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 
Fic. 89.—A transverse section through the pericardium and dorsal vessel of a Blow- 
fly nymph just before its escape from the pupa case. cz, cuticular layer of the 
abdominal integument; /, fat bodies; g, ganglion cells; 4, hypodermis; m, 
muscular coat of the dorsal vessel ; f, pericardial septum ; fs, pericardial sinus ; 
s, blood sinus opening into pericardial sinus ; ¢, ¢v, tracheal vessels. 
pupa of the Blow-fly, the fibrille 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. 
