584 THE ANATOMY OF INVERTEBRATED ANIMALS. 



of the ovum, or may appear later. In this case, it usually 

 happens that the smaller and more rapidly-dividing blasto- 

 meres belong to the epiblast, and the larger and more slowly 

 dividing to the hypoblast. Moreover, no blastocoele may 

 arise, and the process of inclusion of the hypoblast within the 

 epiblast may have the appearance of the growth of the latter 

 over the former, or what is termed eplholy ; while the archen- 

 teron may not be formed within the hypoblast till very late. 



When, in cases of epiboly, the blastoderm is small in rela- 

 tion to the vitellus, the epiblast and hypoblast, at their first 

 appearance, necessarily adapt themselves to the surface of the 

 yelk; and thus the gastrula, instead of having the form of a 

 deep cup, becomes more or less flattened and discoidal. 



I am inclined to believe that all the various processes b}'' 

 which the gastrula or its equivalent are produced, are reduci- 

 ble to epiboly and emboly. Even when the epiblast and the 

 hypoblast appear to be formed by delcmiinatioTi, or the split- 

 ting into two layers of cells of a primitively single-layered 

 blastoderm, there seems little doubt that what happens is 

 either the very early inclusion of the hypoblastic blastomeres 

 within those which give rise to the epiblast, or a very late 

 and inconspicuous ingrowth, or invagination, of the hypoblas- 

 tic region of the blastoderm. 



If we employ the term gastrula in the broad sense defined 

 above, it may be truly said that every raetazoon passes through 

 the gastrula stage in the course of its development. The 

 question whether the mode of development of the gastrula 

 by emboly is primitive, and that by epiboly secondary ; or 

 whether epiboly is primary and emboly secondary; or whether 

 the two processes have originated independently, is of sec- 

 ondary importance, and belongs to the debatable ground of 

 phylogeny.^ 



The meaning of the differentiation of the aggregate of 

 cytodes, of which the body of a simple metazoon is composed, 

 into a hypoblastic, or endodermal, and an epiblastic, or ecto- 

 dermal, group, is to be sought in the physiological division of 

 labor, w^hich is the primary source of morphological changes. 

 It is a separation of the aggregate of morphological units into 

 one set with a specially nutritive, and another set with a spe- 

 cially motor and protective, function. It is quite possible to 

 conceive of an adult metazoon having the structure of a sponge- 



^ Compare Haeckel, " Studien zur Gastraa-Theorie," in his " Biologisehe 

 Studien," 1877. 



