Ontogeny of Endocrine Correlation 



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in pig fetuses of 46 to 50 days of age, thy- 

 roxine and di-iodotyrosine are first detected 

 a little later, i.e., in 52-day-old fetuses (Ran- 

 kin, '41). The stage at which the amount of 

 organic iodine compounds becomes promi- 

 nent is apparently correlated with the ap- 

 pearance of acini and colloid. Moreover, the 

 first appearance of organic iodine is accom- 



Fig. 208. Section of thyroid of a normal 11 -day 

 chick embryo showing marked sinusoidal vascular- 

 ity at time of rapid formation of discrete follicles 

 with colloid. Carotid artery to the left. Initial X 

 135. 



panied by marked increase in vascularity of 

 the gland (cf. Fig. 208). In the bovine fetus 

 from 53 days to term there is a progressive 

 increase in thyroxine-like compounds of 

 iodine and non-thyroxine iodine. These io- 

 dine fractions increase proportionately with 

 increasing fetal size and age (Wolff et al., 

 '49). The rate at which each of these sub- 

 stances accumulates in the fetal thyroid 

 shows an exponential relationship to body 

 weight and length as well as to age of the 

 fetus. Moreover, the increase in their ac- 

 cumulation cannot be accounted for solely 

 on the basis of increase in thyroid growth, 

 since the percentage growth rate of the 

 thyroid declines steadily with increasing age. 

 Apparently as the growth rate declines with 

 age there is a progressive increase in the 



