122 FRANKLIN P. REAGAN 



tionally polyphyletic. If it were not so, the process of differen- 

 tiation would never proceed beyond the point at wliich our in- 

 tuition tells us that differences exist, though we can not see them. 

 It would seem that ''all the polj^lwletic school would ask" (65, 

 p. 283) and again that all the polyph\*letic school would demand 

 is the privilege of reasoning in a circle. 



The attempt has often been made to prove that two tissues 

 could never have come from a common anlage for the reason 

 that when differentiation has taken place the products of dif- 

 ferentiation have different appearances. Thus Clark (9) believed 

 himself to have proved that endothelium could not develop from 

 mesenchyme for the reason that the nucleoli and certain other 

 fine structures of the differentiated tissues were different. This 

 finding should not be surprising if the grosser structures them- 

 selves were so different as to render possible the distinguishing 

 of the two tissues.'* 



If one were to neglect the transitional stages from mesenchyme 

 into cartilage he might convince himself that the latter could 

 never have come from the former. But should he recognize 

 these transitional stages he might then claim that transforma- 

 tion took place because the transforming mesenchyme cells were 

 destined to that fate and were therefore specific. Yet is there 

 no other alternative? The work of Lewis^ and my own work^ 

 has shown that in one case at least, preformation need not be 

 assumed; it was shown that otic cartilage arises in response to 

 the presence of a sensory epithelium. The work of Burr^ has 

 since shown the same to be true in case of nasal cartilage. Lewis 

 showed that if the otocyst of an anuran be transplanted to the 

 mesenchjane of a urodele, there developed a cartilaginous cap- 

 sule tj^ically urodelan in character. Here would seem to be a 

 case in which the realized outcome was surely not an expression 



* In Prentiss's Text-book of Embryology (Saunders Co., 1915), p. 287, a great 

 deal is made of the fact that F. T. Lewis (Am. Jour. Anat., vol. 5, 1905) observed 

 that "lymphatic spaces do not resemble mesenchyma." 



5 Lewis, W. H., Anat. Rec, vol. 1, 1907, p. 148. 



^ Reagan, F. P., Proc. Am. Assn. Anat., Anat. Rec, vol. 9, no. 1. 



7 Jour. Exp. Zool., vol. 20, no. 2, 1916. 



