INTERNAL SECRETIONS 87 



whereas almost all other organs are bilaterally symmetrical. 

 The parathyroids are formed on the two sides in connection 

 with certain of the branchial pouches. In its earliest develop- 

 ment the thyroid gland resembles any other gland a salivary 

 gland, for example. Until a late stage it retains its connection 

 with the back of the mouth. Occasionally indications of this 

 primitive connection persist throughout life. In most cases 

 the place where the duct of the thyroid gland used to open 

 is clearly marked. At the back of the tongue too far back to 

 be seen without the aid of a dentist's mirror there is a V- 

 shaped row of large papillae (papillae circumvallatae). Just 

 behind the meeting-point of the two limbs of the V a pit is to 

 be seen foramen caecum. This pit is the vestige of the mouth 

 of the duct of the thyroid gland which opened into the pharynx 

 in the ancestors of fishes. It is an inconceivably long time since 

 fishes diverged from other races of animals. We do not know 

 which of the various orders of invertebrate animals now 

 existent most nearly resembles our prepiscine ancestor. The 

 organ which has developed into the thyroid body of mammals 

 may possibly have disappeared from all the other descendants 

 of the common stock from which vertebrates and their nearest 

 relatives in the invertebrate sub -kingdom were evolved ; but 

 it is much more likely that it has been preserved, and is still 

 performing its prime function in the higher invertebrate 

 animals. Probably it is a functional organ in a cuttle-fish or 

 a scorpion or a worm, but so unlike the thyroid gland of verte- 

 brates that we fail to recognize its homology. There are other 

 instances in the body of the persistence of an organ long after 

 it has fallen into such ruin that not even archaeologically-dis- 

 posed biologists can guess what it was like, or what purpose it 

 served in the days when it was at the height of its efficiency ; 

 but perhaps there is none other which so pregnantly illustrates 

 the physiological doctrine of functional interdependence. 

 Nature shows herself amazingly conservative in retaining 

 primal organs the pituitary body, the thymus gland, the 

 thyroid gland, the suprarenal capsules organs which millions 

 of years ago forgot the very rudiments of their craft ; but her 

 conservatism is not mere force of habit. Although she no 

 longer has any use for the wares which she created these 

 pieces of apparatus to make, she cannot do without their 



