884 THE DEVELOPING ENDOCRINE GLANDS 



the adult body form). Similar results were obtained as a result of hypophy- 

 sectomy (i.e., removal of the hypophysis). (See Allen, '29, and Smith, '16 

 and '20.) The work of these observers clearly demonstrates that the thyroid 

 and pituitary glands are instrumental in effecting the radical transformations 

 necessary in the assumption of definitive body form in the Anura. 



2. Thyroid and Pituitary Glands in Relation to the 

 Development of Other Vertebrate Embryos 



a. Chick 



1) Thyroid Gland. Studies relative to the possible effect of the thyroid 

 gland upon the developing chick embryo are complicated by the fact that 

 the yolk of the chick egg is composed of many other factors besides fats, pro- 

 teins, and carbohydrates. The yolk is a veritable storehouse for vitamins and 

 for thyroid, sex, and possibly other hormones. Just what effect these sub- 

 stances have upon development is problematical. Some experiments, however, 

 have been suggestive. Wheeler and Hoffman ('48, a and b), for example, 

 produced goitrous chicks and retarded the hatching time of chicks from eggs 

 laid by hens which were fed thyroprotein. Thyroprotein feeding seemingly 

 reduced the amount of thyroid hormone deposited in the egg with subsequent 

 deleterious effects upon the developing chicks. In normal development, the 

 thyroid gland of the chick starts to develop during the third day and produces 

 follicles which contain colloid by the tenth and eleventh days of incubation. 

 Furthermore, Hopkins ('35) showed that thyroids from chick embryos of 10 

 days of incubation hastened metamorphosis in frog larvae. From days 8 to 

 14 the chick embryo undergoes the general changes which transform it from 

 the larval form which is present during incubation days 6 to 8 into the 

 definitive body form present at the beginning of the third week of incubation. 



The foregoing evidence, therefore, while it does not demonstrate that thyroid 

 secretion actually is being released by the developing thyroid gland into the 

 chick's blood stream, does suggest that the thyroid gland may be a factor 

 in chick development and differentiation. If the chick's thyroid gland is se- 

 creting the thyroid hormone into the chick's blood stream during the second 

 week of the incubation period, it is evident that the developing chick during 

 the period when it is assuming the definitive body form has two sources of 

 thyroid hormone to draw upon: 



(1) that contained within the yolk of the egg and 



(2) that produced by its own thyroid gland. 



2) Pituitary Gland. Relative to the development of the pituitary gland in 

 the chick, Rahn ('39) showed that the anterior lobe develops both acidophilic 

 and basophilic cells by the tenth day of incubation. Also, Chen, Oldham, and 

 Ceiling ('40) demonstrated that the pituitary of chicks from eggs incubated 



