For in the first place, the functions of assimilation are much fewer in number; the molecules before they arrive within the organs which they are destined to create, are not submitted to so many actions; they penetrate the fœtus already elaborated by the digestion, circulation, and respiration of the mother. Instead of traversing the apparatus of the digestive organs, which at this age appear to be almost inactive, they enter at once into the system of the circulation; the road which they have travelled is less, it is not requisite that they should be presented to the influence of respiration; and accordingly the fœtus of the mammalia has in its preliminary organization a near analogy with that of the adult reptile, in which but a small part of the blood at its issuing from the heart, is sent into the vessels of the lungs.[48] The molecules of nourishment in this way pass almost directly from the circulating torrent into the nutritive system. The general process of assimilation, then, is much less complicated than that of the following age. On the other hand, those functions which habitually decompose the organs, which clear the system of substances already become injurious and foreign to its nature, are at this age but very inactive. Neither the pulmonary exhalation, nor sweating, nor transpiration have as yet commenced: the bile, urine, and saliva are but small in quantity, if compared with what they are destined at a future time to be, so that the portion of blood from which they are to be made in the adult, in the fœtus is almost entirely expended on the system of the nutritive organs. The organic system of the fœtus, then, is remarkable--on the one hand, for the extreme promptitude of its assimilation, a promptitude depending on the very small number of the functions concurring to that end; and on the other, for the extreme inertia of its decomposition, an inertia depending on the little activity of the different functions, which are the agents of this great process. It is easy from the foregoing considerations to account for the rapidity which characterizes the growth of the fœtus; a rapidity which is manifestly out of all proportion with that which takes place at any other age. Indeed, while every thing is in favour of the progression of the nutritive matter towards the parts where it is destined to be put down, every thing at the same time seems to oblige such matter to remain in the place where it has been deposited, the emunctories of the system being wanting. To the great simplicity of assimilation in the fœtus, we may add the great activity of the organs which contribute to it an activity, which depends upon the more considerable sum of vital power which they then partake. All the powers of the economy, indeed, appear to be concentrated upon the system of the circulation and nutrition; the functions of digestion, respiration, secretion and exhalation, are exercised but obscurely.[49]