THE EMBRYOLOGY OF INSECTS AND ARACHNIDS. 9 



of the procephalic lobe (LB in Fig.) is the paired outgrowth which forms the upper 

 lip. The dorsal surface has been partly covered by the amnion (AM' in Fig.) Yolk 

 spherules are seen filling the body cavity. 



Figure XIII' is a drawing of a section through the procephalic lobes of a more 

 advanced embryo. Here it will be observed that the inner cells of the lateral 

 portions of the procephalic lobes (I in Fig.) have become specialized as nerve cells. 

 Later these specialized cells are separated from the outer unspecialized ectoderm, 

 and form on each side the halves of the supra-cesophageal ganglion. Figure XXII 

 represents a good longitudinal section of the supra and subcesophageal ganglia. 



The Supra-oesophageal ganglion consists of two portions ; an anterior portion 

 (I in Fig.) innervating the antennae, which are not represented in the figure, and a 

 posterior portion (No. I' in Fig.) which sends a nerve (LBN in Fig.) to the labruni 

 or paired lip. 



Posteriorly the second division (No. I' in Fig.) of the supra-ossophageal ganglion 

 forms part of the circum-cesophageal commissure, which is completed by a portion 

 of the mandibular division of the sub-oesophageal ganglion, (No. II in Fig). 



The supra-oesophageal ganglion has its opposite halves united by two com- 

 missures; an anterior commissure (C Fig. XXIX Plate 3) extending beneath the 

 oesophagus, and a posterior commissure (C" Fig. XXXI) extending above the 

 oesophagus. 



The sub-oesophageal ganglion consists of three pairs of closely connected 

 ganglia innervating the mandible, first, and second maxillae, respectively (Nos. I, 

 II, & III Fig. XXII). The three pairs of thoracic ganglia are larger than the fol- 

 lowing ten pairs of abdominal ganglia. This difference in size is apparent on con- 

 sulting figures XVII, XVIII, XIX, which represent good longitudinal sections of 

 an advanced embryo. There are ten pairs of abdominal ganglia, the last or tenth 

 pair being smaller than the preceeding (No. 17, Fig. XVII). It is questionable 

 whether the terminal portion of the abdomen which forms the so called eleventh 

 abdominal somite is to be regarded as a true somite or not. It has no ganglion 

 corresponding to it, and is formed, as stated, by the dorsal flexure of the posterior 

 end of the embryo, and consequently has its body walls formed on all sides at an 

 early period in the manner described. 



