SECT, XXXIX. i. GENERATION. 54? 



infeft, and by the cynips in the bedeguar of the 

 role; and by the young giafshopper on many plants, 

 by which the animal luirounds itfelf with froth. 

 But in no circumllance is extra-uterine geilation fo 

 cxa&ly refembled as by the eggs of a fly, which are 

 depofited in the frontal finus of flieep and calves. 

 Thefe eggs float in fome ounces of fluid collected 

 in a thin pellicle or hydatide. This bag of fluid 

 comprefles the optic nerve on one licje, by which 

 the vifion being lefs diiHncl in that eye, the animal 

 lurns in perpetual circles towards the fide affecled, 

 in order to get a more accurate view of objecls ; 

 for the fame reafon as in fquinting the affected eye 

 is turned away from the object contemplated. Sheep 

 in the warm months keep their nofes clofe to the 

 ground to prevent this fly from fo readily getting 

 into their noftrils. 



The liquor amnii is fecrcted into the womb as it 

 is required, not only in refpcdl to quantity, but, as 

 the digcftive ppwers of the fetus become formed, this 

 jfluid becomes of a different confidence and quality, 

 till it is exchanged for milk after nativity. Haller- 

 Phyfiol. V. i. In the egg the white part, which 

 js analogous to the liquor amnii of quadrupeds, 

 cpniifls of two jdiftincl: parts ; one of which is more 

 vifcid, and probably more difficult of digeftion, ^nd 

 more nutritive than the other ; and this latter is uied 

 in the lafl week of incubation. r \ he yolk of an egg 

 is (till a flronger or more nutritive fluid, which is 

 drawn up into the bowels of the chick juft at its 

 exclufion from the fhell, and ferves it for nourilh- 

 rnent for a day or two, till it is able to digeft and 

 has learnt to choofe the harder feeds or grains, 

 which are to afford it fuftenance. Nothing analogous 

 to this yolk is found in the fetus of lactiferous 

 animals, as the milk is another nutritive fluid ready- 

 prepared for the young progeny. 



The yolk therefore is not neceffary to the fpawn 

 of fifh, the eggs of infedls, or for the feeds of vege- 

 tables ; 



