THE CIRCULATORY SYSTEM. 



unite into an irregular net-work, the nodal points of which are dis- 

 tinguished by an active production of new nuclei. Some of these 

 acquire protoplasm and later become the endothelium of the blood- 

 vessel, while others, more centrally situated, are converted into the 

 primary blood-corpuscles, the intervening tissue undergoing 

 liquefaction to constitute the blood-plasma. These earliest blood- 

 cells, although destined to become the red corpuscles, are at first 

 colorless masses of active protoplasm, provided with nuclei and 

 exhibiting amceboid movements. After a time the protoplasm 

 gradually acquires the characteristic tinge and assumes a discoidal 

 form, the elements then constituting the nucleated red blood-disks 

 of the embryo. The earliest red cells unquestionably increase 

 by the division of the primary corpuscles, the reproduction being 

 attended by the changes of karyokinesis. This multiplication of 

 the early red corpuscles probably ceases in man long before the end 

 of gestation, the embryonal colored corpuscles meanwhile becoming 

 smaller and losing their nuclei, so that at birth all the nucleated red 

 cells have disappeared. The exact details of the metamorphosis 

 from the embryonal to the adult form are still uncertain. There is 

 no evidence at present to establish the descent of the red corpuscles 

 from the colorless cells, the two being distinct elements having 

 independent origins. The liver must probably be reckoned among 

 the situations in which the formation of blood-cells takes place during 

 embryonal life ; in this same category is included the spleen by some 

 authorities, probably, however, with questionable propriety. 



After Birth. Of the many suggested sources for the post-natal 

 production of the red blood-cells, of which great numbers must be 

 formed constantly to replace those continually undergoing destruction, 

 the red marrow of bones is undoubtedly the most important. 



Among the more common elements of the red marrow, cells usu- 

 ally are to be observed which strongly resemble the embryonal red 

 blood-corpuscles, being distinguished from the ordinary marrow- 

 cells by their haemoglobin-colored protoplasm, smaller size, and 

 unstable nuclei. These cells, often called the erythroblasts, are 

 undoubtedly transitional forms of red blood-corpuscles, the nuclei 

 disappearing and the protoplasm assuming the usual appearance of 

 such elements. 



As to the source of the erythroblasts, however, whether they are 

 transformed colorless marrow-cells or distinct elements, the descend- 

 ants of the red corpuscles of the embryo, much uncertainty still 

 exists. There are strong reasons for regarding the latter supposition 

 the true indication of their nature and origin, the production of the 

 red corpuscles both before and after birth being thus closely related. 



Direct transformation of the colorless cells, production within the 



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