■m 



92 THE LARVA OF THE BLOW-FLY. 



of nuclei in its walls.' He dismissed the researches of Biitschli, 

 Dohrn and Jaworowski in the following words : 



' The study of certain embryos leads to the conjecture, which 

 has never been certainly established, that it is formed from a 

 double row of cells, and that each nucleus represents a cell. 

 Thus, it may be said to consist of a series of hollow segments, 

 each formed of two cells, joined in the middle line.' 



Viallanes claims to have demonstrated the junctions of the 

 cells by staining the intercellular substance with nitrate of 

 silver, and to have observed muscular fibrillas, which pass 

 from segment to segment imbedded in the substance of the 

 cells ; and he concludes that it is morphologically a capillary 

 blood vessel with muscle fibrillas imbedded in its cellular 

 walls. 



Weismann [2] regarded it as a hollow muscle fibre. I am 

 inclined to consider it a hollow fibre of peculiar construction, 

 and think that its true nature is as well represented by Weis- 

 mann's as by Viallanes' hypothesis. Its walls are certainly 

 cellular, and consist of muscle fibrillae, but whether the cells 

 should be regarded as a bed in which the fibrillas lie or a lining 

 intima having an endothelial character is a point not easily 

 determined. It is certain that a very slight modification of one 

 of the skeletal muscle fibres with central nuclei would render it 

 practically identical with the dorsal vessel if the valves are left 

 out of consideration. As these are, however, a most important 

 element in its construction, it appears to me that Weismann's 

 view is only an approximation to the truth. 



The nuclei of the dorsal vessel are arranged with great 

 regularity. They are about "i mm. apart, and each measures 

 about 15'^ to 20^ in diameter. The nuclei are surrounded by 

 more or less granular protoplasm. The muscular layer is ex- 

 ternal to this cell substance, and consists of fibrillse 2" to 3^ in 

 diameter. They are chiefly longitudinal in direction. They 

 are most distinctly striated, and the transverse striae surround 

 the whole tube. 



The Splanchnic Nervous System consists of a series of visceral 

 ganglia. Those of the pharyngeal sinus, of the crop and 



