294: EVOLUTION IN BIOLOGY. 



central space, around which they dispose themselves as 

 a coat or envelope ; and thus the morula becomes a ves- 

 icle filled with fluid, the planula. The wall of the pla- 

 nula is next pushed in on one side, or invaginated, 

 whereby it is converted into a double-walled sac with an 

 opening, the blastopore, which leads into the cavity lined 

 by the inner wall. This cavity is the primitive aliment- 

 ary cavity or archenieron / the inner, or invaginated, 

 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 be- 

 tween 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 mid- 

 dle portion of the alimentary canal; and the mesoblast 

 gives rise to all the other tissues, except the central ner- 

 vous system, which originates from an ingrowth of the 

 epiblast, 



With more or less modification in detail, the embryo 

 has been observed to pass through these successive evo- 

 lutional stages in sundry Sponges, Coelenterates, Worms, 

 Echinoderms, Tunicates, Arthropods, Mollusks, and Ver- 

 tebrates ; and there are valid reasons for the belief, that 

 all animals of higher organisation than the Protozoa 

 agree in the general character of the early stages of their 

 individual evolution. Each, starting from the condition 

 of a simple nucleated cell, becomes a cell-aggregate ; and 

 this passes through a condition which represents the 



