io 4 PERIOD IV. 



extraordinary influence, gave a new impetus to the 

 study of development. Pander (1817-8) published an 

 account of the early stages of the chick, illustrated by 

 beautiful plates by D'Alton. Baer (1828-37) carried 

 the work much further, not only greatly extending the 

 knowledge of the developing chick, but discovering the 

 mammalian ovum (1827), and announcing generalisa- 

 tions which down to 1859 were the most luminous that 

 embryology had ever furnished ; we may call him the 

 founder of comparative embryology. He shows that 

 development may supply decisive indications of the 

 zoological position of animals ; it teaches, for instance, 

 that insects are of higher grade than arachnids or 

 crustaceans, and that amphibians ought not to be 

 united with reptiles. He describes the development of 

 an animal as a process of differentiation, the general 

 becoming special, and the homogeneous heterogeneous ; 

 differentiation is, he remarks, the law under which not 

 only animals but solar systems develop. He maintains 

 that the embryo, though gradually attaining complexity, 

 makes no transition to a different type e.g., the verte- 

 brate is never in any stage anything but a vertebrate. 

 All animals, he believes, are probably at first similar, 

 and take the form of a hollow sphere (the gastrcea of 

 modern embryology). There are, he says, no new 

 formations in nature ; all is conversion. When he 

 comes to speak of the pharyngeal clefts of mammals 

 and birds, recently discovered by Rathke, he remarks 

 that their correspondence with the gill-clefts of fishes 

 is obvious. We wonder what is coming next, but our 

 curiosity is not gratified by any memorable deduction. 

 Neither here nor in his miscellanies (Reden), published 

 nearly fifty years later, does he admit that mammals 

 and birds can have descended from gill-breathing 



