2 WHEELER. [Vol. V 1 1 1 . 



v. Neurogenesis in the Insecta 82 



1. The nerve-cord 82 



2. The brain 99 



3. General retitarks on the nervotts systetn 108 



VI. The development of the reproductive organs in the Insecta 113 



1. T/ie gonads 113 



2. The ttiale ducts 116 



3. The female ducts 119 



4. General considerations 126 



VII. The subcesophageal body in Xiphiditim and Blatta 136 



VIII. Technique 139 



IX. Bibliography 143 



Description of the plates 150-160 



The very primitive and synthetic character of the Orthoptera 

 has long been recognized by systematists and comparative 

 anatomists, but the full importance of the group from an 

 embryological standpoint has been but little appreciated, owing 

 to the meagre and fragmentary nature of the observations 

 hitherto published. For this reason I have made the Orthop- 

 tera the starting point of my studies, with a view to determining 

 their relations, on the one hand to the Apterygota and on the 

 other to the higher Pterygote orders. Only a portion of the 

 evidence bearing on these relationships is presented in the 

 following paper ; a number of observations on the Malpighian 

 vessels, corpus adiposum, oenocyte-clusters and abdominal ap- 

 pendages will be published as separate papers. 



I have devoted more attention to XipJiidhim than to other 

 Orthoptera, partly because the Locustidae occupy a somewhat 

 central position in the order, and partly because this curious 

 form exhibits in its embryogeny better than any other insect 

 hitherto studied, the co-existence of certain very ancient with 

 very modern characters. 



My German co-workers in the field of insect development 

 will probably regard my treatment of the literature as rather 

 perfunctory ; but Prof. Graber, Dr. Heider and others have 

 given from time to time such complete resumes of past and 

 current literature that I feel justified in departing from the 

 general custom. If I have failed to give credit where it is due, 



