202 PROCESS OF DEVELOPEMENT; CHAP. IV% 



bursting of the vertical partitions of the cells, the 

 tubes having no existence till the membrane is 

 lacerated.* 



But if the tubes are generated in the manner 

 here supposed, that is by the accidental bursting 

 of the partitions of the cells, it will be difficult to 

 account for the known regularity with which they 

 are formed. The only circumstance giving plausi- 

 bility to the conjecture is that of the occasional 

 occurrence of a transverse membrane interrupting 

 the continuity of the small tubes, which M. Mirbel 

 The sup- regards as a proof of their cellular origin. But 

 beset with allowing this to be a sufficient proof of the truth 

 difficulties. Q f 1 | le SU pp OS ition, which few will be disposed to 

 admit ; how is the formation of the tracheae to be 

 accounted for, which retain no traces of a cellular 

 origin, and are besides twisted spirally throughout 

 the whole of their extent ? They cannot be sup- 

 posed to be merely the result of the accidental 

 bursting of the cells of the pulp ; because there is 

 too much of regularity both in their form and dis- 

 tribution to be the result of accident. If M. Mirbel 

 had even contended that the cells burst open in a 

 regular and determinate manner, and thus give to 

 the tube its spiral or vertical direction, his hypo- 

 thesis would still have been clogged with difficul- 

 ties ; but on the position he assumes the difficulties 

 arc doubled. The most that can be said for it is. 

 that it is perhaps not impossible ; but it cannot 



* Traite d'Anat. ct clc Phys. Ve$ef. 1J V . k 



