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 Pupiparze [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 they 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 20m in 
diameter, contain one and often two nuclei, and have finely 
* For the development of the finer branches of the trachee, see pages 49 
and 273. 
