542 G. 0. BOURNE. 



cirrhi, parapodia, and seta-sacs. All the tissues usually classed 

 as mesoblastic are derived from the ectoderm ; the endoderm 

 appears to form nothing more than the lining of the gut. 



The generalisations which Kleinenberg would found upon 

 these facts may be briefly stated as follows : 



New organs and tissues are always developed in connection 

 with and through the intervention of pre-existing organs and 

 tissues. That this must be true in a phylogenetic sense is 

 obvious on reflection, for a new form with newly specialised 

 organs must always be developed from a form in which the 

 same vital functions were performed, less efficiently it may 

 be, by other or less perfect organs. Hence, if ontogeny is a 

 recapitulation of phylogeny it must also be true in an ontoge- 

 netic sense, but the fact has been overlooked because authors 

 have been too ready to refer developing organs to the undiffer- 

 entiated cells of the two primary layers, ectoderm and endo- 

 derm, or to a mesoblast, falsely assumed to be an undiffer- 

 entiated third germ layer, homodynamous with the two first 

 named. 



In the Ccelenterata the two primary germ layers are present, 

 and form all the tissues of the adult : each of them may give 

 rise to differentiated, tissues or organs, to which special func- 

 tions are appropriate. Among the first tissues to be formed 

 are the nervous and muscular, and since irritability and con- 

 tractility are complementary to one another, these two tissues 

 are invariably developed in such close connection that they 

 may be said to be mutually determinant. But they are not 

 structures developed de novo; they are the result of the 

 specialisation of function of already existing cells, whether of 

 ectoderm or of endoderm, and very frequently arise from the 

 direct transformation of those cells. In the higher Metazoa 

 such tissues derived from the primary ectoderm or endoderm 

 because of the increased functional activity due to their spe- 

 cialisation, give rise to modifications in adjoining cells ; these 

 group themselves round the tissues first formed, and, becoming 

 themselves specialised, lead to the formation of other tissues 

 and organs. In some instances the cells newly formed are 



