gS WHEELER. [Vol. VIII. 



clearly implying that the median cord is set free from the 

 ectoderm along its whole extent he remarks (p. 252): "Be- 

 tween the successive pairs of ganglia the median ingrowth 

 atrophies, and at the time of the closure of the dorsal wall of 

 the body there is seen between the connecting cords of two 

 adjacent pairs of ganglia, a small triangular or cylindrical mass 

 of cells, concerning the fate of which I am not absolutely 

 certain. I believe, however, that they go to form a part of 

 the internal skeleton. The chitinous rods in the thoracic 

 region to which the muscles of the legs and wings are attached 

 probably arise from the remnants of the median invagination, 

 but in the abdominal region they may disappear entirely with- 

 out giving rise to such structures." If I understand this 

 passage correctly, Ayers implies that the chitinous rods are 

 originally interganglionic portions of the median cord. But if 

 this is the case, how can the median cord separate completely 

 from the ectoderm unless we are to suppose that there is a 

 reunion of the interganglionic portions with the integument 

 to form the endoskeletal structures } The chitinous rods are 

 directly continuous with the chitin of the integument so that 

 until observations are forthcoming to show that portions of the 

 integument can loosen and pass into the body-cavity and subse- 

 quently reunite with the integument, I must regard Ayers' 

 account as inadequate. 



I am still in some doubt as to the exact origin of the commis- 

 sures. Grassi (84), Ayers ('84), H eider ('89), and Graber ('9o), 

 all maintain that the commissural fibres arise from the median 

 cord cells. A priori, there are no reasons why the daughter- 

 cells of the median neuroblast should not send out processes to 

 form Punktsubstanz and thus form a commissure. From the 

 position of these cells, however, I regard it as highly improbable 

 that anything but the posterior commissure could be formed in 

 this way. The isolated Punktsubstanz masses in the Coleopte- 

 ran median cord in Graber's and Heider's figures may arise 

 from cells equivalent to the daughter-cells of the median neuro- 

 blasts of the Orthoptera. It is very improbable that the der- 

 matoblastic cells which form the walls of the median cord in 

 the region of the anterior commissure, and which I regard as 



