134 ON THE ELEMENTARY PARTS OF THE HUMAN FABRIC. 



current of the circulation in some of the very transparent aquatic larvae (espe- 

 cially those of the Culicidae), and the lymph-corpuscles of the Frog. Now 

 it is evident from this fact, that, as the Blood of Vertebrata is distinguished 

 from their Chyle chiefly by the presence of red corpuscles in the former, and 

 by the absence of those bodies in the latter, the nutritious fluid of Inverte- 

 brated animals is rather analogous (as Wagner has remarked) to the Chyle 

 and Lymph, than to the Blood of Vertebrata. Or, to put the same idea in 

 another form, the presence of the colourless corpuscles in the nutritious fluid 

 appears to be the most general fact in regard to its character throughout the 

 whole Animal scale; whilst the presence of red corpuscles in that fluid is 

 limited to the Vertebrated classes and the higher Invertebrata. Hence it 

 would not be wrong to infer, that the function of the colourless corpuscles 

 must be of a general character, and intimately connected with the nutritious 

 properties of the circulating fluid ; whilst the function of the red corpuscles 

 must be of a limited character, being only required in one portion of the ani- 

 mal kingdom. 



156. Further, it has been noticed by Mr. Gulliver, that in the very young 

 embryo of the Mammalia, the white globules are nearly as numerous as the 

 red particles: this, Mr. Gulliver has frequently observed in foetal deer of about 

 l inch long. In a still smaller foetus, the blood was pale, from the prepon- 

 derance of the white corpuscles. It is, therefore, a fact of much interest, 

 that, even in the Mammiferous embryo, at the period when growth is most 

 rapid, the circulating fluid has a strong analogy to that of the Invertebrata. 

 It then, too, bears in other respects the most striking analogy to Chyle; since 

 it consists of the fluid elaborated from the organizable matter supplied by the 

 parent, and directly introduced into the current of the circulation. The func- 

 tion of the placental vessels may be regarded as double : for they are at the 

 same time the channel, through which the alimentary materials supplied by 

 the parent are introduced into the circulating system of the foetus; and the 

 medium of aerating the fluid, which has traversed the fetal system. Hence 

 the placenta may be regarded as at once the digestive and the respiratory 

 apparatus of the foetus; and the fluid circulating through the cord, as at once 

 chyle and blood. It is not until the pulmonary and lacteal vessels of the 

 embryo have commenced their independent operation, that the distinction be- 

 tween the blood and the chyle of the fetus becomes evident; and we should 

 expect, therefore, to find that the circulating fluid, up to the time of birth, 

 contains a large proportion of white corpuscles, which is actually the case. 

 There is a gradual decrease, however, in their proportional number, from the 

 earlier to the later stages of embryonic life ; in accordance with the diminish- 

 ing energy of the formative processes. The recent observations of Mr. New- 

 port upon the Blood of Insects,* present a remarkable correspondence with 

 the foregoing. He finds in the circulating fluid of the Larva, a number of 

 "oat-shaped" corpuscles or floating cells; which he regards as analogous to 

 the Colourless corpuscles of Vertebrata. These are most numerous at the 

 period immediately preceding each change of skin; at which time the blood 

 is extremely coagulable, and evidently possesses the greatest formative power. 

 The smallest number are met with soon after the change of skin; when the 

 nutrient matter of the blood has been exhausted in the production of new 

 epidermic tissue. In the Pupa state, the greatest number are found at about 

 the third or fourth clay subsequent to the change; when preparations appear 

 to be most actively going on, for the development of the new parts that are 

 to appear in the perfect Insect. Alter this, there is a gradual diminution; the 

 plastic element being progressively withdrawn by the formative processes; 



* Philosophical Magazine, May 1845. 



