62 HOWELL. [Vol. IV. 



holds good for the nucleated red blood corpuscles in the blood- 

 forming organs of cats of all ages, and presumably for other 

 mammals also. In face of these two facts it is hard to believe 

 that the extrusion of the nucleus is in any sense an accidental 

 occurrence. On the contrary, it is the normal means by which 

 the nucleated corpuscle passes into its non-nucleated form. 



The place and manner of origin of the first red corpuscles 

 of the embryo have given rise to a number of different 

 theories. Reichert (6) taught that the first .corpuscles are 

 formed from the mass of cells from which the heart is devel- 

 oped, the central cells of the mass becoming the blood cor- 

 puscles, while an intra-cellular liquid which forms represents 

 the blood plasma. Kolliker (3) afterwards extended this theory 

 so as to take in the great blood-vessels as well as the heart. 

 He believed that the first blood corpuscles of the embryo are 

 colorless nucleated cells like the other embryonic cells, and 

 at first are found in the solid cords or masses from which the 

 heart and first blood-vessels are developed. The peripheral 

 cells form the walls of the blood-vascular organs, while the 

 central cells are floated away in the plasma which forms 

 between the cells. The red corpuscles are spherical nucleated 

 cells which multiply by division in the blood stream. Within 

 recent years a somewhat similar view has been proposed by 

 Ziegler and by Wenckebach. Wenckebach's (7) first observa- 

 tions were made upon embryos of Perca fliiviatilis. These 

 embryos are so transparent that they can be examined entire 

 under the microscope. Ziegler (8) worked chiefly with the 

 salmon. Both state that the heart begins to beat and forces 

 into circulation a colorless plasma before the red corpuscles 

 appear. These are seen later, and are formed first, according 

 to Wenckebach, from a mass of cells lying in cross-section 

 between the aorta and intestine, and outlining the position of 

 the future posterior vertebral or cardinal vein. The blood 

 plasma, percolating through this mass of cells, washes off the 

 central ones, to form the first red corpuscles, while the periph- 

 eral cells form the walls of the future vein. 



Ziegler also dscribes a mass of cells found in cross-section 

 between the notochord and intestine, which he calls the inter- 

 mediate cell mass. The central cells of this mass become the 

 first blood corpuscles in the way described, while the peripheral 



