekic canal, just as in 

 birds and other Vertebrates with nieroblastic ova. 



