74 SCIENCE PROGRESS. 



one time under a cloud of disrespect, and but for the loyalty 

 of the minority it might have languished. To-day the cloud 

 has passed and the reaction is subsiding — only to show that 

 much that is developmental is, after all, adaptive, and that 

 ontogeny is certainly not so strictly recapitulatory as was 

 originally surmised. The conception of the mesoderm as a 

 distinct embryonic layer, equal in importance with the re- 

 maining two, has been challenged, 1 and other of the most 

 fundamental propositions of the germ-layer theory are now 

 laid under arrest. 2 Dalamination has been discovered in 

 Ophinroids 3 — the Plakula and Parenchymella theories of 

 Metazoan development have now to be measured against 

 the Gastrsea one 4 — and the discovery of Trichpplax* and 

 Salinella* (should the latter be confirmed) suggest altogether 

 new lines of approaching the vitally important study of the 

 structural relationships between the Protozoa and Metazoa. 

 The doctrine of the substitution of organs 7 at least offers 

 a provisional method of solving some of the most formid- 

 able difficulties which beset the evolutionist, and renders 

 intelligible much that is most curious and perplexing 

 in nature. 1 The earliest stages in Metazoan development 

 are now proving to be as susceptible to secondary modi- 

 fication as the later ones, and signs are now dawning that 

 even the respective value of the blasto-meres must be 

 determined by their ultimate fate. 8 The brilliant researches 

 of Julin 9 and others, now progressing, which are fast bringing 

 us to regard the protozoan meganucleus as at least the 

 functional counterpart of the nucleolus of the Metazoa, and 

 its micronucleus as that of their remaining nuclear mass, 

 once more revive the conception of the structural unity of 



1 To wit, the appearance of the electric organ in the Raiidrt. The con- 

 version of the tail of the non Torpedine Batoids into either a vestigial 

 stump (Pterophitea), an elongated whiplash, or a barbed organ {T?y- 

 onidce) strongly favours the conclusion that the development of an 

 electrical organ within it is but one of a series of changes which it has 

 undergone upon the usurpation of its function as the chief propeller by the 

 expanded pectoral fins. The case appears akin to that of the independent 

 development of pharyngeal sacs (proved by experiment to be indispensable 

 to respiration) under accompanying reduction of the gills in the Murse- 

 noid Amphipitous (here reduced to one pair) and the siluroid Sarrohaiichus. 



