BLOOD-FORMING GLANDS 



167 



plasma, we must confine our studies to the cellular portions in the few 

 forms in which their origin is at all understood, merely saying that the 

 plasma is probably a secretion of some of the cells that line the channels. 

 We shall study some of the few known blood glands and then speak of 

 the first appearance of blood in the embryonic tissues. 



In general, it may be said that the primitive mode of producing 

 new blood cells is by a proliferation of cells from the inner endothelial 

 walls of the blood channels, especially of the peripheral parts of the 

 system. Such a process has often been described in many forms, usually 

 in very general and unsatisfactory terms, how r ever. 



The first step in the specialization and organization of this function 

 would be to restrict it to some favorable region of the channel. Such 

 a region would be more favorable if the channel were modified hi such a 

 manner as to slow the current and provide a quiet place for the new cor- 

 puscles to be formed. This would be easiest done by an imagination 

 of the endothelial wall of the channel, and such simple structures are to 

 be seen hi the crustacean blood glands. We shall examine one of these 

 first. 



On the arteries, especially the ophthalmic artery of the crayfish, may 

 be found a number of small irregular glands, each composed of several 

 short acini opening by a common and short duct into the lumen of the 

 artery. The duct is lined by the same intima as the artery, but this 

 structure is absent at the entrance to each acinus. Its place as a lining 

 is taken by the blood-forming cells, which probably represent theLeidig's 



bl.c. 



FIG. 149. Section of a blood gland from the ophthalmic artery of a crayfish Aslacus. int., in- 

 tima of blood vessel extending through the duct and partly into the acinus of the gland ; ep., 

 epithelium of gland one of whose cells is beginning a mitotic division at (mil.) ; bl.c., young 

 blood cells passing out of the gland into the vessel, v. (After SCHNEIDER.) 



cells of the third order that line all blood -channel surfaces and secrete the 

 ultima. These blood-forming cells divide by mitosis and thus produce 



