IN THE LIMBS OF MAMMALIA. H 



sin Hominis et Mammalium," figures, and briefly describes, the skeleton of an anencephalous 

 monster, in which one arm appears attached to the base of the skull, as if by arrest of de- 

 velopment, while the other is in its normal position on the side of the thorax. With a view 

 to ascertain whether, at any period, the shoulders of the mammalian foetus are in actual 

 contact with the cranium, I made careful examination of large numbers of foetal pigs, and 

 in the very smallest, just when the limbs begin to protrude from the sides as little fleshy 

 buds, it is always at some distance from the head ; so that, in the Mammalia at least, the 

 fact of actual contact must be regarded as doubtful. 



3d. But in most fishes they are firmly attached to the cranium, and in the tadpoles of 

 the bull-frog (Rana pipiens) I have found the scapula closely connected with the poste- 

 rior part of the cranium, either by muscle or ligament, which elongates as development 

 proceeds. 



Now the three terms, laterality, tergality, and cephality, are more or less complete expres- 

 sions of the arrangement of organs at the two poles of the three axes of a sphere, the lateral, 

 the vertical, and the longitudinal, one of which is specially prominent in each of the higher 

 types, Mollusks, Articulates, and Vertebrates, while the Radiates are represented by the sim- 

 ple sphere itself, with no one axis more prominent than another ; since the members of this 

 type are not geometrical figures, but organic, living beings, they must have a structural 

 axis around which their diverging segments or spheromeres are arranged ; but this assumes 

 such a variety of directions, being reversed between the Polyps and the Acalephs with most 

 Echinoderms, and becoming horizontal in the Holothurians, as to entirely negative the idea of 

 its having any such morphological significance as the axes of the other three types. These 

 three are the main axes of a sphere, the only ones possible at right angles with each other ; 

 and they also correspond with the three dimensions of a solid, breadth, thickness, and 

 length, while the sphere may be regarded as having no dimension, yet as capable of all. 

 (See also Professor Agassiz's " Contributions to Natural History of the United States," vol. iii. 

 chap. ii. sect. iv. on Morphology and Nomenclature, p. 76.) This gives us four plans, four 

 morphs on which the animal kingdom is built, and this coincides with the number now be- 

 lieved to exist. 



A strong corroboration from a different source is contained in the views of Professor Arnold 

 Guyot, expressed in a course of lectures delivered during the winter of 1862, at the Smith- 

 sonian Institution, on the " Unity of Plan in Animals and Plants." 



He presented, as an indication of the existence of no more nor less than four grand divis- 

 ions among animals, the idea that the four types represent the four grand epochs in the life 

 of a single animal ; the Radiates are the starting-point, the germ, the simple cell, with life, 

 but this of a low, indeterminate character, and inhabiting the water, the lowest medium ; 

 then comes a partial progress in one direction, with the development of the nutritive sys- 

 tems of organs, and this second stage is represented by the Mollusks, with their heavy bodies, 

 devoted to digestion and circulation, and confined to the earth ; then comes a partial prog- 

 ress in the opposite direction, with the development of the respiratory and motory apparatus, 

 and this is well represented by the Articulates, chiefly inhabiting the air ; and, finally, in the 

 Vertebrates is typified the animal in its perfect state, with a more equal combination of 

 both classes of functions. 



Again, for the existence of four classes in the Vertebrates the same reason holds good : 

 the Fishes are the starting-point, and, like the Radiates, dwell in the water ; then come the 

 Reptiles, with their heavy bodies attached to the earth, and characterized by special promi- 

 nence of the nutritive functions, thus corresponding to the Mollusks ; then the Birds, the 



