616 JOHN BEARD, 



If this reproach were true of the standpoint, from which the 

 reproductive problems in von Baer's mind at the time of writing were 

 generally regarded, it applies with equal force to our accepted, but 

 utterly erroneous, notions, concerning those now about to be under 

 consideration. 



Modern embryology regards animate nature as a huge machine 

 of the crudest construction. 



An egg is fertilised, it segments, forms a blastula, an invaginate 

 gastrula, it may be, ensues. Some embryologists even speak of this 

 as an embryo! Anon a third layer appears, and endless have been 

 the disputes as to how, where, and whence this comes into existence. 

 From these three layers, epiblast, hypoblast, and mesoblast, all the 

 parts of the animal take their origin. Indeed, paradox though it be, 

 although the embryo really only gradually manifests itself, the embryo 

 is there and may be spoken of as such, from the moment, that the 

 mesoblast makes its appearance. 



To-day there is not a single Metazoon, whose development has 

 been at all adequately investigated, in which by some observer or 

 other serious deviations from the above crude scheme have not been 

 witnessed. Sometimes he has described them, at others they have 

 been ignored. Transient organs appear and disappear, the final nervous 

 system, it may be, replaces an evanescent one, the alimentary canal 

 undergoes a profound metamorphosis, etc. The real embryo gradually 

 appears on the side of a highly organised or degenerate "larva", or 

 within it; or within a sac, comparable to an amnion and found in 

 such diverse and widely separated animals as some Hydrozoa, Nemer- 

 tines, Peripatus, Insects, some Echinoderms, and all the higher Verte- 

 brata. All these are to modern embryology mere curiosities. 



Generally speaking, the fate of the blastopore and the mode of 

 origin of the mesoblast are considered as of infinitely greater import- 

 ance than the fate of the larval organs and the mode of birth of the 

 embryo upon the larva. 



Fruitful though the germ-layer theory may have been, and weighted 

 as it is with the dragging incubus of recapitulation, it is quite in- 

 adequate to cope with and explain all the phenomena of the develop- 

 mental history of any single Metazoon. Developmental research has 

 hitherto largely concerned itself with the history of the organs; and, 

 as a result, we possess little knowledge of the mode or modes of 

 development of organisms. Not that the study of the origin of organs 



