42 EPITHELIUM. 



have also be^n to produce pale coi'iiusclcs, and in after periods supersede the 

 liver in that office. These corpuscles, either immediately or after fissiparous 

 multiplication, acquire colour like the first — those from the liver and spleen pro- 

 bably in great part before they leave these organs — and are converted into 

 nucleated red corpuscles. Tlie nucleated red corpuscles thus produced are 

 gradually converted into, or at least succeeded l^y, smaller disk-shaped red cor- 

 ])uscles without nuclei, having all the characters of the blood-disks of the adult. 

 This transition or suljstitution begins early, and proceeds gradually, until at 

 length, long before the end of intrauterine life, the nucleated red coriiusclcs 

 have altogether vanished. 



The disk-shaped red corpuscles are produced, in part at least, in the interior of 

 connective tis.sue cells of the developing mammal in a manner somewhat similar 

 to that described by Klein in the cells of the middle layer of the chick's l)lasto- 

 derm. The cell-nuclei, however, are not involved in the process, which seems to 

 be rather of the nature of a deposit within tire cells. The blood-corpuscles which 

 are at first spheroidal eventually take on the flattened form and Ijccome free 

 within a cavity which is hollowed out in the interior of the cell ; the latter 

 becomes united with neighbouring cells to form the l>lood-vessels of the part. 

 This endogenous mode of cell formation commonly ceases before birth.* 



Throughout life the mass of blood is suV)ject to continual change ; a portion 

 of it is constantly expended, and its place taken by a fresh supply. It is certain 

 that the corpuscles are not exempted from this general change, but it is not 

 known in what nianner they are consumed, nor has the process been fully traced 

 by which new ones are continually formed to supply the i)lace of the old. With 

 regard to the latter question, it may l;)e stated, that the explanation A\-hich has 

 hitherto found most favour with i^hysiologists is, that the corpuscles of the chyle 

 and lymph, passing into the sanguiferous system, become the pale corpuscles of 

 the blooil ; and that these last are converted into red di.sks. Pale cori)uscles are 

 also generated in the spleen, and, after part of them have changed into red dislcs, 

 pass directly into the blood, independently of those derived from the chyle and 

 lymph. A production of Ijlood-corpuscles is also said to take place in certain 

 cells of the marrow of the bones, in which transitional forms to the red coiiius- 

 cles have been observed. (Neumann, Bizzozero.) As to the manner in which 

 the pale corpuscles are transformed into the red, there is considerable difference 

 of opinion. According to one view (adopted by Paget, KoUikcr, Funke, and 

 others), the pale corpuscles gradually become flattened, acquire coloured con- 

 tents, lose their nuclei, and shrink somewhat in size, and thus acquire the 

 characters of the red disks. Wharton Jones, on tlie other hand, arrived at 

 the conclusion that, whilst in birds, reptiles, and fishes, the pale or lymph 

 corpuscle, .suffering jnerely some alteration of form and contents, becomes the 

 red disk, its nucleus alone is developed into the red disk of mammalian blood. 

 According to this view (supported l)y Busk, Huxley, and Gulliver), while the red 

 corpuscle of oviparous vertebrata is the transformed pale corpuscle — its develop- 

 ment not proceeding beyond this stage — the non-nucleated red disk of men and 

 mammalia is. on the other hand, considered to be, not the homologue of the 

 oval nucleated red disk of the oviparous vertebrata, but that of its nucleus. 

 It is not within the scope of this work to enter upon a discussion of the relative 

 merits x)f these opinions, and the reader is referred to jiliysiological works for a 

 consideration of these and other views adopted by various authors upon the 

 point at issue. 



EPITHELIAL, EPIDERMIC, OR CUTICULAR TISSUE. 



General nature and situation. — It is well known, that when the 

 skin is blistered, a thin, and nearly transparent membrane, named tlie 

 cuticle or epidermis, is raised from its surface. In like manner, a 

 transparent film may be raised from the lining membrane of the mouth, 

 similar in nature to the epidermis, although it has in this situation 



* ScLiifer, Proceedings of the Royal Society. 1874, 



