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HOWELL. [Vol. IV. 



way in which the first corpuscles are formed, but also in the 

 fact that the original cells are not derived from the mesoblast. 

 In this last respect Kupffer's theory bears some resemblance to 

 the well-known parablastic theory of His (12). His believes 

 that the mesoderm in the sense of the term used by Remak can 

 be separated into an axial and a peripheral portion. The axial 

 portion is formed in the higher animals in the neighborhood of 

 the primitive streak ; in the lower animals, in the groove of the 

 blastopore. From it are derived the muscles, the chorda, the 

 generative epithelium, the duct of the pronephros, etc. It falls 

 into two layers, which taper off laterally, but do not extend 

 beyond the body proper of the embryo. The peripheral por- 

 tion of the mesoderm forms what His calls the parablast ; and, 

 when first formed, it lies outside of the body of the embryo, 

 arising in fact from the white yolk. The parablast gives rise 

 to the blood corpuscles and blood-vessels as well as the general 

 connective tissues. Though arising outside of the body, it 

 afterwards grows in from the periphery, penetrating the germ 

 layers, so that the parablastic cells become inextricably mixed 

 with the cells of the germ layers. 



To which of these various theories the balance of evidence 

 tends it is impossible to say. In my own work I have not 

 attempted at all to follow the development of the first cor- 

 puscles in the germ layers of the embryo. If we suppose that 

 the method of formation of the first red corpuscles in the germ 

 layers is similar to that which prevails in later embryonic life 

 and in extra-uterine life, then it seems to me highly proba- 

 ble, for reasons which I will give presently, that the general 

 method described by Reichert and Kolliker, and afterwards 

 extended and modified by Wenckebach and Ziegler, is most 

 worthy of belief. According to this view, the primitive blood 

 corpuscles form one variety of mesoblastic cells, which become 

 arranged in masses or strings that mark the position of future 

 blood-vessels (veins). The central cells become red blood 

 corpuscles ; the peripheral cells form the walls of the vessels. 



Development of the Red Corpuscles in the Later Stages of 



Embryonic Life. 



Kolliker (3) first proved that the liver, as soon as it is formed, 

 becomes the seat of production of new red corpuscles. This 



