EVOLUTION IN BIOLOGY 201 
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: of the cell-aggregate thus produced, that all the 
| organs and tissues of the adult owe their origin. 
| In certain animals belonging to every one of 
the chief groups into which the Metazoa are 
divisible, the cells of the cell-aggregate which 
results from the process of yelk-division, and 
Bovhich is termed a morula, diverge from one 
' another in such a manner as to give rise to a 
7. central space, around which they dispose them- 
: ‘selves as a coat or envelope; and thus the morula 
becomes a vesicle filled with fluid, the planula. 
The wall of the planula is next pushed i in on one 
Pe or invaginated, whereby it is converted into 
a double- Walled sac with an opening, the dblasto- 
_ pore, which leads into the cavity lined by the 
- inner wall. This cavity is the primitive alimen- 
tary cavity or archenteron; the inner or inva- 
 ginated layer is the hypoblast ; the outer the 
_ epiblast ; and the embryo, in this stage, is termed 
oa gastrula. In all the higher animals a layer of 
_ cells makes its appearance between the-hypoblast 
| the epiblast, and is termed the mesoblast. In 
the further course of development the epiblast 
i Piiecomes the ectoderm or epidermic layer of the 
| ak ; the hypoblast becomes the epithelium of 
: the middle portion of the alimentary canal; and 
_ the mesoblast gives rise to all the other diene 
except the central nervous system, which origin- 
ates from an ingrowth of the epiblast. 
With more or less modification in detail, the 
