294 LIFE : OUTLINES OF GENERAL BIOLOGY 



complicated by forming little buds or vesicles; it ceases to be 

 connected with anything but blood-vessels; it becomes a very 

 typical endocrinal or ductless gland. 



The key to the evolution of the thjToid is found in the lamprey, 

 for in its larval form (the Ammocatc or "niner") the thjToid is a 

 mid-ventral gutter-like fold of glandular and flagellate cells which 

 remains open to the pharjTix. This is undoubtedly homologous 

 with the ventral pharyngeal fold, called the endostyle, which per- 

 sists throughout life in Amphioxus (lancelet) and Tunicates (sea- 

 squirts). Some zoologists, indeed, would trace it farther back still 

 to the ventral portion of the pharynx in the half-chordate, hajf- 

 wormlike pioneer types known as Enteropneusta, of which Balano- 

 glossus is a common example in many seas. 



As the larval lamprey continues its slow development, the com- 

 munication between the thyroid diverticulum and the pharynx is 

 gradually reduced, till eventually a narrow slit becomes a pore, 

 and even this disappears when the "niner" metamorphoses into a 

 lamprey. The organ divides up into numerous closed vesicles, and 

 it is a suggestive fact that these, like the vesicles of other thyroids, 

 continue making a mucous colloid material like that employed 

 in entangling food-particles in lancelets and Tunicates. The 

 function persists though the material produced can no longer be 

 exuded. 



This interesting chapter in evolution, first outlined by W. Miiller 

 in 1S71, discloses a good instance of change of function. What was 

 originally a glandular organ connected with the food-canal has 

 become from fishes onwards a ductless gland, and that hormone- 

 producing. 



Closely associated with the thyroid in man are four minute 

 "parathyroids", httle studied except in mammals. They develop 

 as outgrowths from the third and fourth gill-clefts on each side, 

 that is, from the clefts that give risQ to another difficult organ 

 — the thymus. The parath^Toids are also endocrinal, and the hor- 

 mones they produce are different from those of the thyroid proper, 

 but their still obscure function will be considered later. 



When the thyroid degenerates in a young animal or is removed, 

 the result is arrest of growth, especially of the skeleton; also an 

 arrest of development in the cells of the cerebral cortex, a flabbiness 

 in the muscular system, a slowing of the development of the repro- 

 ductive organs, and various other symptoms which are summed 

 up in the word cretinism. In the case of children there is an arrest 

 of bodily and mental development; the child remains infantile, 

 even if it ceases to be in years a child. These abnormal conditions 

 may be alleviated, indeed in many cases cured, by adding to the 

 diet preparations of sheep's thyroid, or some form of thyroid 

 extract, or artificially synthesised thyroxin. 



