618 



'Thymus IV', — i. o., the ventral outpocketi 

 embrvos older than 7.5—9 mm. was more 





PH 



U-_ 





:^ 



Fig. 1. Section through the caudal pharyngeal 

 complex, 11 mm (N.B.), embryo No. 26, Cornell 

 collection. Photograph, X 45. P. 2 F. parathyroid 

 IV; C carotid artery ; Th. thymus; Thy. thyroid; 

 U. ultimobranchial body. 



ng toward the thyroid, — iu 

 evident in the series exa- 

 mined by him than in 

 those studied by me. A 

 discussion of this aspect of 

 the matter which involves 

 theorigin and interpretation 

 of thymus bodies may with 

 advantage be postponed 

 until a subsequent paper 

 dealing with the pharynx 

 as a whole. 



In the next succeeding 

 stages the caudal pharyn- 

 geal complex becomes se- 

 parated from the ectoderm 

 (cervical vesicle), and from 

 the pharyngeal cavity. The 

 former occurs first. In the 



-PET 



10 mm (2) embryo the con- *- " '*^ 



nectiou with the cervical 

 sinus is drawn out to a de- 

 licate tube, which is inter- 

 rupted iu the 11.5 mm. 

 embryo and subsequent 

 stages. In the 11 mm. em- 

 bryo the connection of tho 

 complex with the pharynx 

 has become quite atten- 

 uated, and in the 14.5mm. 

 embryo it is interrupted. 

 The caudal pharyngeal 

 complex, thus isolated by 

 the growth of the sur- 

 rounding structures may 

 be regarded as consisting 



of three parts, — a head made up largely of the Parathyroid IV, a neck 

 connecting it with a body which mainly at least is made up of that 



"--^j/ 



Th 



'?-.\v*n.5ij-''- 



Fig. 2. Section from the same series, nine sections 

 farther caudad. Photograph, X 45. As above. 



