10 THE EMBRYOLOGY OF INSECTS AND ARACHNIDS. 



The nervous substance consists principally of round nuclei. The commissures 

 consist of the granular material before described. The nerve fibres consist of 

 similar granular material (Figs. XXII Si XXIII). After the separation of the 

 nerve ganglia from the ectoderm, the blastopore closes entirely (Fig. XV). A closer 

 examination of figures XVII, XVIII, XIX, may make clearer the number and 

 relations of the nerve ganglia. 



Figure XVII is a nearly true median longitudinal section The cesophageal 

 invagination (OE in Fig ) will be seen directly posterior to the upper lip (LB in Fig). 

 Following the three large thoracic ganglia are the ten smaller abdominal ganglia, 

 (Nos. 8-12, Fig). The last abdominal ganglion will be seen to be smaller than the 

 preceeding ganglia. 



The nature and origin of the last or eleventh abdominal somite has already 

 been described. Its dorsal wall extends forward beyond its lateral walls. This dor- 

 sal extension of the last somite will be seen, on referring to figures XVII, XVIII, 

 & XIX, to be deeply grooved. This groove is a continuation of the median groove 

 or blastopore over the dorsal surface. It becomes deeper, and is finally invaginated. 

 On being separated from the dorsal surface of the body, the invaginated portion 

 forms the anal invagination. The cesophageal invagination also occurs in the 

 middle line of the body ; unlike the anal invagination, however, it is formed not by 

 the infolding of a portion of the median groove, but by a simple, vertical ingrowth 

 (OE Fig. XVII). The cephagus is formed before the anal invagination is completed. 



Back of the oesophagus there occurs an aggregate of migratory mesoderm 

 cells, wrongly regarded by Hatscheck as endoderm. (1> The cesophageal invagination 

 occuring in the median line of the body pushes in the cells of the inner layer at 

 that point, and extends dorsally and posteriorly, invested by these cells, (Figs. 

 XVIII, XXII). On the dorsal surface of the cesophageal invagination there is 

 formed a thick string of mesoderm cells. The outer cells of this string adhere to- 

 gether, while the inner cells separate from them ; thus is formed a tube containing 

 cells. This tube is the heart (H in Figs. XXVIII Plate 2, & XXIX, XXX, XXXI 

 Plate 3). 



Probably the inner cells of the tube form the blood corpuscles and plasma. 



Beyond the cesophageal invagination the heart does not appear to be formed 

 from a solid string of cells, but is apparently formed from separate mesoderm cells 



