474 



TEOUMENTARY ORGANS. 



plexities whose solution appears to me to be 

 essential to any philosophical treatment of 

 the subject, and to the consideration of which 

 1, therefore, propose to devote the following 

 Preliminary Section. 



1 . My first difficulty was to find an answer 

 to the question, What constitutes a tegu- 

 mentary organ as distinguished from any other? 



The most obvious definition of an integu- 

 ment or tegumentary organ is, of course, 

 that which forms the external covering of any 

 animal viscus, on the other hand, being 

 that which is contained. More strictly, it 

 may be said that the integument constitutes 

 that free surface of an animal which is ex- 

 ternal to the edges of the oral and anal aper- 

 tures, or where the former alone exists, to its 

 edge. Now these definitions are perfectly 

 sufficient so far as surface is concerned ; but 

 suppose we make a section perpendicular to 

 the surface, where does integument cease, and 

 where does viscus begin ? So far as I am 

 aware, no elucidation of this point has hither- 

 to been undertaken, and yet, for want of it, 

 the greatest confusion prevails in the nomen- 

 clature of those organs which constitute the 

 outer wall of the animal frame. 



Intimately connected with this question, and 

 indeed forming a part of it, is a second. In man 

 and the higher animals, there is an universally 

 recognised distinction of the integument into 

 two portions, the epidermis and the derma ; 

 and these terms have been extended to all 

 animals. But, if we inquire what constitutes 

 an epidermis, and what a derma, no definite 

 answer is to be met with. It may be said that 

 the derma is vascular, while the epidermis is 

 nonvascular ; or that the epidermis is a simple 

 cellular horny structure, while the derma is 

 complex and fibrous,- but these characters, 

 applicable enough among the higher animals, 

 fail completely with the lower. 



Thus, in the majority of the Invertebrata, 

 the derma cannot be said to be vascular, 

 while, on the other hand, the epidermis, or 

 its representative, assumes the structure of 

 fibrous tissue, bone, cartilage, dentine, and 

 enamel, acquires, in fact, the utmost 

 complexity, and, instead of possessing a horny 

 nature, contains chitin, cellulose or calca- 

 reous salts. 



Following Mr. Bowman, who, of course, 

 when he wrote his well-known article on 

 " Mucous Membrane," in this Cyclopedia, 

 could not contemplate the new questions to 

 which the progress of ten years would give 

 rise, many regard that which is external to 

 a " basement membrane " as epidermic, that 

 which is internal to it, as dermic structure. 

 This test, however, fails us where we most 

 want it ; for among the lower animals, and in 

 some integumentary organs among the higher, 

 membranes identical in structure, or rather 

 in htructurelessness, with " basement " mem- 

 branes, may be met with, forming the surface 

 of what are assuredly epidermic organs. 



I believe that here, as elsewhere, the only 

 ultimate appeal lies to development, both as 

 it occurs in the embryo and as it goes on in 



the adult. What, in fact, is the first process 

 which takes place in the embryo, when the 

 germinal disc is once formed ? It is a sepa- 

 ration into two layers, by the setting up within 

 the outer portion of the primitive germ of a 

 process of growth independent of that in the 

 inner portion. Where these two areae or 

 planes of growth, as they might be called, 

 meet, the germ readily separates into two 

 portions, the outer of which is the so-called 

 serous layer, the primordial tegumentary 

 system ; while the inner is the mucous layer, 

 the primordial viscus. Of course each of 

 these, while actually integument and intestine, 

 represents potentially a great deal more, the 

 former, for instance, in the higher animals 

 becoming eventually differentiated into the 

 proper tegumentary system and a great part 

 of the nervous, the muscular, and the vas- 

 cular systems ; but what I wish to direct 

 attention to at this moment, is the fact, that 

 this first differentiation into integument and 

 viscus proceeds from the setting up of two 

 independent lines, or rather planes of growth, 

 in the germinal membranes. 



In the Hydra and Hydroid Polypes gene- 

 rally, we have the essence of this embryonic state 

 as a persistent condition. If, in fact, the body 

 or almost any organ of one of these animals 

 be examined, it will be found (see Memoir on 

 the Structure of the Medusa?, Phil. Trans. 

 1849) to be composed of two distinct mem- 

 branes, an inner and an outer (fig. 303. A). 

 The junction between the two is distinctly 

 marked by a clear line, which would elsewhere 

 be called a basement membrane (). External 

 and internal to this, there is a layer of young 

 tissue, consisting of a homogeneous periplast 

 with minute imbedded endoplasts (" nuclei"). 

 As we proceed towards the free surface, we 

 find that a process of vacuolation and cel- 

 lulation takes place in the periplast, until the 

 coarsely cellular appearance with which every 

 one is acquainted is produced. 



Fig. 303. 



A, hydra ; b, outer membrane ; c, inner mem- 

 brane. 



B, young mammal ; b, epidermis ; c, derma. 



In the Hydra, then, we have the whole 

 thickness of the body divided into two por- 

 tions by a line, on each side of which, inwards 

 and outwards, there is an increasing histo- 

 logical metamorphosis or differentiation. 

 There is a median plane of no differentiation, 



