276 JAMES ROLLIN SLONAKER 



'00). They state that this mode of development of the dilator 

 muscles was first observed by Grynfeltt ('99) in the rabbit, in 

 which they were first seen at the age of fourteen days after birth. 

 This mode of development was later verified by Nussbaum ('99) 

 in the mouse and in birds. This view is also held by Szili ('01), 

 Herzog ('02), Collin ('03), and Lewis ('03). 



Opposed to the ectodermal origm of the muscles of the iris 

 is the view of Hertwig ('90), who maintains that they are devel- 

 oped from the mesenchyme. In describing their development 

 in mammals, he says (p. 399): ''Mit der Flachenausbreitung der 

 beiden Epithellamellen halt die ihnen von aussen anliegende 

 Mesenchymschicht gleichen Schritt. Sie verdict sich und liefert 

 das mit glatten Muskelzellen und Gefassen reich versehene 

 Stroma der Iris." 



Which of these views is correct I am unable to state, as my 

 sections throw no light on the subject. The first appearance of 

 these muscles as more dense masses in the stroma portion of the 

 iris rather favors the view that they are of mesenchymatous 

 origin. It does seem strange that these muscles are almost 

 the only ones in the whole organism not derived from the mesen- 

 chyme. The only others I know of are the smooth muscles of 

 the glandulae sudoriferae, which Kolliker ('98), Heidenhain ('93), 

 and Stohr ('02) claim are of ectodermal origin. Authorities 

 agree that the ciliary muscles are derived from the mesenchyme. 

 Why they should be developed from mesenchymatous tissue 

 when the conditions for ectodermal origin are just as favorable 

 as in the iris muscles is difficult of explanation. 



It is interesting to note that complete striation of the ciliary 

 muscles and the muscles of the iris occurs but a short time 

 before they are to function in aiding the bird in its flight. Why 

 these muscles are striated in birds and smooth in mammals has 

 been explained (Slonaker, '18) as due to the difference in the 

 rate and manner of locomotion in the two groups of animals. 

 The rapid flight of the bird requires quick changes in accom- 

 modation and in the size of the pupil which could not be accom- 

 plished by the slow-acting smooth muscle. 



