Vi EVOLUTION IN BIOLOGY 201 



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 

 which is termed a monda, diverge from one 

 another in such a manner as to give rise to a 

 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 in on one 

 side, or invaginated, whereby it is converted into 

 a double-walled sac with an opening, the Uasto- 

 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 

 a gastrula. In all the higher animals a layer of 

 cells makes its appearance between the hypoblast 

 and the epiblast, and is termed the mesoblast. In 

 the further course of development the epiblast 

 becomes the ectoderm or epidermic layer of the 

 body ; the hypoblast becomes the epithelium of 

 the middle portion of the alimentary canal ; and 

 the mesoblast gives rise to all the other tissues, 

 except the central nervous system, which origin- 

 ates from an ingrowth of the epiblast. 



With more or less modification in detail, the 



