60 HISTORY OF THE HUMAN BODY 



gastrulse this latter cavity is completely obliterated by the com- 

 pletion of the process of invagination, but often remains as a 

 space between the two layers, the ectoderm and endoderm. 



When this type is completed and becomes an adult animal 

 it often assumes a considerable complexity of structure but 

 never gets far away from the original plan and does not de- 

 velop more than the two primary layers. The fresh-water 

 hydra_js jmjexample of one of the simplest coelenterates or 

 gasjtrulaj : ajiirrial^ and the coral polyps and medusae^ represent 

 the more complex ones. In none^oFThese does a ISastocoele 

 appear, in the simpler forms ectoderm and endoderm are 

 everywhere in contact, and in the more complex medusae the 

 space between them is filled by a gelatinous tissue developed 

 from the other layers, and termed mesenchyme.* 



Up to this point the course of development is the same for 

 all Metazoa, allowing for the adaptive modifications always 

 met- with in the application of a general plan to a group of 

 organisms. 



From this point on, however, there is a divergence in 

 the course of development, and the various branches of the 

 higher Metazoa proceed along different paths, yet all de- 

 velop, although through different means, the three following 

 attributes, which differentiate them from the lower Metazoa, 

 the Ccelenterata : 



1. The formation of a third germ element, the mesoderm, 

 situated between ectoderm and endoderm. 



2. The formation of a new cavity or system of cavities, the 

 metaccele, lined wholly by the mesoderm. 



3. The attainment of a new body axis, and a bilateral, in- 

 stead, of a radiate, symmetry. 



Omitting all further reference to the other branches, it ap- 

 pears that in the branch leading to the vertebrates the gas- 



* This is carefully to be distinguished from the mesoderm, or middle 

 layer, which appears first in animals above the coelenterates and is always 

 in the form of a definite layer. The mesenchyme never appears as a 

 layer, but its cells serve to fill in the spaces between the true germ layers, 

 and the structures formed from this source are thus determined by the 

 form of the surrounding tissues. 



