ON THE SUPRA-RENAL, THYMUS, AND THYROID BODIES. 69 



tema already alluded to as forming at its posterior part the 

 supra-renal capsule. This portion of the blastema becomes 

 much larger at the side of the heart, round the ductus Cuvieri, 

 behind the lateral parts of the pericardium, and in the future 

 situation of the lungs, which have not yet left their median 

 position. Each lateral portion of the blastema stretches from 

 the heart forwards along the internal side of the jugular vein, 

 par vagum, and carotid arteries. These two anterior portions 

 of the lateral blastema, from the narrow portion forwards to 

 the skull, are the lateral portions of the thymus and thyroid, 

 which have not yet joined across the middle line. 



In embryos a little further advanced, the two portions of 

 blastema join across the trachea in a line extending from the 

 base of the heart to the lower end of the larynx, which has 

 now appeared as an oblong oval swelling behind the tongue. 

 Previous to, and also contemporaneous with, this cross junc- 

 tion, a change has occurred in the position of the lungs and of 

 the ductus Cuvieri. 



As the lungs proceed in development, they pass in a 

 direction from behind forwards and from within outwards, 

 moving from their original median position to a lateral one : 

 they at the same time increase both absolutely and relatively. 

 At the same time, a somewhat similar change takes place in 

 the two ductus Cuvieri. They pass forward so as to appear 

 to enter the anterior instead of the posterior extremity of the 

 auricle, becoming in this way the anterior vense cavse, this 

 change of position being produced apparently by a semi- 

 revolution of the whole heart, coinciding with its elongation 

 and the altered arrangement of the bulbous aortse. 



Coincident with this change in the ductus Cuvieri is a 

 corresponding change in the position of the lateral masses of 

 the blastema. These pass forward, become grouped around 

 the auricles and anterior venae cavse, and join across the 

 middle line as already stated ; but a narrow portion, particu- 



