IMPORTANCE OF THE CHICK. 33 



independent and isolated body, but in a small egg, which, 

 until maturity, remains enclosed and concealed in the body 

 of the mother. For this reason it is very difficult to pro- 

 cure all the stages of development in any large number, 

 for the purpose of making connected investigations, not 

 to mention external reasons, such as the great cost, the 

 technical difficulties, and the many other obstacles, which 

 lie in the way of any extended series of researches into 

 fecundated mammals. For this reason, from that time to 

 the present day, the Chick during the process of incubation 

 has been the subject oftenest and most closely investigated. 

 The perfection of hatching-machines has made it yet easier 

 to obtain embryo-chicks in any required stage of evolution 

 and in any quantity, in order to examine the whole process 

 of formation step by step. 



About the end of the seventeenth century the history of 

 the evolution of the incubated Chick had already been 

 advanced as far, and its more essential, external, and less 

 delicate conditions were as well knowTi, owinir to the 

 labours of Malpighi, as investigations with the imperfect 

 microscopes of that time rendered possible. Of course, the 

 perfection of the microscope and of technical methods of 

 research was a necessary condition for more accurate em- 

 bryological research. For vertebrate embryos in their 

 earlier stages are so small and delicate, that it is impossible 

 to examine them without a good microscope, and without 

 applying peculiar technical methods. But these means 

 were not applied, and the microscope was not essentially 

 'perfected till the beginning of our century. 



Throughout the whole of the first half of the eighteenth 

 century, during which time the systematic Natural History 



