TEGUMENTARY ORGANS 417 



the indifferent tissue of the pulp in exactly the same manner as those 

 of an ordinary horny, cellular ecderon from that of the rete mucosum : 

 that is to say, the periplast increases, and becomes marked out into 

 polygonal arete ; it then requires a horny consisten-ce, and a stronger 

 and stronger definition along the lines of demarcation, until polygonal 

 " cells " (as in_/%-. 317. b, a) are formed. The walls of the latter now 

 thicken and become granular ; the endoplasts disappear, and at 

 length nothing is left but the honey-combed perfect medullary 

 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 (c) with recently formed cortical substance ib), it is observable 

 that the endoplasts do not become surrounded by cell cavities, but 

 that the periplast acquires a granular, longitudinally fibrous, appear- 

 ance ; while the endoplasts, though they are occasionally visible in 

 the striated mass, soon completely disappear.^ The elongated cavities 

 or tubuli do not at first exist in the cortex, but are the result of a 

 secondary vacuolation, 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 fenestrated root- 

 sheath, and the canaliculi of bone, must be regarded as the results of a 

 :secondary vacuolation. The feather sac resembles that of the hair 

 in all essential points of structure 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 fenestrated 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. 

 f), 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 cornification 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 "striated membrane" of the feather sac 

 ■{suprcl, § 2). It is a sheet of horny matter, in which traces of closely 

 set endoplasts are discoverable. The inner {fig. 317. E, a) "striated 

 membrane " is a membrane having a similar structure, possessing 

 .similar relations to the inner surface of the feather, and which is 



^ Compare Schwann, Untersuchungen, &c. 



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