DEVELOPMENT OF DEFINITIVE BODY FORM 885 



for five days possessed a melanophore-expanding principle when administered 

 to hypophysectomized frogs. 



This general evidence, relative to the developing pituitary gland in the 

 chick, suggests that the cells of the pituitary gland may be active functionally 

 during the latter part of the first week and during the second week of incu- 

 bation. If so, the pituitary gland may be a factor in inducing the rapid growth 

 and changes which occur during the second week of incubation. It suggests 

 further, that a possible release of a thyrotrophic principle may be responsible 

 for the presence of colloid within the developing thyroid follicles during the 

 second week of incubation. 



b. Mammal 



As in the chick, the developing embryo of the placental mammal is in 

 contact with hormones from extraneous sources. Hormones are present in 

 the amniotic fluid, while the placenta is the seat of origin of certain sex and 

 gonadotrophic hormones. Also, the maternal blood stream, which comes in 

 contact with embryonic placental tissues, is supplied with pituitary, thyroid, 

 adrenal, and other hormonal substances. This general hormonal environment 

 of the developing mammalian embryo complicates the problem of drawing 

 actual conclusions relative to the effect of the embryo's developing endocrine 

 system upon the differentiation of its own organ systems and growth. Never- 

 theless, there is circumstantial evidence, relating to possible activities of the 

 developing, embryonic, endocrine glands upon development. 



1) Thyroid Gland. Colloid storage within the follicles of the developing, 

 human, thyroid gland is evident at 3 to 4 months. In the pig embryo, Rankin 

 ('41 ) detected thyroxine and other iodine-containing substances in the thyroid 

 at the 90-mm. stage, and Hall and Kaan ('42) were able to induce meta- 

 morphic effects in amphibian larvae from thyroids obtained from the fetal 

 rat at 18 days. The foregoing studies suggest that the thyroid gland is able 

 to function in the fetal mammal at an early stage of development. (For further 

 references, consult Moore, '50.) 



2) Pituitary Gland. Similarly, in the pituitary gland, granulations within 

 the cells of the anterior lobe are present in the human embryo during the 

 third and fourth months (Cooper, '25). Comparable conditions are found in 

 the pituitary of the pig from 50 to 170 mm. in length (Rumph and Smith, '26). 



c. Fishes 



The relationship between the thyroid and pituitary glands in the develop- 

 ment of fishes is problematical. There is evidence in favor of a positive in- 

 fluence of endostylar cells and of the cells of the developing thyroid gland 

 in the transformation of the ammocoetes larva of the cyclostome, Petromyzon, 

 into the definitive or adult body form. Similar evidence suggests a tli^roid 

 activity relationship in the transformation of the larvae of the trout and 



