IO 6 BIOLOGICAL LECTURES. 



with respect to what ? The events of ontogenesis may con- 

 veniently (though quite arbitrarily) be divided into four series 

 of stages, viz., (i) cleavage-stages, (2) gastrular stages (includ- 

 ing the formation of the germ-layers as distinguished from 

 . their further differentiation), (3) embryonic (differentiation of 

 the germ-layers, early organ-formation), and (4) larval (the free 

 immature stages). These stages, of course, overlap more or 

 less, and a special larval stage often does not exist. Which, 

 now, of these periods shall be taken as a starting-point ? 

 This question may best be viewed from an historical stand- 

 point. The early embryologists, chief among them von Baer, 

 were impressed mainly with the embryonic and larval stages ; 

 and it was the study of these stages that led von Baer to the 

 enunciation of his celebrated law. It was the same stages, again, 

 that were taken years afterwards by Agassiz, Bronn, Darwin, 

 and Fritz Miiller as the basis of the recapitulation theory ; 

 and they have always formed the basis of popular exposition, 

 which has worn threadbare the subject of gills and gill-slits, 

 visceral and vascular arches, notochord, tails, and teeth. Simi- 

 larities in development are here not only clear and striking, 

 but obviously have some palingenetic meeting, since they give 

 irresistible evidence of ancestral reminiscence. But embryol- 

 ogists did not stop here. Huxley's brilliant comparison be- 

 tween the layers of the embryo and those of the coelenterate 

 body, and the ultimate demonstration of the universality of 

 the germ-layers, pushed the basis of comparison back into the 

 gastrular stages, and the germ-layers came to be taken as the 

 real starting-point in the embryological study of homdlogy. 

 Meanwhile, however, it was found that the " mesoblast " shows 

 so many contradictions in its mode of origin, that by common con- 

 sent it was regarded as of subordinate value. Thus the primary 

 germ-layers, ectoblast and entoblast, came to be regarded 

 are perhaps still generally regarded as the ultimate standard 

 for the comparison of one form with another ; and the embryo- 

 logical criterion became in the long run a question of similar 

 relations to the primary germ-layers. " In every case homolo- 

 gies between organs must be reduced to similar relations to 

 the two layers of the coelenterate body (due allowance being 



