93 



with the vessels of the mother. It hence appears that the acquisi- 

 tion of fluid from the uterus by the placenta must be exceedingly 

 slow, or it would be disproportionate to the growth of the 

 fffitus. It is probable that the fluid is thus obtained from the 

 uterus by the slow and well-adapted process of absorption, either 

 animal or capillary; the latter may perhaps be preferred, by 

 reason that fluid is absorbed before absorbent vessels are found 

 in the ovum. 



72. That red blood is not absorbed by the maternal portion 

 of the placenta, appears probable from the circumstances, 1st, 

 that injections, which usually pass into all vessels carrying red 

 blood, cannot be made to pass through the maternal portion of the 

 placenta; and, 2nd, that in the crustaceous ovum red blood is 

 formed, as in the chick, unequivocally without any absorption of 

 this fluid from the mother; it is formed from a substance which 

 supplies nourishment to the chick, but which is nothing like red 

 blood. Assuming the probability that blood is formed by the 

 foetal system in the human ovum, and it being acknowledged that 

 such is the fact in the crustaceous ovum, it remains that we in- 

 quire how this conversion is accomplished. 



73. That red blood is formed by the foetus from other fluids, 

 is an assumption which rests partly upon analogy; but if the 

 analogy should not be unexceptionable, the reasonings founded 

 upon it will at least be explanatory of some processes of the 

 crustaceous ova. 



74. If an egg which has lost its vital principle, or which 

 was never endowed with the fecundating principle (in effect the 

 same), be exposed to a proper temperature (i.e. such a one as 

 would lead to the development of the chick, in an egg disposed 

 for the living processes), the egg in this condition will no more 

 form blood than it will form the textures or commence the other 

 functions. As it has been shewn that the latter are attributable 

 to the operation of vital properties, so it must also be inferred, by 

 parity of reasoning, that the conversion of substances which differ 

 materially from blood, into blood, is a result of properties which 

 belong to the organic spirit. 



75. But we have reason to believe that a fluid may contain 

 all the materials of growth, indeed w r e have many direct examples 

 of it, without answering to the description of blood. And it may 

 be supposed that nutrition may be performed by such a fluid, 

 imbibed in the one case through the placenta, and in the other 

 from the albumen of the egg; the constituents of which fluid, as 

 those of other things, are infinite; requiring only the means of 

 analysis, of which we are ignorant, to shew them to be so. Then 

 the question seems to be, not how a fluid containing the elements 

 of the structures comes to be possessed by the embryon, but how 

 this fluid is made red blood? 



76. This has been attributed to the agency of mere atmos- 

 pherical oxygen, but erroneously, as appears from the following 



