PERIOD OF GROWTH AND DEVELOPMENT. 1017 



ism should be so much reduced as no longer to suffice for the maintenance 

 of its own integrity ; and whenever such is the case, the termination of its 

 existence as a Jiving body must be the necessary result. Hence we find that 

 there is a natural limit, not only to the size and development of the organ- 

 ism, but also to the duration of its life. And although that limit in each 

 case is subject to variation amongst individuals, partly in consequence of 

 diversity of external conditions, but partly (it may be surmised) through 

 differences in the measure of germinal capacity possessed by each, yet there 

 is a limit also to these variations, so that the character of the species is never 

 departed from. 



863. Period of Growth and Development. The general history of the first 

 part of this period, that of Embryonic existence, has already been so fully 

 given, that it is only necessary here to remark briefly in regard to the char- 

 acter of its vital operations, that the whole uisus of its activity is directed 

 rather to the performance of the vegetative or organic than to that of the ani- 

 mal functions; the action of the heart, and the occasional reflex movements 

 of the limbs, being its only manifestations of nervo-muscular power. And 

 thus it seems to be that the formative capacity is greater during embryonic 

 life, than at any subsequent period, and greater in its earlier than in its 

 later stages; so that we have not only evidence of an extraordinary power 

 of regenerating parts which have been lost by disease or accident, as seen in 

 attempts at the reproduction of entire limbs after that "spontaneous ampu- 

 tation" ( 361); but there is also not uufrequently an absolute excess of pro- 

 ductive power, as shown in the development of supernumerary organs, which 

 may even proceed to the extent of the complete duplication of the entire 

 body, by the early subdivision of the embryonic structure into two inde- 

 pendent halves ( 357). It is to be noticed, also, that the embryo derives 

 its supply not merely of food, but also of heat, from its maternal parent; and 

 it is probably owing especially to the constancy with which this force op- 

 erates, that the period of embryonic development is so uniform in Man (as 

 in warm-blooded Animals generally), by comparison with the corresponding 

 developmental periods in Plants and cold-blooded Animals, these being en- 

 tirely determined by the degree of heat to which the embryos are subjected. 



864. It is frequently of great importance, both to the Practitioner and to 

 the Medical Jurist, to be able to determine the age of a Foetus from the 

 physical characters which it presents ; and the following table has been 

 framed by Devergie 1 in order to facilitate such determination. It is to be 

 remarked, however, that the absolute length and weight of the Embryo are 

 much less safe criteria than its degree of development, as indicated by the 

 relative evolution of the several parts which make their appearance succes- 

 sively. Thus it is very possible for one child born at the full time to weigh 

 less than another born at 8 or even at 7 months; its length, too, may be in- 

 ferior; and even the position of the middle point of the body is not, taken 

 alone, a safe criterion, since it is liable to variation in individuals. 2 



Embryo three to four weeks. It has the form of a serpent; its length from 3 to 5 

 lines ; its head indicated by a swelling ; its caudal extremity (in which is seen -a white 

 line, indicating the continuation of the medulla spinalis) slender, and terminating in 

 the umbilical cord ; the mouth indicated by a cleft, the eyes by two black points; 

 members begin to appear as nipple-like protuberances; liver occupies the whole ab- 

 domen; the bladder is very large ; the chorion is villous, but its villosities arc- still 

 diffused over the whole surface. 



Embryo of six weeks. Its length from 7 to 10 lines; its weight from 40 to 75 grains ; 



1 Medecine Legale, 3eme edit., torn, i, p. 279. 



2 See, on this fast point, Moreau in Lancette Fran9aise, 1837; and Dr. A. Taylor 

 in Guy's Hospital Keports, 1842. 



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