1888.] MORPHOLOGY OF SUPERNUMERARY PHALANGES. 499 



III. HISTOLOGICAL AND DEVELOPMENTAL. 



Dixey, in a paper on the ossification of the terminal phalanges (3), 

 incidentally describes and figures the syndesmosis in Proteus. He 

 deals with that interposed between the penultimate and terminal 

 phalanges — with that, that is to say, which would appear to represent 

 the supernumerary phalanx of the Anura. He writes (p. 68) : 

 " another instance of arrested development in the digit of Proteus is 

 afforded by the inter-phalangeal joint .... the cartilage, with a 

 sHght alteration in the size and relative number of its cells, is seen to 

 be quite continuous between the heads of the two phalanges, nor 

 does it exhibit the least sign of an articular cavity." He believes 

 his specimen to have been "fully adult " (p. 70) ; but in this he was 

 mistaken, for in the larger of our specimens of the same {cf. p. 504) 

 ossification had proceeded much further than in his. 



Microscopic examination of this syndesmosis with its associated 

 parts in Proteus reveals, under the treatment which we have adopted 

 (p. 496), the following facts. The matrix of the epiphysial cartilages 

 of the phalanges stains uniformly and feebly, while that of the less 

 resistant syndesmosis takes the dye much more readily, becoming 

 thereby sharply differentiated {cf. Plate XXV. fig. 1 2). The latter 

 shows traces of a fibrillar structure, but it is for the most part 

 homogeneous. The cells which are present are well defined, and 

 their peculiarities in structure, disposition, and size are common to 

 both epiphysis and syndesmosis. Each is irregular in contour, and 

 carries a large nucleus, while it is seen to lie within a spacious 

 lacuna, the boundary of which is smooth and well defined ; and such 

 differences as are met with between the corpuscles of the epiphyses 

 and of the syndesmosis are seen to be entirely due to pressure 

 under apposition of the first named. The cells of the syndesmosis 

 are more numerous and more closely aggregated than those of the 

 hyaline epij)hyses, and the intensity of colour of the former under 

 the action of reagents is, to a large extent, due to this crowding. 

 The details of histological structure here described hold good, with 

 but slight modifications, for all conditions in which the super- 

 numerary phalanx and its homoiogue remain non-hyaline {cf. Proteus, 

 fig. 12, and Uyla peronii, fig. 14). Its cells are cartilage corpuscles, 

 and the tissue to which they give rise is, in its most elementary 

 form, a nascent cartilage. 



The whole digital skeleton is invested in a continuous and well- 

 differentiated fibrous tunic, and in Proteus this is, at any rate 

 ventrally, incompletely marked off from the syndesmodial pad. 

 There lie buried up in the former at this point {sh., fig. 12) cells 

 which closely resemble those of the syndesmosis in size and struc- 

 ture ; the question, therefore, naturally arises as to whether some of 

 these might not have migrated into the inter-phalangeal region, there 

 to give rise to the pad in question, or that that might conceivably 

 have been an ingrowth of the tunic itself. It will be seen, how- 

 ever, that the corpuscles of the tunic lie buried in a coarsely fibrous 

 matrix, in which there is a total absence of the lacunae so charac- 



