THE WONDERS OF LIFE 



morphological division. There are, however, exceptions 

 in both groups of the metaphyta. The large and highly 

 developed fucoidea among the algfe exhibit siihilar differ- 

 entiations of organs to those we distinguish as stalk and 

 leaves in the higher cormophyta. On the other hand, 

 they are wanting in the lower liverworts, which form a 

 thallus like many of the algse; thus, for instance, the 

 liverwort riccia fluitans is just like the brown alga 

 dictyota dichotoma. Other primitive liverworts (such 

 as the anthoceros) have also a very simple thallus; but 

 most of them have a separation of the thallus into an 

 axial organ (stalk) arid several lateral organs (leaves). 

 In the distribution of labor among the leaves there 

 then emerge the differences between the lower leaves, 

 foliage leaves, higher leaves, and flower leaves. A sim- 

 ple poppy-plant (pdpaver) or a single-flowered gentiana 

 ciliata, which has only one bloom at the top of its 

 branchless stalk, is a good example of a highly developed 

 culmus. 



To the plant-sprout corresponds in the animal world 

 the person. All the tissue-animals pass in the course of 

 their embryonic development through the important 

 stage of the gastrula, or "cup-shaped embryo." The 

 whole body of the tissue-animal at this stage forms at 

 first a simple gut-sac or gastric sac (the primitive gut), 

 the cavity of which opens outward by a primitive 

 iriouth. The tliin wall of the sac is formed by two super- 

 imposed layers of cells, the two primary germinal layers. 

 This gastrula is the simplest form of the "person," and 

 the two germinal layers are its sole organs. 



The diverse animal forms which develop along differ- 

 ent lines from this common embryonic form of the gas- 

 trula may be grouped into two sub-kingdoms, the lower 

 (ccelenteria) and the upper (caslomaria) animals. The 

 former correspond in the simplicity of their structure in 

 maiiy respects to the thallophyta, and the latter to the 



i66 



