-43- 



Rxplanation of Plates. 

 All sections drawn with a Zeiss Camera Luclda, 



• • Fir,, 1 a. Larva of P. planerl, 11 days after ferti- 

 lization; to show the external appearance of the larva at the 

 time of the first trace of the thyroid. (Mag. 65 diam. ) 



Fig. 1 b. Transverse section of 12-day larva of P. 

 planerl, through the anterior end of the thyroid, just anterior 

 to the openlnp Into the pharynx. Shov/s the gland as a circular 

 cavity, lined with columnar cells, lying under the digestive 

 cavity. (I'ag. 325 diam.) 



Fig. 1 c. Transverse section posterior to Fig. 1 b 

 through the anterior part of the opening of the thyroid into 

 the pharynx. (Mag. 325 di&nu ) 



Fig. 2 a. Transverse section through the extreme an- 

 terior end of the thyroid of a 14-day P» planeri iscsata. 

 (I.:ag. 325 diam.) 



Fig. 2 b. Transverse section posterior to Fig. 2 a 

 but anterior to the opening of the gland into the pharynx. 

 Shows beginning of lateral invagination. (Mag. 325 diam.) 



Fig. 2 0. Transverse section posterior to Fig. 2 b 

 through the anterior part of the opening of the gland into the 

 pharynx. (Mag. 325 diam.) 



Fig. 2 d. Transverse section posterior to Fig. 2 c 

 through the posterior part of the opening of the gland into the 



