822 On the Corpuscles of the Blood, 



4. On first seeing the large cells in the mammiferous ovum,* 

 I was struck with the resemblance they bore to the corpus- 

 cles or cells of the blood in, for instance, the Batrachia ; which 

 waiS also remarked by Dr Roget on seeing my delineations of 

 the former : and I have since (§ 1, 2) shewn them to be per- 

 petuated by the same means. Finding also in the blood of the 

 mammiferous embryo corpuscles or cells (figs. 4, 5) like the 

 ordinary blood- corpuscles or cells of the adult Batrachia, &c., 

 I conceived that the difference between the condition of the 

 blood corpuscles in the embryo and in the adult of the same 

 animal, was referable to a difference in the degree of their 

 development as cells.-[* 



5. Now, there are facts, I think, which leave little doubt 

 that the blood-corpuscles — not only in the embryo, but at all 

 periods of life— are descended from the two cells constituting 

 the foundation of the new being in the ovum ; cells arising 

 out of previously existing cells, by self- division of the nuclei. 



6. When tracing the early stages in the formation of the 

 embryo, I shewed that, as the cells thus increase in number, 

 they diminish in their size. Have we any proof that this 

 diminution in size ceases in later stages ? Is it not rather to 

 be presumed that it continues \ and, indeed, does not the dif- 

 ference in size between the corpuscles of adult and foetal 

 blood render it probable that this progressive diminution in 

 size goes on \ If so, the younger the larva is, the larger may 

 be its blood-cells. And as a larval state in the Batrachia, &c., 

 is indicated by a retention of the gills, is it surprising that we 

 find their blood-corpuscles large in proportion to the length of 

 time during which they retain the gills \ 



7. I cannot doubt that a law of the kind now mentioned — 

 progressive diminution in the size of cells — is general in its 

 operation ; and if so, it may regulate the magnitude of the cor- 

 puscles in other blood. 



* Researches in Embryology, Second Series. Phil. Trans. 1839, pi. 16, fig.' 

 loOi, &c. 



t On the Corpuscles of the Blood, Part II Phil. Trans. 1811, p. 206. 



