124 Mr. PTA. Buxton on the 



Histologically the whole of the protocerebral lobes are 

 very uniform in structure, and consist of Punktyubstanz of 

 a moderate degree of density. The tunuilus, however, is 

 very nuich closer in structure, and the parts of the proto- 

 cerebral lobes inmiediately below and around it are very 

 loosely formed. The lobes are penetrated in all directions 

 by bundles of axons (Fasersubstanz), some of which are 

 enumerated below. 



III. The Mushroom Body. 



(Stalked body — Packard. Pilz — Kiihnle, etc. Gestielte 

 Korper — Leydig. Les Corps Pedoncules — Dujardin.) 



Before I describe the nmshroom body of Microjpteryx it 

 may not be out of place to state that the organ consists 

 typically of a cup-shaped or globular head [colice, PiJzkul, 

 Becker, lobe a convolutions) supported by a stem {Stamm, tige, 

 cauliculus, pedunculus) which divides below into a number 

 of roots or branches. The word Stiel is used by Kiihnle to 

 denote the stem and roots together. An early worker, 

 Newton, described the brain of the cockroach. In this 

 insect the head and stem of the mushroom body are double, 

 and Newton named the two stems the cauhculus and 

 pedunculus, respectively. A small number of insects have 

 their mushroom body formed on a simpler plan, with only 

 one liead and one stem. It is better, therefore, not to use 

 the terms cauliculus and pedunculus, which are responsible 

 for the notion that two supports of the mushroom body are 

 to be looked for in the typical insect brain. As will be seen 

 later the homologies of the roots of the organ are very 

 obscure. 



This exceedingly simpUfied accomit of the least complex 

 type of mushroom body may serve to remind the reader 

 of the essential characters of that organ. A full account 

 of that of Forficnla, together with a painstaking summary of 

 previous work, is given by Kiihnle. This is valuable, but 

 as I shall explain later I believe that Kiihnle has made 

 a fundamental mistake in homology. The summary in 

 Packard's text book is out of date and most difficult to 

 understand. 



At first sight it appears that the mushroom body of 

 Micropteryx is formed on a plan not altogether identical Avith 

 that found in other insects. This is not very surprising, 

 for nothing is yet knowai of the brains of the Lepidoptera 

 or TricJioptera. I hope, however, to show that the difR- 



