ENDO-SKELETON THE NERVOUS SYSTEM. 323 



B. Endo-skeleton. 



The endo-skeleton of the head {tentorium) develops out of two 

 pairs of ectodermal invaginations ; the anterior invagination develops 

 on the inner side of and somewhat in front of the mandihle, the 

 posterior within and somewhat in front of the second maxilla. 

 The anterior pair becomes connected with the posterior and gives 

 off two supporting columns which ascend on the inner surface of the 

 clypeus towards the dorsal side. The median fusion of the posterior 

 pair leads to the bridging over of the sub-oesophageal ganglion and, 

 in many Insects, a transverse trabecula is thus formed in the cavity 

 of the posterior part of the head (Tichomiroff, Grassi, Patten, 

 Heider, Carriere). 



Similar ectodermal invaginations bring about the development of 

 a chitinous tendon for the flexor mandibulae and a similar smaller 

 tendon for the antagonistic muscle. 



Hatschek (No. 36), who was unaware of the relation of these invaginations 

 to the hard structures of the head, thought them to be tracheal invaginations. 

 They have been regarded in the same way recently by Carp-jerk (No. 13). 

 Since this kind of endo-skeleton is found in other groups {e.g., the Crustacea, 

 Vol. ii., p. 160), and the hypothetical transformation of a trachea into an endo- 

 skeletal structure of this kind involves the idea of a considerable change of 

 function, we do not consider the homology between the invaginations under 

 consideration and tracheal invaginations sufficiently well established. We are 

 inclined to regard the former as structures of a distinct character, all the more 

 so that they do not by any means everywhere agree so closely in position with 

 tracheal stigmata of the following segments as they do in Chalicodoma* 



C. The Nervous System. 



All the parts of the nervous system are derivatives of the 

 ectoderm and appear in the embryo as ectodermal thickenings. 

 The rudiment of the ventral chain of ganglia is found, as was first 

 shown by Hatschek, soon after the gastrula-invagination closes, in 

 the form of two longitudinal ectodermal thickenings running on 

 either side of the median line. These are the so-called primitive 

 swellings (Fig. 147 A, p. 297), which extend from the cephalic 

 re&ion to the terminal segment and show between them a median 

 depression, the primitive groove (Fig. 158 C, pr and pw). Soon 

 after the appearance of the primitive swellings, the first signs of 

 segmentation can be seen on] them, the swellings being thicker near 

 the middle of the body-segments than at their boundaries. The 

 primitive swellings pass anteriorly at the sides of the oesophageal 



* [Cf. footnote, p. 78.] 



