TEGUMENT All Y ORGANS. 



409 



of a coarsely granular horny substance exca- 

 vated by polygonal cavities of about TT >\7jj 

 inch in diameter, frequently if not invariably 

 containing air, which adds to the dark hue 

 (by transmitted light) arising from the 

 granular opacity of the horny matter. At its 

 edges, this tissue passes into the cortical sub- 

 stance, which, in a transverse section (Jig- 317. 

 n) appears as a clear, homogeneous or slightly 

 granular mass, dotted over by minute aper- 

 tures, about TOO <TO ' m - i diameter, and g^Vo 

 in. apart. In a longitudinal section, on the 

 other hand (Jig. 317. c, k), the general mass 

 appears obscurely striated in a longitudinal 

 direction ; and in the place of the circular 

 apertures, we see elongated fissures, some- 

 what narrowed at each extremity, whose 

 transverse sections constituted these aper- 

 tures. The pointed ends of the fissures 

 were continued by a line which could fre- 

 quently be traced into some other fissure 

 above or below, so that I conceive the fis- 

 sures are in reality more or less complete 

 canals. 



The quill of the feather is entirely com- 

 posed of cortical substance ; the barbs have 

 the same structure as the shaft ; the barbnlcs 

 present both cortical and medullary sub- 

 stances in a rudimentary condition. Each 

 barbule in fact (Jig. 317. E,e) exhibits along 

 its axis a series of oval cavities, the remains 

 of cells like those of the medulla, while its 

 lateral portions are composed of striated 

 horny matter like that of the cortex, and are 

 produced into the curved and hooked lateral 

 processes (/") 



The polygonal cells of the medullary sub- 

 stance are produced from the indifferent 

 tissue of the pulp in exactly the same 

 manner as those of an ordinary horny, cel- 

 lular ecderon from that of the rcte mucosum : 

 that is to say, the periplast increases, and 

 becomes marked out into polygonal areas ; it 

 then acquires a horny consistence, and a 

 stronger and stronger definition along the 

 lines of demarcation, until polygonal "cells" 

 (as m fig. 317. B, ) are formed. The walls of 

 the latter now thicken and become granular; 

 the endoplasts disappear, and at length no- 

 thing is left but the honey-combed perfect me- 

 dullary substance. The mode of formation 

 of the cortical substance is the inverse of this. 

 On examining the line of junction (fig- 317. B) 

 of the pulp (V) with recently formed cor- 

 tical substance (b), it is observable that the 

 endoplasts do not become surrounded by cell 

 cavities, but that the periplast acquires a 

 granular, longitudinally fibrous, appearance ; 

 while the endoplasts, though they are oc- 

 casionally visible in the striated mass, soon 

 completely disappear.* The elongated ca- 

 vities or tubuli do not at first exist in the 

 cortex, but are the result of a secondary va- 

 cuolation, and so far as I have been able to 

 observe, have no relation with the pre-existing 

 endoplasts. In fact, these canals, like those 

 in the hair-shaft, the clefts in the fencs- 



* Compare Scbwann, Untersuchungen, &c 



trated rootsheath, and the canaliculi of bone, 

 must be regarded as the results of a second- 

 ary vacuolation. The feather sac resembles 

 that of the hair in all essential points of struc- 

 ture, except that the relations of the layers 

 of the inner rootsheath are different. As in 

 the hair, two layers may be distinguished in 

 the inner rootsheath, an outer, strong, dark, 

 horny membrane corresponding with the fe- 

 nestrated membrane, and an inner delicate 

 flexible layer, corresponding with the inner 

 horny rootsheath. The former has a structure 

 intermediate between that of the two layers 

 of the inner rootsheath in the hair, consisting 

 of irregular polygonal plates, which retain the 

 remains of their endoplasts (_fig. 317. p),as in 

 the inner layer of the horny rootsheath, and 

 do not become separated by fissures; while 

 they resemble the plates of the outer horny 

 rootsheath in their thickness, complete cor- 

 nification and striated appearance. 



The inner layer of the horny rootsheath is 

 a delicate, often granular membrane, which 

 closely invests the outer surface of the feather, 

 and from presenting a cast of its elevations 

 and depressions, has been called the outer 

 "striatetTmembrane" of the feather sac (supra, 

 2.) It is a sheet of horny matter, in which 

 traces of closely-set endoplasts are discover- 

 able. The inner (fig. 317. E, d) " striated 

 membrane " is a membrane having a similar 

 structure, possessing similar relations to the 

 inner surface of the feather, and which is con- 

 tinuous with the so-called "pith" in the quill 

 of a fully formed feather. The mode of de- 

 velopment of these rootsheaths is identical 

 with that of those in the hair, and therefore 

 requires no further elucidation here. 



Tcgumentary glands. The other con- 

 versionary productions of the ecderon which 

 we have to consider, are the glandular ap- 

 pendages, which are always diverticula of 

 the cellular ecderon inwards.* Under this 

 head I include only those small glandular 

 organs which, so far as we know, have no 

 reference to any other functions than that of 

 cutaneous transpiration or fatty secretion, 

 referring to the articles on special divisions 

 of the animal kingdom for an account of 

 those organs, such as the " water vessels " of 

 Echinoderms and Trematoda, the nidamental 

 glands of Mollusks, the genital glands of 

 Vertebrata and Insecta, which might strictly 

 be regarded as productions of the integument. 



Tegumentary glands in this limited sense 

 are somewhat rare among the Invcrtebrata. 

 They have, however, been observed in the 

 Annelids, where they consist of delicate tubes, 

 terminating internally by a blind extremity 

 containing a single nucleated cell. Such 

 glands exist on the ventral surface of the head 

 and foot discs in Piscicola, and are scattered 

 all over the body in Clepsine and Nephelis. 

 Similar glands are found opening upon the 

 ventral surface of Argulus foliaceus. 



* Unless, indeed, these simple " mucous cells," 

 described by Clark and Leytlig in Fishes, mid 

 which are merely modiiied cells of the cellular 

 ecderon, should be regarded as glands. 



K K 2 



