GASTRULATION, FORMATION OF GERM LAYERS 67 



between ectoderm and entoderm or from numerous mesenchyme (;ells 

 which are separated from the walls of the blastosphere or of the gastrula, 

 as in the Echinoderms. In some forms, as in Sagitta and Balanoglossus, 

 coelomic pouches — sac-like outgrowths from the entodermic lining of 

 the gastrula-cavity — develop into mesoderm (Figs. 39, 40). The meso- 

 derm comes to lie between ectoderm and entoderm, one layer of the 

 mesoderm which attaches itself to the ectoderm forming the somatic or 

 parietal layer; the other, clinging to the mid-gut, forming the visceral, 

 or splanchnic, layer. 



With the Arthropoda, in which, for the most part, the blastula is 

 formed by the outward migration of cleavage cells from the center, the 

 process of gastrulation is obscured by the manner of blastoderm forma- 

 tion and the presence of a large amount of yolk. 



The difficulties in interpreting the homologies of the germ layers, 

 especially in insects, are so great that writers from time to time have 

 questioned the validity of the germ-layer theory. According to this 

 theory, which is based on a large number of observations on the develop- 

 ment of various animals, the material from which the mid-gut epithelium 

 is formed should correspond to entoderm, and the efforts of many investi- 

 gators have been bent toward establishing this homology and deriving 

 the conditions found in the insect egg from the typical gastrula. This 

 has proved so difficult a task that Nelson (1915) quoting Weismann says: 

 "It becomes more and more evident that nowhere in the entire animal 

 kingdom is the ontogeny so distorted and coenogenetically degenerate, as 

 in the insects, so that scarcely anywhere are the germ layers so difficult 

 to recognize as here." 



The germ-layer theory dates back for much more than a century. 

 Von Baer (1828), who is generally regarded as the founder of the theory, 

 himself assigns the credit for the discovery of germ layers to Pander, 

 although it should be pointed out that he acknowledged the fact that 

 Wolff (1759) had already proposed the theory of epigenesis, a forerunner 

 of the germ-layer theory. Haeckel's biogenetic law, which states that 

 ontogeny, or the development of the individual, is a short recapitulation 

 of phylogeny, or the development of the race, led embryologists to search 

 in the development of individuals for stages that could be regarded as 

 ancestral. It is generally stated that the blastula and gastrula stages 

 at the end of cleavage represent forms that are ancestral to all Metazoa 

 and that one is justified in the assumption that in these two stages there 

 exists a repetition of ancestral forms which are common to all Metazoa 

 (Korschelt and Heider, 1895). Earher writers saw in the gastrula a form 

 common to all metazoan embryos, but Brachet (1921) points out that 

 there may be no gastrula in the Haeckelian sense in the development of 

 certain forms. 



