C 1438 ) 
no continuity J ot commnnication of Veffels of any kind, du- 
ring the whole time of Incubation. 
But thefe Objeftions carry neither the Weight nor DiflS- 
culty along v/ith 'em, that they may^be fuppos'd to do^ for 
in thofe days there is neither Blood nor Blood VeJfeU^ and 
confequently there can be no Circulation of the Blood 5 and 
the Embryo^ of what Species foever, is no more than a Ve- 
getable at that time ^ nor does the FtKtHi of any Viviparous 
Cresitmc enpY 2iny Circulation^ or (hew any figns of Animal 
Life, till after thofe Veffels, as well as others requifire to the 
Circulation, are compleated. 
It muft be confefs'd,^ that Oviparous Animals are denied 
the Benefit of this Communication : But that want is fuffi- 
ciently compenfated by a peculiar Mechanifm, which di* 
reftly anfwers the ends of Rejfiration^ arid the Prejfure of 
the Atmofphere u^on the Foetus* There is at the ObtufeQnd 
of an Egg a fmall Cavity fiird with Air, which is the Suc- 
cedaneous Inftrument to the Refpiratorj Organs. For as 
foon as the Contents begin to be warm'd by the Incubation 
of the Hen, or any analogous Heat of Furnace or Dunghil^ 
the feveralHumour^ of theEgg require a FermenUitive Motion, 
and th^Air contain*d in the Cavity or Veficle at the obtufe end of 
the Egg is rarefied^and, the Veficle extendedand enlarged, and 
confequently the other contents arecompreft 5 to which the 
Fermentative Motion naturally refifts. But both Bodies being 
as well comprejjible 2iS dilatable^ and both having an Expan- 
fve Motion impreft upon 'em by Incubation^ the compreflion 
and renitency will be mutual 5 but varied in degree, accord- 
ing as either, thro the variation of Circumftances, fhall pre- 
~ vail. By this means, an Alternation of ComprelSon and Dila- 
tation will be produc'd in both anfwering the Refpiratory Mo- 
tion,by which aMotion will be communicated,which,as foon 
as the Organs by which it fhoud be regulated are compleat- 
ed, will in the Body of the Rullus be regular and circulatory. 
Fabritius . ab Aquapendente^ and after him our Great 
Dr Harvey, have aflign'd divers ufes to this Cavity or Air 
Veficle, the Extravagance of which have perhaps deterr'd 
others 
