The more perfect or forward eggs then, about the end of the fourth day, contain a double or bipartite pulsating vesicle, each portion reciprocating the other's motion, in such order and manner that whilst one is contracting, the other is distended with blood and ruddy in colour ; but this last contracting anon forces out its charge of blood, and, an instant being interposed, the former rises again and repeats its pulse. And it is easy to perceive that the action of these vesicles is contraction, by which the blood is moved and propelled into the vessels.
" On the fourth day," says Aldrovandus, 1 " two puncta are perceived, both of which are in motion ; these, undoubtedly, are the heart and the liver, viscera which Aristotle allowed to eggs incubated for three days."
The Philosopher, 2 however, nowhere says anything of the kind; neither, for the most part, are the viscera mentioned con- spicuous before the tenth day. And I am indeed surprised that Aldrovandus should have taken one of these pulsating points for the liver, as if this viscus were ever moved in any such manner ! It seems much better to believe that with the growth of the embryo one of the pulsating points is changed into the auricles, the other into the ventricles of the heart. For in the adult, the ventricles are filled in the same manner by the au- ricles, and by their contraction they are straightway emptied again, as we have shown in our treatise on the Motion of the Heart and Blood.
In more forward eggs, towards the end of the fourth day, I have occasionally found I know not what cause of obscurity intervening and preventing me from seeing these pulsating vesicles with the same distinctness as before; it was as if there had been a haze interposed between them and the eye. In a clearer light, nevertheless, and with the use of magni- fying glasses, the observations of one day being further col- lated with those of the next succeeding day, it was discovered that the indistinctness was caused by the rudiments of the body, a nebula concocted from part of the colliquament, or an effluvium concreting around the commencements of the veins.
Aldrovandus appears to have observed this : " On the fifth day," says he, " the punctum, which we have stated to be the
Op cit. p. 217. 2 De General. Animal, lib. iii, cap. 4.
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