562 SUMMARY OF CURRENT RESEARCHES RELATING TO 



formation constituting the otic capsule ; whether the stapedial plate is to 

 be considered a part of the otic capsule, or as an independent formation 

 in the fenestra ovalis, is purely a matter of interpretation. The greater 

 part of the stapedial plate seems to be structurally independent of the 

 otic capsule, while a portion of it is fused with the latter, probably from 

 a very early time, seemingly having arisen from a part of the otic 

 capsule. It seems reasonable to conclude that the avian stapedial plate 

 is not a part of the visceral skeleton. At any rate, if its constituent 

 mesenchyme is a derivative of the second visceral arch, that mesenchyme 

 is powerless to form a stapedial plate unless stimulated to do so by the 

 auditory epithelium. 



It is of interest to note that if a very small portion of the otocyst be 

 left in the mesenchyme, the latter will chondrify in the region of the 

 small abandoned sensory epithelium and follow its contour very closely. 

 Whatever may be the configuration of the sensory epithelium, the 

 developing otic capsule always conforms to the shape of the former. It 

 seems well established that the cartilages forming about the three main 

 pairs of sensory epithelia in the head are formed in response to the 

 presence of those epithelia. 



The important general result is that the removal of the otocyst from 

 young chick embryos seems entirely to do away with the stimulus to the 

 later development of the otic capsule. Embryos devoid of otic capsules 

 fail to develop stapedial plates. The columellar portion of the stapes 

 attains its full length and normal proportions in the absence of a 

 stapedial plate. The stapes-homologue in birds seems to be of mixed 

 origin, cranial and visceral. 



Dependent Development.*^ — Franklin Pearce Reagan has shown, as 

 above recorded, that the development of the otic capsule and of the 

 stapedial plate belongs to that class of phenomena which have been 

 designated " the interaction of parts," or " dependent development." 

 He briefly discusses these phenomena. Hertwig was one of the first to 

 attach importance to those developmental processes which are due only 

 indirectly to the original constitution of the fertilized ovum, and like- 

 wise those due only indirectly to the external environment. These he 

 described as due to the " perpetually changing mutual relations in which 

 cells of an organism are placed to one another." According to Herbst 

 all movements, tropic or tactic, and many processes of differentiation, are 

 responses of a formative, as well as a directive, nature. Thus far the 

 clearly demonstrated cases of interaction of parts are relatively few. 

 Loeb has shown that the position occupied by the pigment-cells on the 

 yolk-sac of Fimdidus is an oxygenotactic reaction. Several cases have 

 been reported in which early displaced embryonic cells have resumed 

 their original position. 



The development of certain parts of the vertebrate eye has been 

 shown to be of a " dependent " sort. Spemann showed that if the 

 formation of the optic vesicle of the frog be inhibited by injury to the 

 medullary plate, or that if its approximation to the ectoderm be pre- 

 vented, a lens will not form. Lewis showed that ectoderm from other 



* Journ. Exper. Zool., xxiii. (1917) pp. 104-6. 



