ON THE CELLULAR THEORY OP DEVELOPMENT. 99 



be observed ; that the fibres in the embryonic mesoderm are 

 not processes of mesoderm cells (as they are always figured), 

 which have no existence, but are parts of the reticulum which 

 has always existed from before cleavage onwards^ connecting 

 together the various parts of the developing ovum, and that 

 this reticulum is not separate from ectoderm and endoderm, 

 but freely continuous with both, they being but parts of it. 

 The almost universal practice of drawing this reticulum as 

 composed of separate branched cells is a most remarkable 

 instance of the manner in which a theory can blind men's eyes 

 to the most obvious facts. 



Before concluding this general account of my work, I may 

 mention one or two other points of general interest which I 

 have noticed. Firstly, I may mention thatin Scyllium there 

 are a number of anterior roots next the head, varying in 

 number from three to five, according to the age of the embryo, 

 without posterior roots. They no doubt give rise, as has been 

 suggested by others, to the so-called anterior roots of the 

 vagus. Secondly, Balfour was quite correct in the account he 

 gave of the origin of the sympathetic ganglia in Elasmobranchs.^ 

 The ganglia arise as swellings on the posterior roots of the 

 spinal nerves, and soon become removed from the latter, so as 

 to form isolated masses connected with the spinal nerves by a 

 cord. These masses eventually become united longitudinally 

 into a chain. I may add to Balfour's account this fact, viz. 

 that no sympathetic ganglia are found within the area of 

 extension of the vagus ganglion. Or, if I am not correct in 

 applying the term " vagus ganglion " to the posterior part of 

 the vagus — the part which lies dorsal to the gill-slits and gives 

 off the branchial nerves, it would be better to say that sym- 

 pathetic ganglia are not found in the region of the branchial 

 slits, but begin immediately behind these structures. Thus, 

 in an embryo of 22 mm. the vagus ganglion and branchial 



' I have not examined mammals on this point, but I think Paterson's 

 memoir (' Phil. Trans.,' 181) does not carry conviction. On the contrary, 

 there is, I think, in it internal evidence which inclines me to the view that lie 

 lias not got to the bottom of the matter. 



