EMBRYOLOGICAL INVESTIGATIONS 251 



similarity between embryos of forms zoologically 

 akin, have not hesitated to fill the gaps in their 

 knowledge of the developmental history of certain 

 animals, by reference to the known processes in 

 allied forms. More especially is this the case with 

 regard to man himself : material for direct observa- 

 tion is difficult to obtain, and it is only too common 

 to find that descriptions purporting to be of human 

 embryos are really founded on observations derived 

 from the study of pigs and rabbits, or even of 

 animals so remote zoologically as chicken, lizards, 

 or dog-fish. Of late years however there has 

 been a marked reaction in this respect, and the 

 pendulum has shown a tendency to swing over to 

 the opposite side. More exact observations on 

 many groups of animals have proved that even 

 in allied forms the course of development may be 

 markedly different ; in some cases indeed not only 

 genera and species, but even the eggs of the same 

 brood, may develop in ways curiously unlike one 

 another. 



This inconstancy in the mode of development, 

 more especially in the earliest stages, is one of the 

 most striking results of recent embryological inves- 

 tigation, and may well claim attention on the 

 present occasion. If at first sight it appear be- 

 wildering or even unwelcome to those who, from 

 the study of development of existing animals, 

 would seek to unravel the past history of the race, 

 it is to be remembered that facts must always be 

 accepted ; and it may even be that in this varia- 

 bility, by which his labours are so greatly extended, 



