THE BLOOD. 77 



larger than others, and the smallest consisting almost entirely 

 of a nucleus surrounded by a faint and delicate envelope. 

 Between the two extremes of size every possible gradation is 

 presented. (See Plate IX. fiy. 1.) 



This variation in the dimensions of the corpuscles becomes 

 scarcely less apparent if they be immersed in water, in which 

 they become perfectly spherical. (See Plate IX. fig. 2.) 



A diversity of size almost as remarkable as that which 

 exists between the red blood corpuscles of the embryo fowl 

 will be observed also in those of the young frog which has 

 but just emerged from its tadpole state. If a drop of the 

 blood of this young frog be compared with that of a full- 

 grown frog, the corpuscles in the former will be remarked to 

 vary greatly in dimensions, while in the latter they will be 

 seen to present a much greater uniformity of size. (See 

 Plate IX. figs. 4, 5.) 



Now, the inferences to be deduced from this great diversity 

 of size are palpable, and are, first, that the red blood corpuscle 

 is at its origin small, and only attains its full dimensions after 

 a given period ; and, second, that the nucleus is the part of 

 the corpuscle which is first formed, the coloured investing 

 and perfectly smooth portion of it being gradually developed 

 around this subsequently. This view is inconsistent with the 

 notion entertained by many that the red blood corpuscles 

 result from the gradual assumption by the white globules of 

 the characteristic distinctions of the red blood discs, for were 

 this really the case we should be at a complete loss to account 

 for the remarkable differences of size to which we have 

 adverted. 



A similar mode of development to that which has been 

 described as belonging to the red blood corpuscles of the 

 embryo fowl appertains also, I believe, to that of all the 

 Oviparous Vertebrata. 



The development of the coloured blood corpuscle of the 

 Mammalia, I conceive to agree also with that of the other 

 Vertebrata in the fact of their being small at first, and sub- 

 sequently and gradually attaining their normal proportions, 



II 



