636 



Metamorphosis 



man, '46). With regard to the pituitary hor- 

 mones, no evidence to show a change in 

 thyrotrophin level in the blood in relation 

 to metamorphosis is known to the writer. 



In the case of the thyroid the lack of 

 evidence on the level of hormone in the 

 blood was compensated for to some extent 

 by extensive and clear-cut cytological evi- 



dence showing that the thyroid is activated 

 at the time of metamorphosis. Unfortunately, 

 the histological and cytological study of the 

 pituitary in relation to metamorphosis does 

 not present unequivocal evidence of any 

 change in activity at this period (Clements, 

 '32; Schliefer, '35). In fact, the considerable 

 variation in the degree of cytological differ- 



HEMIMETABOLOUS DEVELOPMENT HOLOMETABOLOUS DEVELOPMENT 



Fig. 220. The life history of a hemimetabolous insect, the grasshopper Melanoplus differentialis, is 

 shown at the left. Holometabolous development in the giant silkworm moth, Platysamia cecropia, is shown 

 at the right. (From C. D. Turner, General Endocrinology.) 



