W PRELIMINARY ESSAY ON GENERATION. 
culate through the lungs. In fourteen hours more, the chicken has acquired additional firength ; the head, howevcf* 
is ftill bent downwards ; the veins and arteries begin to branch, in order to form the brain ; and the fpinal marrow is feen 
Itretching along the back-bone. 
In three days the whole body of the nafcent animal appears bent; the head, with it's two eye-balls, and their different 
humours, now diftinftly difclofe themfelves to view ; and five other veficlcs are feen, which foon unite to form the rudi- 
ments of the brain. The outlines alfo of the thighs and wings begin to be diftinguilhed ; and the body feems to gather 
flefh. 
At the end of the fourth day, the veficles that compofe the brain approach each other ; the wings and thighs appear 
more folid ; the whole body is covered with a gelatinous fubilance refembling flelh ; the heart, that was hitherto ex- 
pofed, is now concealed within the body by a very thin tranfparent membrane; and at the fame time the umbilical 
vefiels, which unite the animal to the yolk, now appear to proceed from the abdomen. 
After the fifth and fixth days, tiie veffels of the brain begin to be covered over ; the wings and thighs lengthen and 
expand ; the belly is clofed up and becomes tumid ; the liver is perceived within it very diflinftly, llili of a very duflcy 
white colour; both the ventricles of the heart are difcerned, as if they conftituted two feparatc hearts ; the whole body 
of the chicken is covered over; and the traces of the incipient feathers are already perceptible. 
The feventh day, the head appears enlarged ; the brain is wholly cloathed over; the bill begins to appear; and the 
wings, the thighs, and the legs, have acquired th ir perfeft figure. 
Hitherto, however, the animal appears as if it had two bodies ; the yolk is united to it by the umbilical veffels pro- 
ceeding from the belly, and is funiiflied with it's veffels, through which the blood circulates as through the body of the 
chicken, forming a greater bulk than that of the animal itfelf. But towards the end of incubation, the umbiiical vt-fiels 
fhorten the yolk; and with it the inteftines thruft up into the body of the animal, by the aftion of the mufcles of the 
belly; and the two bodies, as they appear, are thus formed into one. 
During this ftate, all the organs are obi'erved to perform their fecretions ; the bile is found to be feparated, as in grown 
animals; but it is fluid, tranl'parent, and tailelefs : the chicken then appears alfo to have lungs. 
On the tenth day, the mufcles of the wings appear, and the feathers begin to be protruded. On the eleventh, the heart, 
which had hitherto feemcd divided, begins to unite ; and the correfponding arteries unite, like the infertion of the finiferj 
into the palm of the hand. 
By the eleventh day, the animal being thus compleatly formed, begins to gather ftrcngth, becomes more une;ify in it's 
fituation, and exerts it's vital powers with encreafing force. For fome time before it is able to break thn ugh it's 
prifon, it is heard to chirrup, receiving a fufficient quantity of air for this purpofe from the cavity lying between ths 
membrane and the fhell, and which muft neceffaiily contain air to refiftthe external prefiure. 
At length, on the twentieth day in fome birds, and later in others, the inclofed animal breaks the faell within which it 
had been confined, with it's beak ; and, by repeated efforts, at lail procures it's liberation. 
From this minute hiftory ofincipient life, in a humble racein thefcaleof creation, we may readilv perceive, thatthofe 
parts which are moft conducive to life are firft begun : the head and the back-bon", which encloie the brain and the fpii al 
marrow, though both are too limpid to be difcerned, are the firfl: that appear to exifl ; tlie beating of the he. rt is per- 
ceived foon after; the 1 fs noble parts feem to fpring from thele ; the wings, the thighs, the feet, and, laftiy, ihe bill. 
Whatever, therefore, the animal poffeffes double, or whatever it can 1 ve wiih' uc the immediate ufe of, are latell in p.o- 
du(5lion : Nature thus appears fedulous in applying to the formation of the nobler organs, without which life woulu be but 
of fliort continuance, and would be begun in vain. 
From analogy we may infer, that the refemblance between the incipient animal in the egg, and the embryo in the 
womb, is very itriking ; and this fimilitude has induced many to afl'ert, that all animals are produced in i;ke mannfr Ir. m 
eggs. Thofe who maintain the ovarious fyftem, confiJer an egg excluded from the body by fome, and iep . rated into tho 
womb by others, as aftions of the fame kind ; with this only difference, that the nourifhment of the one is retained within 
the body of the parent, and encieafes as the embryo happens tovvant he fupply; the nourifhment of the other is prepared 
all at once, and furnifhed by the producing animal as a flock entirely fufficient for the future fupport of it's ofFspiing. 
Bu% leaving this to the dilcufTion ol anatomifts, let us proceed rather with f afts than opinions ; and, as we have feeu the 
progrefs of an oviparous animal, lei us likewife trace that of a viviparous one. 
In the inveitigation of this fubjeft, De Graaf has, with almoft unexampled patience and fedulity, attended the progrefs 
and increafe of various animals in the womb, and minutely marked the changes they undergo. iJaving dill'rfted a 
rabbit half an hour after impregnation, he perceived the horns of the womb, that embrace and communicate with the 
ovary, to be redder than before; but no other change was vihblein the other parts. Having diffefted another, nx hours 
after, he perceived the follicules, or the membrane covering the eggs contained in the ovary, to becc^mc reddilh. fn a 
rabbit difieCled after twenty-four hours, he perceived, in one of the ovaries, three follicules; and, in another, five, thcit 
had undergone fome change, exhibiting a dark and reddilh appearance, whereas they ufually are tranfparent. In o.ne 
difTefted after three days, the horns of the womb were obferved to embrace the ovaries very clofely ; and he obferved 
three follicules in them, much larger and harder than before. 
Purfuing his inquifition, he alfo found two fubilances refembling eggs adlually feparated into the horns of the womb, 
each about the fize of a grain of muftard-feed : thefe were fepaiately inclcfed in a double membrane, the inner parts 
being replenifhed with a very limpid liquor. 
After four days, he found in one of the ovaries four, and in the other five follicules, emptied of what he calls their 
eggs; and, in the horns correfponding with thefe, he obferved an equal number of eggs thus feparated, which were 
grown IHII larger. In five days time, they were ftill more encreafed in lize : in feven, they were found as big as a piliol- 
bullet, each covered with it's double membrane, and thefe much more dilHnft than before. 
At the expiration of nine days, having examined the liquor contained in one of thefe eggs, he found it was become 
Jefs fluid, and limpid. In ten days, a cloud feemed to thicken on it's furface, and to form an obiong body, of the figiire 
of a little worm; and in twelve days, the figure of the embryo was diftindlly perceptible, and even it's parts were di- 
Hinguifhablc. In the region of the breaft, he perceived two bloody, and two whitifh fpecks. 
Fourteen days after impregnation, the head of the embryo was become large and tranfparent, the eyes prominent, 
the mouth open, and the rudiments of the ears began to appear: the back-bone, which was of a whitiih tulour, inclined 
towards the breail; the two bloody fpecks, obferved before, now exhibited the outlines of the two ventriclei of the heart; 
and the two whitifh fpecks on each fide, appeared to be the rudiments of the lungs. Towards the regicn. of the belly, 
the liver began to difclofe itfelf, and to aii'ume a reddilh colour; and a little intricate mafs, like r.ivclled thread, foon 
appeared to be the origin of the ftomach and inteftines. The legs fpeedily became vifible, and affunied their natural 
polition; and from that time forth, all the parts being formed, every day ferved to develope and expand them ftill more, 
till the thirty.firft day arrived, on which the rabbit brought forth her youngin due courfe of nat'.re. 
Thus having furveyed the difterent modes and ftages of generation in the more ignoble animals, let us ta'ce a view of it's 
progrefs in roan, and inveftigate the feeble beginnings of our own exiftence. 
An account of the iowiinefs of our own origin, if it fliould not prove entertaining, will, at leaft, ferve to humble us; 
and it may diminilh our pride, though it fails to gratify our curiohty. In this cafe we cauno: fo accurately tr<!ce the 
commencement of the incipient animal, as in the former inftances; for the opportunities of infpedion are but few and ac- 
cidental. We muft therefore content ourfelves to fill up the blanks of our hiftory with ration.il conjeiflure, Vv'bere ocu- 
lar demonftration has never been procured. 
Immediately after conception, we have no regular idea of the ftate of the embryo in the womb. However, anatomifts 
inform us, that four days after conception, an oval fubilance is perceived in the womb, about the magnitude of a fmall 
pea, but longer one way than the other. This little body is formed by an extremely fine membrane, inclofing a liquor 
bearing a ftrong refemblance to the white of ac egg: and in this, feveral fmall fibres, united together, have been diftin- 
guilhed, which unqueftionably form the firft rudiments of the embryo. Befides, there are feen another fet of fibres, which 
foon after form theaifeives iato the placenta, or that body by which the incipient being is fupplied with nourifhment. 
At 
