FROM MONER TO MAN. 581 



amoeba to be capable, is the fundamental fact in the functions of the 

 fully developed muscle, nerve, and brain of the highest organisms. 



The embryon, in its condition of a three-layered sac, soon begins 

 to show a slight bilateral symmetry, and a chorda dorsalis appears. 

 Its rank, as a vertebrate, is thus established in the dawning of that im- 

 portant structure, a backbone. 



Allusion has been made to the ascidian as introducing the verte- 

 brate type. Whatever may be thought of the claims of this animal 

 to so important a place in the genealogical tree, there can be little 

 doubt about the position of the amphioxus with its dorsal cord dis- 

 tinct and persistent throughout life. Though classed, on this ac- 

 count, among vertebrates, it is singularly wanting in vertebrate char- 

 acteristics, having neither heart nor brain in the true sense of these 

 words. It is also destitute of limbs, even of the most rudimentary 

 kind, such as are found in the very lowest fishes. In fact, it is dis- 

 tinctly neither vertebrate nor invertebrate, thus admirably filling the 

 position of a connecting link between these two great subdivisions of 

 the animal kingdom. 



At the chordonian stage of its development, the human embryon 

 is equally destitute of a true heart, brain, and limbs, thus correspond- 

 ing to a sub-type of the vertebrates called by Haeckel, Acrania, of 

 which the amphioxus is the best-known representative. There is, nev- 

 ertheless, in this heartless, brainless, limbless, and almost shapeless mass 

 of but slightly differentiated protoplasm, that wonderful impulse of 

 evolution by which its destiny, as an individual of the highest organic 

 rank, is assured. 



Along the line of the chorda dorsalis, rudimentary nerve-centers 

 and spinal vertebrae gradually appear, the embryon thus entering on a 

 grade of development comparable to that of the lowest fishes, in which 

 the spinal column is cartilaginous rather than bony. 



The budding limbs resemble budding fins ; arches similar to those 

 which, in water-breathing animals, support the gills are seen ; and the 

 rudimentary lungs are mere air-bladders. 



Next arises the amnion stage, so named from an important though 

 temporary nutritive organ whose development begins at this period ; 

 it is an extension of the yolk-sac, and contains a highly nutritious 

 fluid. 



The gill-arches gradually disappear, developing into more ad- 

 vanced structures ; the heart becomes subdivided into four chambers ; 

 the air-bladders give place to true lungs ; and, with the complete 

 formation of a placenta, the mammalian stage of development is fully 

 established. The embryon is henceforth recognizable as belonging 

 to the class mammalia, the highest of the vertebrates. 



As the growing organism becomes more and more complex, its 

 progress is more and more gradual. We have seen how the germ 

 passes, almost at a single step, from the gastrula to the rudimentary 



