84 ADAM SEDGWICK. 



the embryonic stage^ is the same as it is in the adult. It may 

 be altered relatively more or it may be altered relatively less ; 

 the point is that it is altered in the same direction as the 

 adult organ. And this is surely what we should expect when 

 we remember that embryonic development is the preparation 

 of the free form in the most perfect state and at the least 

 expense. If this view is correct that variations are present in 

 the embryo — that an organ which is enlarged, diminished, or 

 suppressed in the adult is correspondingly, or nearly so, en- 

 larged, diminished, or suppressed in the embryo, — then I ask, 

 how are we to account for those cases which most undoubtedly 

 occur in which records of previous states of structure are 

 present in the embryonic history, e. g. the pharyngeal slits of 

 Sauropsids, the tubular heart, the vascular arches, the em- 

 bryonic kidney of the same group, and many such. The point 

 is this : organs which have been recently altered show a 

 similar alteration in the embryo, whereas some organs, like the 

 gill-slits, which must have been altered very far back, do not 

 show a corresponding embryonic alteration, but persist more 

 or less in their old form without discharging the original 

 functions or being of any use to the embryo. In other words, 

 some ancestral organs persist in the embryo in a functionless 

 rudimentary (vestigial) condition and at the same time without 

 any reference to adult structures, while other ancestral organs 

 have disappeared without leaving a trace. The latter arrange- 



' It appears that in some cases, at least, it is less in the embryo. E. g. 

 sternal ribs of ostrich are generally five in adult, rarely six ; in embryo, 

 they appear always to be six. In birds the fibula reaches the tarsus in 

 embryos, but very rarely does so in adults. 



A case of this kind which might be investigated is this : — In the golden 

 plover the hallux is entirely absent, whereas in other plovers it is present. 

 Has the golden plover any trace of it in the embryo ? 



I am aware that it is often held — Darwin held it — that rudimentary organs 

 are, relatively to the adjoining parts, larger in the embryo than in the adult. 

 But unless this fact can be shown to be universal, it has but little value 

 because it applies to many other organs in the embryo which are not rudimen- 

 tary, e. g. brain, eye, heart, and kidney. This difference in relative size is 

 probably simply owing to the fact, that the bulk of the skeletal, muscular, 

 and connective tissues of the embryo is relatively less than in the adult. 



