354 THE TRACHEAL SYSTEM OF THE IMAGO. 



A careful perusal of Kowalevski's work and a comparison of 

 his figures with his text has convinced me that these statements 

 are made on insufficient evidence. Kowalevski's sections do 

 not apparently bear out his conclusions, and it is clear that, 

 unless all the observations already cited, concerning the origin 

 of the tracheal vessels from the parablast, are incorrect, 

 Kowalevski must have been mistaken. 



Weismann gives the most definite account of the manner in 

 which the tracheal vessels are developed in the blow-fly embryo, 

 and his observations agree with those of Leuckart [20] and with 

 the more recent observations which derive the true tracheal 

 system from the parablast (see p. 273). 



According to Weismann, the longitudinal tracheal trunks of 

 Musca appear in the embryo as solid cell-strings of mesoblastic 

 origin, and Leuckart makes the same statement with regard to 

 their origin in the Pupipara; [20]. 



Weismann says [2, p. 76]: 'As Meyer has already remarked, 

 the tracheal trunks and their finer end-branches have a different 

 origin.* First, as regards the trunks, there cannot be the 

 slightest doubt they first appear in the embryo as thick solid 

 strings of spheroidal embryonic cells.' 



He continues : ' In the earliest observed condition, thick 

 strings of loosely-compacted spheroidal embryonic cells extend 

 forwards from the neighbourhood of the stigmatic furrow ; 

 these can be isolated, for a short distance only, from the 

 general mass of mesoblastic cells with which the)- are united 

 by numerous lateral branches, consisting of several cell layers, 

 passing over into the general cellular mass. A little later it is 

 possible to isolate the tracheal trunk with a number of its 

 lateral branches, which can easily be recognised as the rudi- 

 ments of the future tracheal network by their form and dis- 

 position, and the later the observation the further these 

 branches extend towards the periphery.' 



'The cells of which these strings are formed measure 20/4 in 

 diameter, contain one and often two nuclei, and have finely 



' For the development of the finer branches of the traclnn- ^e<- pa^t-- .\-i 

 and 273. 



