ON GENERATION. 241 



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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