150 



DISCOVERY 



in a comparatively short time (three to four weeks) 

 \\ hen axolotls are fed on raw th\Toid gland from the ox. 

 Tliis was first shown independently by Laufberger 

 (1913) and Jensen (1917), whose work was later con- 

 firmed and extended by Huxley and Hogben (1920). 



Now there is an interesting circumstance relating 

 to the th}Toid-induced metamorphosis of the axolotl 

 which has emerged from the work of the last-named. 

 Iodine has no effect whatever on this species. This 

 appears to indicate that the thyroid of the axolotl is 

 not in working order, though as a matter of fact it is 

 structurally well developed. Does the thyroid gland of 

 the amphibia store iodine and secrete thyroid hormone 

 continuously, or does the activity of the thyroid depend 

 intimately upon internal stimuli ? This question leads 

 us on to a consideration of the other important duct- 

 less gland mentioned earlier in this article, namely 

 the pituitary. 



In the tadpole the pituitary gland which arises as a 

 vesicle of tissue budded off from the roof of the mouth 

 is not fully differentiated at the time of hatching, apd 

 it is possible to remove it by the neat insertion of a 

 fine needle without fatal consequences. The technique 

 of this operation was perfected by Bennet Allen (1917) 

 at the same time as his experiments on thyroid 

 removal were in progress, and the results obtained by 

 him have been confirmed by two other workers, Smith 

 and Atwell, who have conducted inquiries on similar 

 lines. Large numbers of pituitaryless tadpoles have 

 now been reared, and the most significant feature they 

 display for present purposes is that they show a retar- 

 dation in the development of the thyroid, and — Hke 

 thyroidless tadpoles — a failure to metamorphose. 

 Later Allen (1920) and Swingle (1921) have published 

 preliminary indications of the possibility of inducing 

 metamorphosis precociously b}' ingrafting the pituitary 

 gland of other individuals into normal tadpoles. 



Here the pituitary gland, or, to be more precise, its 

 anterior portion, seems to exercise an influence on 

 development as well as the thyroid ; and recently 

 the writer (Hogben) has induced Axolotls to trans- 

 form by injections of pituitary extracts. Its precise 

 function in man and the higher animals is not under- 

 stood, though clinical evidence points to a connection 

 with growth. Of its posterior half much more is 

 known. This portion of the pituitary secretes a hor- 

 mone (or more probabh' a group of hormones) which 

 assist in regulating blood pressure, child labour, and 

 the flow of the milk by excitatory action on involuntary 

 muscle and glands. One of these hormones plays an 

 important role in regulating the colour of the tadpole. 



Most readers will be familiar with the power of 

 certain animals, such as fishes, amphibia, reptiles, and 

 some molluscs, to respond to stimuli — in some cases to 

 the colour of their surroundings — by appropriate changes 

 of colouring of the skin. The chameleon, whose 



aptitude in quick-change artistry is perhaps a little 

 exaggerated, is the proverbial example ; but more 

 familiar animals like the trout and frog are capable of 

 considerable pigmental changes in quite a short time. 

 A frog that has been kept in the shade and exhibits a 

 coal-black tint \\ill within an hour of removal to white 

 flagstones in the sun dev-elop a pale j'ellow or flesh- 

 coloured hue. Colour changes of this nature are 

 brought about by the contraction and expansion of 

 certain corpuscles charged with pigment granules and 

 situated near the surface in the skin. 



The responses of these microscopic pigment cells are 

 in certain cases adaptive, being co-ordinated in such a 

 way as to render the animal inconspicuous among its 

 surroundings. This is well seen in the case of albino 

 axolotls which have very little pigment, and, if kept 

 in a tank with white glazed sides illuminated from 

 above, remain perfectly white. When transferred to a 

 container with blackened sides similarly illuminated 

 from above, they becomg in a very few minutes quite 

 dark. This change will not, however, take place in 

 animals whose eyes have been removed, blinded, or 

 covered with an opaque substance. The response — 

 " colour adaptation " — depends upon the appropriation 

 of stimuli by the retina. Nervous impulses probably 

 pass from the brain direct to the pigment cells. But 

 in all probability an important factor in regulating 

 colour change is brought about more in directly by 

 increased secretion of certain of the ductless glands. 



Of these the posterior portion of the pituitary gland 

 is of the utmost significance. The frog tadpole is 

 usually dark, in fact black, in colour. Allen and 

 other workers agree in stating that after pituitary' 

 removal their tadpoles assumed a pale silvery appear- 

 ance. This extreme pallor was found, on microscopic 

 observation, to be due to the contraction of the pigment 

 cells. The nature of the sjTnptoms of pituitary removal 

 points to the conclusion that the secretion of this gland 

 maintains the pigment cells in an expanded condition 

 under normal circumstances. This has been directly 

 verified quite recently by Hogben and Winton (1922), 

 who have found that an adult frog can be changed 

 from a pale yellow to a coal-black tint within twenty 

 minutes by injection of an extract equivalent to less 

 than a hundred thousandth of a gram of the substance 

 of the posterior portion of the pituitary. 



The secretion of another gland (suprarenal), lying in 

 man above the kidney and acting on involuntary muscle 

 in a manner somewhat similar to the pituitary hormone, 

 when administered to fishes or amphibia alike induces 

 a sharp contraction of the pigment cells, the animal 

 becoming extremely pale. A similar result follows 

 treatment with extracts of another organ — the pineal 

 body, at least in the case of tadpoles. The pineal is a 

 small structure lying on the roof of the brain. In 

 some forms — for instance, the archaic New Zealand 



