718 SPECIAL HISTOLOGY. 



mative cells of every part of the young embryo, and arise in the origi- 

 nally solid rudiments of the heart and great vessels, in some situations 

 very early, in others somewhat later, by the separation of the central 

 cells contained in the rudiments, in consequence of the development of 

 a fluid (the first blood-plasma) between them. The first perfect blood- 

 corpuscles arise from these colorless cells, which lose their granules, 

 and, except the nucleus, become filled with Jiematin. These colorless, 

 nucleated, primary blood-cells are spherical, of a deeper color than the 

 blood-corpuscles of the adult, and larger (in a foetal Lamb, 3^ lines 

 long, most of them were 0-005-0-0065, the minority 0-0025-0-0035 

 of a line ; in a human embryo, 4 lines long, according to Paget, 

 0-004-0-007 of a line), but in all other respects present the same condi- 

 tions, and, with their colorless formative cells, at first constitute the 

 sole elements of the blood. But many of them soon begin to multiply 

 by division ; to this end they grow into elliptical, or even flattened 

 cells, 0-009 of a line long, 0-004-0-006 of a line broad, bearing a decep- 

 tive resemblance to the blood-corpuscles of the Amphibia; produce 2, 

 Fig. 295. rarely 3 or 4, rounded nuclei, and afterwards 



divide by one or several annular constrictions, 

 into 2, 3, or 4 new cells. When the liver begins 

 to be formed, this multiplication of the blood- 

 cells in the entire mass of the blood ceases, and 

 in a short time (in the foetal Lamb, 11 lines 

 l n g) a ^ trace even of their development out of 

 colorless formative cells is lost ; whilst at the 

 same time, as Reichert supposed and I have directly proved, a very 

 active formation of blood-cells is set up in the liver, a reason for which 

 may be found in the circumstance that, at this time, all the blood from 

 the umbilical vein, which supplies the embryo with new organizable 

 materials, first flows into the liver, instead of entering the general cir- 

 culation as before. In proportion to the extent in which this cell- 

 formation in the hepatic vessels is carried on, does the self-multiplication 

 of the blood-corpuscles become less and less considerable ; and instead 

 of it, a new formation of free colorless nucleated cells, having a mean 

 size of 0-003-0-004, and an extreme diameter of 0-0015-0-006 of a 

 line, is observed to take place in the blood and immediately around 

 nuclei, which also occur in the free condition ; and which cells after- 

 wards, still for the most part in the liver, are transformed, by the de- 

 velopment of coloring matter in their contents, into colored, nucleated 

 blood-cells, either immediately or after they have multiplied in a similar 

 way to that which the colored corpuscles had previously followed. This 



FIG. 295. Blood-corpuscles of a fcetal Lamb, 3^ lines long: a, bi- and tri-nucleated, large, 

 colored blood-globules, in various stages of division; 6, larger spherical, colored blood-cells, 

 one with a nucleus undergoing spontaneous division; c, a smaller one of the same kind. 

 Magnified 300 diameters. 



