THE CCELOM AND DORSAL VESSEL. 91 



•25 mm. in diameter, and is smallest at its ends. It dips 

 downwards from its origin, and its termination is near the 

 central axis of the body. 



It is not enclosed in a pericardial cavity, but the pericardial 

 septum is continued forwards as a fringe on either side of the 

 vessel, consisting of a double or triple row of cuboid cells. 

 The cell fringes give insertion to small alar muscles at intervals 

 which arise from the lateral tracheal trunks. They terminate 

 in front in Weismann's cellchaplet. When the dorsal vessel is 

 removed from the body the intermediary portion contracts and 

 throws the cell fringes into very regular plications. 



The cells which form the fringes and cell chaplet are 25" to 

 50*^ in diameter. The marginal cells are cuboidal, and those 

 nearer the dorsal vessel are spheroidal, and usually contain 

 from two to five nuclei. 



Similar cell chaplets exist in the imago, and are undoubtedly 

 young fat bodies ; and in many insects fat bodies are found 

 attached to the dorsal vessel in the place of cell fringes. I am 

 inclined to regard these structures, therefore, as the young fat 

 cells of the nymph (see Development of the Nymph). 



The intermediary portion of the dorsal vessel contains 

 several double valves opening forwards. These are pouches of 

 the lateral walls. The muscle fibrillse have a somewhat spiral 

 arrangement at the valves, which gives the appearance of a 

 St. Andrew's cross when both sides of the tube are seen 

 superimposed (see Fig. 18, zf. and 5). 



The third part of the dorsal vessel, or aorta (Fig. 18, 2), 

 measures about i mm. in length. It lies upon the neuroblast 

 and terminates in the ring, from which a number of fine muscle 

 fibrillse are prolonged forwards, and are attached to the meso- 

 blast of the cephalic discs and to the posterior extremity of the 

 cephalo-pharynx. These fibrillse lie in the cephalo-pharyngeal 

 blood sinus, and were described by Weismann. 



Structure and Morphology. — Viallanes [27, p. 58] gave a resume 

 of what was certainly known of the dorsal vessel in 1882, and I 

 do not find that anything has been added to our knowledge 

 since. He said : ' It consists of a tube with two lateral rows 



