THE DEVELOPMENT OF THE DORSAL VESSEL 657 



Jaworowski, however, says that the dorsal vessel in the 

 newly-hatched larva of Chironomus ' consists of two lateral 

 halves, in which every nucleus indicates a muscle-cell. Each 

 muscle-ring, which is gradually differentiated from a pair of cells, 

 therefore consists of two lateral halves. For a long time both 

 the cells and the enclosed muscle-fibres are seen.' He further 

 states that ' the valves are developed from the same muscle- 

 cells.-' 



From the manner in which the nuclei are arranged in the 

 dorsal vessel, I think there can be no doubt it is at first a tube 

 formed by two lateral rows of cells. Sections show that in the 



tr 



FIG. 89. A transverse section through the pericardium and dorsal vessel of a Blow- 

 fly nymph just before its escape from the pupa case, en, cuticular layer of the 

 abdominal integument ; f, fat bodies ; g, ganglion cells ; /i, hypodermis ; m, 

 muscular coat of the dorsal vessel ; /, pericardial septum ; ps, pericardial sinus ; 

 s, blood sinus opening into pericardial sinus ; tr, tr, tracheal vessels. 



pupa of the Blow-fly, the fibrillse which surround the dorsal 

 vessel are gradually differentiated from the cell substance of 

 large cells, which surround the lumen of the tube. The nuclei 

 of these cells are very large, and project into the interior of the 

 tube. As to the origin of the muscle-cells, either in the embryo 

 or pupa, nothing is certainly known ; but so far as is known, 

 the manner in which the dorsal vessel originates is in favour of 

 Weismann's view that it is a hollow muscular tube, and not a 

 complex organ consisting of connective and muscular tissue. 

 For further details on the development of the dorsal vessel of 

 the pupa see p. 341. 



