I04 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 gastrcBa 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 



