82 ON THE DEVELOPMENT OF THE ANIMAL TISSUES. 



as Schwann remarks, remains attached to the inner surface of their 

 membranous parietes, when they are rendered turgid by the action of 

 water. The red colouring matter of the corpuscles is to be regarded as 

 the contents of the cells. The lymph corpuscles, the globules of mucus, 

 and those of pus, belong to the same class. They are all nucleated 

 cells. 



To the second class belong the horny tissues, the pigment membranes, 

 and the tissue of the crystalline lens. 



1 . Epithelium. Generally composed of round cells, to the inner surface 

 of whose parietes a nucleus containing one or two nuclei is attached. 

 When united into a membrane, they are polyhedral. In the epithelium 

 of the skin of a frog, Schwann saw two nuclei in one cell, and also a 

 nucleated epithelium cell within another larger cell ; a fact which Henle 

 has not observed in Mammalia. The epithelium cells, at first globular, 

 undergo modifications of form in one or two directions. Either they 

 acquire the form of perpendicular cylinders, as in the epithelium of the 

 intestinal mucous membrane, described by Henle ; or they become flat- 

 tened into laminae, which have the nucleus in the middle of one surface, 

 and which sometimes are elongated or riband- shaped, as in the epithe- 

 lium of blood-vessels according to Henle. In the latter case, it is ob- 

 served, that the young cells are found beneath the older ones, and are 

 at first globular, but become more and more flattened as they approach 

 the free surface of the epithelium. 



2. Pigment cells. These have a nucleus at one part of their parietes, 

 which produces the well-known white spot in the middle of some pig- 

 ment cells. The nucleus has usually one or two nucleoli. Many pig- 

 ment cells in the progress of their growth, send out hollow fibre-like 

 processes in different directions, so as to become stellate cells. 



3. Nails. The nail of a fully developed human foetus consists of 

 laminae lying horizontally one upon the other. These laminae become 

 less and less distinct at the inferior surface of the nail, in proportion as 

 the part examined is nearer to the root of the nail which is inserted 

 into the fold of the skin of the finger ; and the posterior half of this 

 portion of the nail presents nothing of a laminated structure, but con- 

 sists merely of polyhedral cells, with distinct nuclei. Laminae of the 

 nail treated with acetic acid, separate into scales, in which an indistinct 

 nucleus can in very rare cases be observed. The polyhedral cells of 

 the root of the nail must become flattened into these scales, and the 

 nail ought consequently to become thinner towards its free margin. 

 This is probably prevented by the formation of laminae of epithelium, 



