598 ARTHUR WILLEY. 



serosa took place by substitution, and my meaning can best be 

 illustrated by an observation of my own with regard to the 

 endoderm in the embryos of P. novse-britannise. The tro- 

 phic cavity of the embryo becomes the gastral cavity of the 

 adult, but the exceedingly thin layer of endoderm lining the 

 trophic cavity does not become the definitive endoderm by 

 simple continuous growth. On the contrary, the embryonic 

 endoderm undergoes histolytic changes, the cells losing their 

 mutual contiguity, rounding up and migrating as free tro- 

 phocytes into the trophic cavity. After this remarkable 

 histolysis, the endoderm reconstitutes itself, secretes a basal 

 membrane, and produces the high columnar epithelium of the 

 gut. 



Thus, while the gastral endoderm of the adult is derived 

 from the trophic endoderm of the embryo, it is not exactly the 

 same as the latter since histolytic changes intervene. 



The three principal phases in the development of the endo- 

 derm in the embryos of P. novse-britannise may be sum- 

 marised as follows : 



I. II. III. 



Trophic endoderm. Trophocjtes. Gastral endoderm. 



I regard this method of transformation of trophic endo- 

 derm into gastral endoderm as a special case of substitution, 

 and well adapted to elucidate the significance of the assump- 

 tion that the transformation of the trophoblast into the serosa 

 has taken place by substitution. 



Many other still clearer cases of epithelial substitution could 

 be adduced. In insects, after the eversion of the embryo and 

 consequent reflection of the amnion over the yolk, the amnion 

 forms the provisional dorsal wall of the embryo. But it 

 becomes replaced later by true ectoderm which grows out from 

 the pleural edges of the embryo, while the amnion itself under- 

 goes disintegration and absorption (Wheeler, 11, 12; Hey- 

 mons, 2). The origin of the definitive hypodermis of holome- 

 tabolous insects from the imaginal discs is another case in 

 point. 



