444 EDGAR H. NORRIS 



resolution of the plates parallel the increasing length of the 

 embryo up to 65 mm. that it is very improbable that individual 

 variations could explain the presence of these structures in such 

 great numbers at the relatively late stage of 86 mm. Further, 

 as shown in figure 16, forms of all degrees of complexity may be . 

 found from simple spheroidal follicles to those of extreme com- 

 plexity. The conclusion is therefore reached that these branch- 

 ing structures, which are better described as folhcular complexes 

 than as folhcles, are developmentally only follicles which have 

 grown excessively and attained a high degree of complexity. 



Such a rapid increase in the number of thyroid follicles as 

 occurs in fetuses between 65 and 158 mm. in length might be 

 expected to produce a marked increase in the size of the gland. 

 But according to Jackson ('09) the growth curve for the pre- 

 natal thyroid gland shows no remarkable increase in the size 

 (weight) of the gland during this period. These two obser- 

 vations, which at first may appear contradictory, are readily 

 explained when the size of the follicles is taken into account. 

 The secondary folhcles formed by budding and division of the 

 primary folhcles are very small and arise from folhcles which 

 are in most cases relatively of much greater size. So that while 

 the number of folhcles is greatly increased during this period, 

 the gland mass is not correspondingly larger. As previously de- 

 scribed, the formation of secondary folhcles becomes less rapid 

 before the fetus has reached 163 mm. in length. From this 

 point on, the number of thyroid folhcles apparently increases 

 but slowly, the subsequent growth of the gland being due rather 

 to the increase in the size of the individual folhcles than to a 

 further increase in their number. 



The significance of the large cyst-like follicles described in 

 four of the fetuses is uncertain. Klirsteiner ('99) has described 

 the presence of similar folhcles in four fetuses. The remarkable 

 regularity with which they were found, in his cases as well as in 

 those of the present series, located in the lower and posterior 

 (dorsal) part of the lateral lobe of the gland, is a striking fact. 

 Possibly they may be related to the cysts of the thyroid gland, 

 frequently met in pathological conditions of postnatal life. 



