2 ON EMBRYOLOGY. 



history we find that the study of it sprang out of the various 

 attempts to solve the problems of why and how living beings 

 come into existence. 



It would be beyond the scope of this work to enter at 

 all fully into any account of the earlier of these inquiries 

 from those of Aristotle downwards ; but it may be of some 

 use to point out the chief steps by which in modern times 

 embryology has been established as a distinct branch of 

 knowledge. 



From the very first, incubated bird's eggs, and especially 

 hen's eggs, owing to their abundance at all seasons, and the 

 ease with which they could be examined, became special 

 objects of study. Aristotle examined the growing chick 

 within the egg, and gave the name of punctum saliens to the 

 'bloody palpitating point,' which marks the growing heart in 

 the early days of incubation. Since his time all observers 

 have had recourse to the hen's egg; and though it may be 

 urged that the highly specialised characters of the avian 

 type unfit it for so general a purpose as that of serving as 

 the foundation of embryology, the practical advantages of the 

 bird's egg over either the mammalian or any other ovum, are 

 so many, that it must always continue to be, as it has been, 

 a chief object of study. 



From the time of Aristotle down to that of Fabricius of 

 Aquapendente so little progress in real observation of facts 

 had been made, that we find the latter anatomist [De Forma- 

 tione Ovi et Pulli, 1 621) describing the chick as being formed 

 out of the chalazaa of the white of the egg; a view which 

 lived long afterwards, and whose influence may still be 

 recognized in the names 'tread' or 'treadle' which the 

 housewife sometimes gives to those portions of thickened 

 albumen. 



Harvey was the first to clearly establish that the essential 

 part of the hen's egg } that out of which the embryo pro- 



