TEGUMENTARY ORGANS 387 



the differences presented by different tissues in this respect have 

 given rise to the estabhshment of a distinction between what is called 

 the process of conversion and that of excretion. For instance, in the 

 development of a hair or of a nail, the elements of the protomorphic 

 layer evidently pass, as such, into the perfect substance of these 

 organs ; the periplast simply becoming horny, and the endoplasts 

 remaining for a long while, or even always, visible in the cornified 

 tissue. This is therefore a process of " conversion '' of the protomorphic 

 tissue. On the other hand, the chitinous coat of the lower Annulosa 

 and the shells of the lamellibranchiate and gasteropod Molluscs arise 

 in a totally different manner. The elements of the protomorphic layer 

 do not pass into them entire, but they are formed, like the cuticula of 

 a plant, or like the dentine and enamel of the teeth, by the successive 

 outgrowth of layers of the outer portion of the periplast. No 

 endoplasts, therefore, are ever found in them, and there is no con- 

 version of the protomorphic tissue, but a process of excretion} 



At first sight this distinction would appear to be very decided, 

 and likely to afford a good ground for the formation of definite 

 sub-divisions of the integumentary organs into classes. Unfortunately, 

 it is often difficult in practice to assure oneself in what way a given 

 tegumentary organ has been formed. While the presence of endo- 

 plasts in a metamorphosed tissue is good evidence of its having been 

 developed by conversion, their absence is no proof that the tissue has 

 been developed by excretion ; inasmuch as it may simply be due to 

 their very early disappearance. In fact, if any one affirm that the 

 shell of a Unio or of a Crustacean, notwithstanding the impossibility 

 of detecting endoplasts in its youngest lamina, is in reality formed 

 by the successive apposition of entire layers of the protomorphic 

 tissue, in which the endoplasts disappear so early that they cannot be 

 detected, it would be very difficult absolutely to disprove the assertion, 

 though we might ask for evidence of its truth. Disbelieving in the 

 doctrine of the special vital activity of the endoplasts, I confess the 

 question does not seem to me to be of much importance, and I 

 have only enlarged upon the subject because great weight has by 

 high authorities been laid upon these distinctions. It appears to me 

 that the processes of conversion and of excretion grade one into the 

 other, and that no real subdivisions can be based upon the occurrence 

 of either to the exclusion of the other. I will, however, take care to 

 indicate what appear to me to be clear instances of each. I shall now 

 proceed to consider the histological structure of the integuments of 



^ Using the word in the sense of "growth out," not in the common perverted signification 

 of fluid transudation and hardening. 



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