ELEMENTARY MORPHOGENESIS 47 



always understood here with regard to visible morphogenesis 

 proper and does not apply to what may be called elementary 

 in the physiological sense. An elementary process in our 

 sense is a very distinct act of form-building, and an elementary 

 organ is the result of every one of such acts. 



The elementary organs are typical with regard to their 

 position and with regard to their histological properties. 

 In many cases they are of a very clearly different histo- 

 logical type, as for instance, the cells of the three so-called 

 germ -layers ; and in other cases, though apparently almost 

 identical histologically, they can be proved to be different 

 by their different power of resisting injuries or by other 

 means. But there are not as many different types of 

 histological structure as there are typically placed organs : 

 on the contrary there are many elementary organs of the 

 same type in different typical parts of the organism, as all 

 of you know to be the case with nerves and muscles. It 

 will not be without importance for our future theory of 

 development, carefully to notice this fact, that specialisa- 

 tion in the position of embryonic parts is more strict than 

 in their histology. 



But elementary organs are not only typical in position 

 and histology, they are typical also with regard to their 

 form and their relative size. It agrees with what has been 

 said about histology being independent of typical position, 

 that there may be a number of organs in an embryonic 

 stage, all in their most typical positions, which though all 

 possessing the same histology, may have different forms or 

 different sizes or both : the single bones of the skeleton of 

 vertebrates or of adult echinoderms are the very best instances 

 of this most important feature of organogenesis. If we look 



