166 



HISTOLOGY 



so many that one is at a loss where to begin. Blood should be studied 

 both by itself and in situ in the tissues. For the latter, the ordinary 

 methods are sufficient, excepting that the special blood stains, to be 

 mentioned later, should be used here too. For study outside of the tissue, 



FIG. 148. Five stages in the free life of red blood cells of a salamander, Diemyctylus. A, the 

 youngest free stage (spindle-cell) found in the capillaries of the lungs. B and C, two suc- 

 cessively older stages showing the slow accumulation of htzmoglobin in the peripheral cyto- 

 plasm. D, an older stage than C, undergoing mitotic division. E, fully developed and 

 functional red blood cell. X 1000. 



the blood is ordinarily taken fresh in small drops and spread, by capil- 

 lary attraction, between two cover glasses, which are then slid apart 

 and the remaining films dried and fixed at the same time by passing them 

 through the flame of a spirit lamp. For the staining, various combina- 

 tions of some of the aniline dyes are used to get a differential stain of the 

 various sorts of blood cells that are to be found in the preparation (see 

 Lee). 



LITERATURE 



ENGEL, S. " Zur Entstehung der korperlichen Elemente des Blutes," Arch.f. mik. Anat., 



Band XLII, S. 217. 

 WIELOWIESJSKI, H. v. " Uber das Blutgewebe der Insecten," Zeit. fur Wiss. Zool., 



Band XLIII, S. 512. 



BLOOD-FORMING TISSUES 



The blood must not only take its origin from the differentiating tis- 

 sues of the body and increase in amount to suit the needs of the growing 

 embryo ; but provision must also be made for producing further supplies 

 of a tissue that is so apt to be lost in the adult by accident if not worn 

 out by use. This idea applies to the plasma as well as the corpuscular 

 portion, but, owing to our lack of knowledge as to the production of the 



