886 THE DEVELOPING ENDOCRINE GLANDS , 



the bony eel. However, this evidence is not indisputable, and more study 

 is necessary before definite conclusions are possible. (Consult Lynn and 

 Wachowski, '51, for discussion and references.) 



3. General Conclusions Relative to the Influence of the 

 Thyroid and Pituitary Glands in Vertebrate Embryology 



These conclusions are: 



(a) Positive activities of the thyroid and pituitary glands are demonstrated 

 in the transformation of the larval form into the definitive or adult 

 form in the Anura. 



(b) Suggestive evidence in favor of such an interpretation has been ac- 

 cumulated in fishes. 



(c) Circumstantial evidence, relative to the possible activities of the thyroid 

 and pituitary glands during the period when the embryos of the chick 

 and mammal are transforming into the adult form, is present. With 

 the evidence at hand, however, it is impossible to conclude definitely 

 that these glands are a contributing factor to a change in body form 

 (metamorphosis) in chick and mammalian embryos (fig. 256). 



D. Possible Correlation of the Endocrine Glands with Sex Differentiation 



1. Differentiation of Sex 



a. General Sex Features in the Animal Kingdom 



Many animal groups are hermaphroditic, that is, both sexes occur in the 

 same individual. Flatworms, roundworms, oligochaetous annelids, leeches, 

 many mollusks, and certain fishes are representatives of this condition, whereas 

 most vertebrates, insects, and echinoderms are bisexual. If one examines the 

 developing gonads in insects or vertebrates, it is evident that, fundamentally, 

 the potentialities for both sexes exist in the same individual. As observed 

 previously (Chap. 18), the early gonad is bipotential in most vertebrates, and 

 two sets of reproductive ducts are formed. As sex is differentiated, the gonadal 

 cortex and the Miillerian duct assume dominance in the female, while the 

 gonadal medulla and Wolffian duct become functional if the animal is a male. 

 Generahty, therefore, gives way to specificity. Conditions thus are established 

 in the developing reproductive system, similar to the generalized conditions 

 to be found in other systems. If we take into consideration the fact that in 

 a large number of animals both sexes are present in a functional state in one 

 individual and in many bisexual species both sexes are present in a rudi- 

 mentary condition in the early embryo, we arrive at the conclusion that both 

 sexes are fundamentally present in a large majority of animal species. Sex, 

 therefore, tends to be an hermaphroditic matter among many species of 

 animals. The problem of sex differentiation, consequently, resolves itself into 

 this: Why do both sexes emerge in the adult condition in a large number of 



