48 The Alligator and Its Allies 



mandible. The other pair is inside of the lips of 

 the cloaca. 



The Histology of the Integument. To understand 

 the structure of the integument of the Crocodilia 

 it is well to begin with the embryo. A cross section 

 of the epidermis of such an embryo will show the 

 rete Malpighii as a single layer of short, cylindrical 

 cells; over these are found more or less flattened, 

 disk-shaped cells formed by transverse division of 

 the underlying cells of the rete. On the outside lies 

 the epitrichial layer which consists of a mosaic of 

 polygonal cells, near the middle of each of which 

 lies an oval nucleus. Between the epitrichial cells 

 are small oval holes, not unlike the stomata in the 

 epidermis of plant tissues. Bronn thinks these are 

 not artifacts, but he does not suggest any explana- 

 tion of their occurrence. 



In the epidermis of young and half -grown ani- 

 mals the rete Malpighii is seen still as above noted. 

 On these cylindrical cells are found flattened cells 

 that gradually become very flat and lose their 

 nuclei as they pass over into the horny layer. 



The stratum corneum consists of strongly flat- 

 tened cells in which the nuclei can no longer be 

 clearly seen, though their location can usually be 

 determined by the groups of pigment granules. On 

 the cells of the more superficial layers of the stratum 

 corneum are seen straight, dark lines, perhaps 

 ridges caused by pressure of the over- or underlying 

 polygonal cells. The individual cells of the horny 



