132 KARL KHNST VON BAKU 



The vegetative parts, however, develop on the whole unsym- 

 metrically (cf. Bichat). These elements which von Baer dis- 

 tinguishes are morphological units, as he himself points out, 

 contrasting them with organs which are not usually units 

 in a morphological sense. " We call organ," he writes, "each 

 part that has by reason of its form or its function a certain 

 distinctiveness, but this concept is very indefinite, and 

 possesses, from a morphological point of view, little value. 

 For this reason it seems necessary to introduce into scientific 

 morphology the concepts of morphological elements and 

 divisions" (ii., p. 84). 



Von Baer exercised a very considerable influence upon 

 the subsequent trend of morphological theory. By his 

 criticism of the Meckel-Serres theory, he rid morphology for 

 a time of an idea which was leading it astray ; by his 

 substitution of the law that development is always from the 

 general to the special, he set morphologists looking for the 

 archetype in the embryo, not in the adult alone, and made 

 them realise that homologies could often best be sought in 

 the earliest stages of development ; by formulating the germ- 

 layer theory he supplied morphologists with a new criterion 

 of homology, based upon the special relations of the parts 

 (germ-layers) which are first differentiated in all develop- 

 ment. He made the study of development an essential 

 part of morphology. 



