The Morphology of Cosniobiu 3G5 



ment in a certain direction impossible. Such is the case of the pro- 

 boscis-like nose given above. Still more serious M'ould be the difficulty 

 if the encroaching organ should hinder the full efficacy of some system of 

 functional importance to the embryo, as for example the circulatory or 

 lymphatic organs. Thus there might arise those evidently pathological 

 beings that fill the pages of general works on teratology, yet in which 

 may still be traced a definite original plan capable of finding a place 

 in some series of symmetrical and non-pathological beings. It is prob- 

 able, indeed, that certain types of primarily symmetrical monsters have 

 such a mechanical configuration as to render development impossible 

 without becoming secondarily pathological through this cause, and in 

 such cases one must either learn to eliminate the secondary modifica- 

 tions as remarked above or, perhaps, by some fortunate chance, as rare 

 as the discovery of some long-sought link among the strata of the earth's 

 crust, obtain and study an embryo of the type in question representing a 

 stage previous to the appearance of the causes of the pathological con- 

 dition. 



Aside from the above and probably other causes affecting the develop- 

 ment of abnormal embryos in general, certain groups of vertebrates are 

 undoubtedly subject to special hindrances due to some peculiar and 

 specific mode of development. The result of this may be that only 

 certain t3''pes are possible within a certain group of vertebrates; hence, 

 in completing a given series of symmetrical monsters it may easily happen 

 that a certain stage sought cannot be found in mammals but may be of 

 frequent occurrence among amphibians, or that a stage in the series that 

 always appears in a secondarily distorted condition in birds may be found 

 without such modifications in reptiles. This suggests an explanation 

 of the fact that certain types of monsters are of far more frequent 

 occurrence in some animals than in others, a field of inquiry that would 

 undoubtedly yield much if it were carefully investigated. For example, 

 birds have at least two developmental peculiarities that would tend to 

 modify the development of monstrous embryos, namely, the enormous 

 yolk and the early twisting of the embryo, and as a matter of fact, avian 

 monsters, when hatched, are more limited in variety than those of 

 mammals and are usually unilateral and somewhat distorted. All of 

 these suggested causes of malformation have this in common, that they 

 exert their modifying influence at some time during embryonic life 

 upon what must be at first an undeformed, though abnormal, embrvo. 

 It follows, therefore, that we would eliminate these secondary modifi- 



