PRELIMINARY ESSAY ON GENERATION. 
V 
At the.fpace of feven .days after conception, we can readily diftingnifh by the eye the firft lineaments of the child hi 
the womb. However, they are as yet without form ; being a fmail gelatinous mafs, yet exhibiting the rudiments of the 
head; the trunk being alfo in fome meafure diftinguifhable, together with a fmall affemblage of fibres iffuing from the 
body of the infant, which afterwards become the blood-veffels that convey nourifhment from the placenta to the child 
while inclofed in the womb. 
Fifteen days after conception, the hs&d becomes more diftinftly vifible, and even the moil prominent features of the 
vifacre beo-in to appear. The nofe is a little elevated; there are two black fpecks in the place of eyes; and two little 
apertures, where the ears are afterwards formed. The body of the embryo is alfo grown larger; and both above and be- 
low, are two little protuberances, which mark the places from whence the arms and legs are deftined to proceed. The 
whole length of the foetus at this time is lefs than half an inch. 
Three weeks after conception, the body has received but very little encreafe; but the legs and feet, with the hands and 
arms, are become unfolded. The growth of the arms is more rapid than that of the legs; and the feparation of the 
fingers takes place fooner than the toes. About this time, the internal parts are found, on dill'etlion, to be flightiy di- 
itinguilhable. The places of the bones are marked by fmall filiform fubftances, that are yet more fluid than a common 
jelly. Among them, the ribs are perceivable, exhibiting the fame appearance, difpofed on each fide of the f^me; and 
£ven the fingers and the toes fcarcely exceed the thicknefs of a hair. 
The embryo, in about the fpace of a month, is nearly an inch long; the body is bent forward, a fituation it almofi; uni- 
formly afl'umesin the womb; either becaufe this polture is moft convenient, or becaufe it requires lealt room. The hu- 
man figure now becomes extremely evident; every part of the face is diftinguifhable; the body is Iketched out; the 
bowels appear like threads; the bones are ftill quite foft, but in fome places begin to affume a greater rigidity; the vef- 
iels communicating with the placenta are plainly feen iffuing from the navel, and beginning to fpread themfelves in the 
placenta. If we may give credit to Hippocrates, that great father of phyfic, the male embryo developes fooner than the 
female: he adds, that the parts of the body of the male are diiiinguifliable at the end of thirty days; while thofe of the fe- 
male are not fo evident till the expiration of forty. 
In fix weeks, the embryo has acquired the length of two inches; the human figure daily becomes more and more 
diftinft; the motion of the heart is almolc perceptible by the naked eye; and, in an embryo of fifty days old, it has been 
obferved to palpitate for fome time after it's removal from the womb. 
In two months, the ofiification is perceivable in the arms and thighs of the embryo, and in the tip of the chin, the under 
jaw being greatly advanced before the upper. Thefe parts, however, may lull be conlidered as bony points, rather than 
as real bones. The umbilical veffcls, which before went fide by fide, now begin to be twilled over each other, like a 
rope, and go to join with the placenta, which as yet remains very fmall.' 
At the expiration of three months, the embryo is above three inches long, and weighs as many ounces. Hippocrates 
obferves, that the mother does not become fenfible of the child's motion till that period; and he adds, that in female 
children this motion is not perceptible till the end of four months. However, this is not an obfervation that can be 
uniformly relied on ; for fome women affert, that they perceive themfelves to be quick v/ith child (to ufc their own mode 
of e,vpr;(iion) at the end of two months; fo that this quicknefs feems rather to originate from the proportion between the 
child's ilrength, and the mother's fenfibility, than from any determinate fpace of time. At all times, indeed, the child 
is equally alive; and confequently, thofe juries of matrons, which in this country frequently determine on the pregnancy 
of criminals, Ihould not enquire whether the culprit be quick, but whether flie be enfient; if the latter be perceivable, the 
former follows of cr)urfe. 
Four months and an half after conception, the embryo is from fix to feven inches in length; and all the parts have re- 
ceived a proportionable encreafe, The very nails begin to appearon the fingers and toes; and the ftomach and inteftines 
affume their fundtions of receiving and digefting. A liquor is found in the ftomach limilar to that in which the embryo 
£oats; in one part of the inteflines, a milky fubftance; and, in the other, an excrementitious. There is alfo found a 
fmall quantity of bile in the gall-bladder; and fome urine in it's proper receptacle. 
By this time alfo, the pollure of the embryo feems determined and fixed. The head is bent forward, fo that the chia 
feems to reft on the breaft; the knees are raifcd up towards the head; and the legs are bent backwards, fomewhat re- 
fembling the attitude of thofe that fit on their haunches. Sometimes the knees are raifed fo high as to touch the cheeks, 
and the feet are croffed over each other; the arms are laid on the breaft; while one of the hands, and often both, touch the 
vifage; fometimes the hands are fhut, and fometimes alfo the hands depend by the fides. Thefe are the moft ufual 
poftui'cs that the embryo affumes; but there it is frequently known to change; and from thefe alterations arife thofe fre- 
qnent uneafy fenfations which pregnant women are very liable to feel. 
Thus fituated, the nafcent being is furniflied by nature with all things proper for it's fupport; and, as it encreafes in 
iize, it's nourifhment is found alio to encreafe with it. When it firft begins to grow in the womb, that receptacle, from 
being naturally very fmall, becomes proportionably larger; and, what is more extraordinary, at the fame time becomes 
thicker alfo. The fides of a bladder, we well know, the more they are diftended, the thinner they are ; but, in this cafe, the 
larger the v/omb grows, the more it thickens. Within this the embryo isftill farther involved, in two membranes, called 
the chorion and amnios; and floats in a thin tranfparent fluid, on which it feems, in fome meafure, to fubfift. However, 
the great ftore-houfe, from whence it's chief nourifliment is derived, is the placenta; a red fubftance, ibmewhat refemblin?- 
a fponge, which adheres to the infide of the womb, and communicates by means of the umbilical veffels with the embryoT 
Thefe umbilical veffels, confiftiiig of a vein and two arteries, iffue from the navel of the child, and are ramified on the 
placenta, where they ieem aftually to conftitute it's fubftance; and, if it may be fo expreffed, to fuck up their nourifliment 
from the womb, and it's contained fluids. The blood, thus received from the womb by the placenta, and communicated 
by the umbilical vein to the body of the embryo, is conveyed to the heart; where, without ever palling into the lunp-s, as 
in the infant born, it takes a fliorter ccurfe: for, entering the right auricle of the heart, inftead of palling up into the pul- 
monary artery, it feems to break this partition, and proceeds direftly through the body of the heart, by an aperture called 
the foramen ovale; and from thence to the aorta, or great artery, by which it is difperfed into all parts of the body. 
Thus, in a confiaerable meafure, we find the placenta fupplying the place of lungs; for as the little being can receive 
no air by infpiration, the lungs are therefore entirely ufelefs. But we fee the placenta converting the fluid of the womb 
into blood, and fending it by the umbilical vein to the heart; from which it is difpatched, by a quicker and Ihorter circu- 
lation, through the v/hole frame. 
After this manner the embryo repofes in the womb, fupplied with that nutrition which it's neceffties require, and fur- 
rilhed with thofe organs which are adapted to it's fituation. As it's fenfations are few, it's wants are in the fame propor- 
tion; and it is probable, that an almoft continuous fleep^ with a few intervals of waking, fills up the earlieft periods of 
exiftence. As the little creature, hotvever, gains ftrength and fize, it feems to become more watchful and reftlefs : even 
in the womb, it begins to feel the want of fomething unpoffeffed; a fenfation that feems coeval with the dawn of human 
life, and which never quits us till death. Even then the embryo begins to ftruggle for the acquifition of a ftate more 
marked bypleafure and pain; and, from about the fixth month, commences it's warnings to the mother of the greater 
pain fhe is deftined to endure. 
The duration of pregnancy in women, is ufually nine month?; but there have been manyinftances of children born at 
feven, that have been perfeft, and exhibited no marks of premature birth; while parturition has fometimes been delayed 
till nearly the expiration of the tenth month. 
When the appointed time approaches, the infant begins to redouble it's efi-orts for liberty. The head is applied down- 
wards, towards the aperture of the womb; and by rsiterated efforts it endeavours to extend the fame. Thefe exertions 
produce the pain which all women in labour in fome degree or other feel: thofe of ftrong conftitutions the leaft ; thofe 
who are weakly, with moft feverity; fince we learn that the African women always deliver themfelve^; and in a few 
hours afterwards are pcrfeftly recovered ; while the European women need affiftance, and require fome time to re-eftabliih 
their health, after the violence of the fliock parturition occafions. 
The efforts of the child at laft prevail: it's head breaks through thofe natural obftacles which are formed to yield, and 
it iffues into life. The blood which had hitherto paffed through the heart, as has been previoufly defcribed, now takers a 
wider 
