Il6 NORMAL HISTOLOGY AND ORGANOGRAPHY. 



may do so. According to some investigators, minute 

 pores in the epithelial wall, called stigmata and sto- 

 mata, allow this migration. Others deny the presence 

 of these spores, in which case the blood elements 

 escape by passing between two adjacent epithelial 

 cells, after which this opening closes. 



Flat view. 



Side view. 



THE BLOOD. 



The blood is derived from the mesoderm; it is a 

 red fluid that consists of (i) a liquid portion, the 

 plasma, and (2) solid constituents, the corpuscles. 

 There are at least three classes 

 of the latter, red corpuscles, white 

 corpuscles, and platelets. 



i. Red corpuscles (erythro- 

 cytes) in the mammalia are non- 

 nucleated, circular, biconcave 

 discs. In all the other verte- 

 brate groups and in all embryos 

 they are nucleated oval and 

 biconvex cells. Each corpuscle 

 consists of a red coloring matter, 

 hemoglobin, and a more substan- 

 tial fabric or reticulum, the 

 stroma. The hemaglobin is the 

 bearer of oxygen, is readily solu- 

 ble in water, leaving the stroma 

 or fabric, which is then known as 

 a ghost corpuscle. 



The red corpuscles are soft and 

 elastic and are covered by an oily 

 film. In a fresh spread they adhere to each other 

 by their concave surfaces forming rouleaux or " money - 



Rouleau. 



Rouleau. 



Fig. 80. Red blood- 

 corpuscles from man. 



