XL] EVOLUTION IN BIOLOGY. 287 



into a double-walled sac with an opening, the blasto- 

 pore, which leads into the cavity lined by the inner 

 wall. This cavity is the primitive alimentary cavity 

 or archenteron; 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 appear- 

 ance 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 

 originates from an ingrowth of the epiblast. 



With more or less modification in detail, the 

 embryo has been observed to pass through these 

 successive evolutional stages in sundry Sponges, 

 Coelenterates, Worms, Echinoderms, Tunicates, Arth- 

 ropods, Mollusks, and Vertebrates ; 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 evolu- 

 tion. Each, starting from the condition of a simple 

 nucleated cell, becomes a cell-aggregate; and this passes 

 through a condition which represents the gastrula 

 stage, before taking on the features distinctive of the 

 group to which it belongs. Stated in this form, the 

 " gasteea theory " of Haeckel appears to the present 

 writer to be one of the most important and best 

 founded of recent generalisations. So far as indi- 



