88 ]E'^VilN^O^, Genninal Layers of Vertebrates. 



which he clearly realised, and it may perhaps be admitted 

 when the exceptions are few and the rule general. But 

 when, as in germ-layer development, exceptions to the 

 rules of theory meet us at every turn, when everything 

 has to be explained away as caenogenetic — how much? 

 or rather how little ! of the palingenetic is left ! 



In the germ-layers, at least, between the conflicting 

 alternatives of origin and destiny, there is no media via. 

 To cleave to origin is to plunge into a quagmire of 

 absurdities ; to follow destiny is to abandon all hope of 

 finding any ultimate criterion in development, and to 

 return to that older conception of morphological simi- 

 larity or homology which is based simply on identity of 

 anatomical relations extending over a large series of forms 

 in the same stage of their life history. 



And in this direction serious embryological thought is 

 steadily trending. Though some still seem to halt between 

 two opinions not a few — notably Driesch, Hertwig, Braem, 

 Child, Conklin, Treadwell, Morgan — have definitely re- 

 jected the ontogenetic criterion of homology and refused 

 any morphological significance, any phylogenetic value to 

 the germinal layers. 



But though thus divested of the claims falsely set up 

 on their behalf the germ-layers remain, from another 

 point of view, the morphogenetic, structures of paramount 

 importance. The aim of the experimental embryologist 

 is to give a causal account of the sequence of develop- 

 mental phenomena, regarding development as one of the 

 functions of the organism to be studied by the ordinary 

 physiological methods ; and the problems which confront 

 him in this effort are, in the main, two. The first is to 

 describe in accurate terms the influence exerted upon the 

 embryo by its environment ; the second is to determine 

 the mutual relations which subsist between the parts of 



