10 THE BLOOD. 



less serrated. Under these conditions, their original form may be restored 

 by adding to the specimen a liquid of about the density of the serum. When 

 they have been completely dried, as in blood spilled upon clothing or on a 

 floor, they can be made to assume their characteristic form by carefully moist- 

 ening them with an appropriate liquid. This property is taken advantage 

 of in examinations of old spots supposed to be blood ; and if the manipula- 

 tions be carefully conducted, the corpuscles may be recognized without diffi- 

 culty by means of the microscope. 



If pure water be added to a specimen of blood under the microscope, the 

 corpuscles swell up, become spherical and are finally dissolved. The same 

 effect follows almost instantaneously on the addition of acetic acid. 



Structure. The blood-corpuscles are perfectly homogeneous, presenting, 

 in their normal condition, no nuclei or granules, and are not provided with 

 an investing membrane. The appearances presented upon the addition of 

 iodine to blood previously treated with water, which have been supposed to 

 indicate the presence of shreds of ruptured vesicles, are not sufficiently dis- 

 tinct to demonstrate the existence of a membrane. The great elasticity of 

 the corpuscles, the persistence with which they preserve their biconcave form, 

 and their general appearance, rather favor the idea that they are homogene- 

 ous bodies of a definite shape, than that they have a cell- wall with semi-fluid 

 contents ; especially as the existence of a membrane has been only inferred 

 and not positively demonstrated. 



Development of the Blood- Corpuscles. Very early in the development of 

 the ovum, the blood-vessels appear, constituting what is called the area vascu- 

 losa. At about the same time, the blood-corpuscles are developed, it may be 

 before, or it may be just after the appearance of the vessels, for this point is 

 undetermined. The blood becomes red when the embryon is about one-tenth 

 of an inch (2*5 mm.) in length. From this time until the end of the sixth 

 or eighth week, they are thirty to one hundred per cent, larger than in the 

 adult. Most of them are circular, but some are ovoid and a few are globular. 

 At this time, nearly all of them are provided with a nucleus ; but from the 

 first, there are some in which this is wanting. The nucleus is ^-g^o to -^^ 

 of an inch (3-1 /* to 3*6 /*) in diameter, globular, granular and insoluble in 

 water and acetic acid. As development advances, these nucleated corpuscles 

 are gradually lost ; but even at the fourth month, a few remain. After this 

 time, they do not differ anatomically from the blood-corpuscles in the adult. 



In many works on physiology and general anatomy, accounts are given of 

 the development of the red corpuscles from the colorless corpuscles, or leuco- 

 cytes, which are supposed to become disintegrated, their particles becoming 

 developed into red corpuscles ; but there seems to be no positive evidence 

 that such a process takes place. The red corpuscles appear before the leu- 

 cocytes are formed ; and it is mainly the fact that the two varieties co-exist in 

 the blood-vessels which has given rise to such a theory. It is most reasonable 

 to consider that the first red corpuscles are formed in the area vasculosa in 

 the same way that other anatomical elements make their appearance at that 

 time, the exact process not being understood. In the later periods of devel- 



