342 SPECIAL HISTOLOGY. 



The development of the blood-corpuscles is pretty accurately 

 known, in the embryo, as concerns its principal stages. The 

 first blood-corpuscles, in the Mammalia and other Vertebrata 

 in general, are nucleated, colourless cells, with granular con- 

 tents ; they are perfectly identical with the formative cells of 

 every part of the young embryo, and arise in the originally 

 solid rudiments of the heart and great vessels, in some situa- 

 tions 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 colour- 

 less-cells, which lose their granules, and, except the nucleus, 

 become filled with hematin. These colourless, nucleated, pri- 

 mary blood-cells are spherical, of a deeper colour than the 

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

 8J'" long, most of them were 0005 — 00065'", the minority 

 00025 — 0-0035"' ; in a human embryo, 4'" long, according to 

 Paget, 0*004 — 0-007"'), but in all other respects present the 

 same conditions, and, with their colourless 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, 0009'" long, 

 0004 — 0006'" broad, bearing a deceptive resemblance to the 

 blood -corpuscles of the Amphibia ; produce 2, rarely 3 or 4, 

 rounded nuclei, and afterwards divide 

 "- by one or several annular constric- 



t(^J| 0^h tions, into 2,-8, or 4 new cells. When 

 jHn ^BF the liver begins to be formed, this 

 *tr multiplication of the blood-cells in the 



j& 4 t entire mass of the blood ceases, and 



B Mfk i ft *L m a 8nort ti me ( m a foetal Lamb, 

 ^K ^|p ^0 ll'" Ion-) all trace even of their 

 development out of colourless forma- 

 tive cells is lost; whilst at the same time, as Reichert sup- 

 posed and I have directly proved, a very active formation 

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



Fig. 295. Blood-corpuscles of a fetal Lamb, 3^'" long : a, bi- and tri-nucleated, 

 large, coloured blood-globules, in various stages of division ; b, larger spherical, 

 coloured blood-cells, one with a nucleus undergoing spontaneous division ; c, a 

 smaller one of the same kind ; x 300 diam. 



