380 Comparative Anatomy — Its History, Aim, and Method 



cod have drifted far foward, but their anchorage certifies to their 

 posterior origin. 



Embryonic origin is generally regarded as of great significance in 

 the judgment of homologies. Leaving aside all controversial issues as 

 to whether ontogeny "repeats" or "recapitulates" phylogeny, it is 

 merely an observable fact that a notochord, pharyngeal pouches, 

 aortic arches, and a cartilaginous internal skeleton are temporarily 

 present in embryos of reptiles, birds, and mammals. Inheritance offers 

 the only reasonable explanation of their presence. It is not safe to say 

 that these temporary structures are "useless vestiges." The results of 

 recent investigations in experimental embryology indicate the possi- 

 bility that these "vestiges" may be essential. It has been proved that, 

 in many cases, the embryonic development of one part is dependent, 

 upon stimuli produced by some other part which has already been 

 formed. Normal development of the lens of the vertebrate eye from 

 I he outer layer of the embryo depends upon the prior development of 

 the "optic cup," which grows out from the embryonic brain and be- 

 comes the retinal part of the eye (Fig. 141). It may be that the noto- 

 chord and other transitory embryonic structures are necessary as 

 sources of stimuli which initiate and control the development of other 

 structures which are to be permanent. If this should prove true, it 

 would make it even more certain that the transitory parts are a herit- 

 age from remote ancestors. 



If it is admitted that inheritance is the basis for the temporary 

 presence of such conspicuous things as the notochord and pharyngeal 

 pouches, then it becomes reasonable to expect that any organ may 

 pass through early stages closely similar to early stages of that same 

 (i.e., homologous) organ in some remote ancestor, even though the 

 adult organ of the recent animal and that of the ancestor may be very 

 different in form and function. When we discover that a shark's spi- 

 racles and the middle-ear cavities of a mammal pass through closely 

 similar early embryonic stages, we may infer that spiracles and middle- 

 ear cavities are the same. Unfortunately, we cannot observe the devel- 

 opment of the spiracle in the remotely ancestral shark. We observe it 

 only in a recent shark. We are accordingly forced to the more elaborate 

 inference that ancient sharks were the starting point of lines of "adap- 

 tive radiation." Along one line, sharks remained sharks and retained 

 spiracles down to the present. Along another line (or lines), sharks 

 became successively amphibians, reptiles, and mammals, and the old 

 spiracles became middle-ear cavities, although still retaining the 

 ancient method of embryonic origin. In terms of the genetic scheme 

 illustrated in Fig. 290, the germ-plasm of ancient sharks contained a 

 complex of factors, s, which determined development of a spiracle, S. 

 Along some lines of descent the germinal s remained s and always 



