ee t— 
ADDRESS. lxix 
menclature equivalent to express his convictions of the different relations of 
similitude. Most difficult and recondite are the questions in face of which 
the march of homology is now irresistibly conducting the philosophic observer. 
Such, for instance, as the following :—Are the nervous, muscular, digestive, 
circulating and generative systems of organs more than functionally similar 
in any two primary provinces of the animal kingdom? Are the homelogies 
of entire systems to be judged of by their functional and structural con- 
nexions, rather than by the plan and course of their formation in the 
embryo? 
In the development of animals the vitellus is observed to have different 
relations to the embryo. But such difference is not always or absolutely 
associated with a difference of plan of structure: the Cephalopods show a 
higher development of the same fundamental plan of structure as that of the 
Gasteropods, but the vitelline phenomena of their development resemble 
those of Vertebrata, not those of Gasteropoda. 
Even in the last-named restricted molluscous group there are striking 
differences in the vitelline relations of the embryo. In most the embryo 
early encloses the vitelline mass; in some,.as in Limaaz, much later: and 
there is what may be termed a temporary vitelline sac. 
The yolk undergoes a complete segmentation in placental Mammals, the 
embryo of which is formed out of the whole vitelline mass, as it is in the 
whelk, the oyster and the star-fish. The bird, the crocodile and the cuttle- 
fish resemble each other in the embryo rising out of the yolk, assimilating 
only a portion, and leaving the rest as an appendage until the period of birth, 
or for a short time after. 
It may be doubted, therefore, if embryology alone is decisive of the 
question whether homology can be predicated of the alimentary canal in 
animals of different primary groups or provinces. The armadillo (Dasypus) 
and the woodlouse ( Oniscus) are good subjects for illustrating this question. 
In both, the alimentary canal begins at the fore part and terminates at the 
hind part of the body: in both, an cesophagus precedes a stomach, as this 
precedes the part of the canal receiving the biliary secretion ; in both, the in- 
testine follows to terminate at the vent. 
Besides the sameness of function, the homologist, confiding in the 
characters of connexion and relative position, would retain the names ¢ ali- 
mentary canal,’ ‘ mouth,’ ‘gullet,’ ‘stomach,’ ‘ gut,’ to express his ideas of 
the veritable answerable character of the parts compared in Oniscus and 
Dasypus: but he who believes embryology alone capable of affording a solid 
basis for determining homologies*, will infer that the different relations of 
the yolk and the intestine in the embryos of the vertebrate and articulate 
* “Bmbryology affords further a test for homologies in contradistinction to analo- 
gies. It shows that true homologies are limited respectively within the natural boundaries 
of the great branches of the animal kingdom.”—Agassiz, Nat. Hist. of the United States, 
Ato, 1857, vol. i. p. 86. 
