Geology and Natural History. 473 
theory of embryonic layers so strongly insisted upon by Heckel, 
they could have no possible relation, the one being a product of 
the entoderm, the other of the ectoderm, the two primitive embry- 
onic layers. . 
“Tt is not known, as is stated by Heckel, that the walls of the 
primitive digestive cavity are invariably formed of the entoderm, 
and when Heckel states the result (the gastrula) to be the same 
whether formed by the ectoderm or entoderm he states what is 
* Heckel and Lankester both seem to think that because the result is a sumilar 
form it must be homologous. 
