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



larly along the left side, still passes downwards and back- 

 wards along the cardinal veins, which have now become the 

 azygos veins. 



While these changes in the veins and blastema have 

 taken place, the lungs have increased in size, and their 

 roots have taken up their proper position. In consequence of 

 this change in the position of the pulmonary roots and of 

 the ductus Cuvieri, the cardinal veins arch over the root of 

 the lungs in the same manner as the azygos vein of the adult 

 does. 



At the same time the blastema of opposite sides unites, as 

 has been stated, across and in front of the base of the heart 

 and root of the neck. 



Shortly after this period, the posterior part of the blas- 

 tema, which has now advanced, as already stated, from the 

 sides of the chest to the front of the heart, becomes separated 

 by a narrow neck from the cervical portion. The posterior 

 part has now become the thoracic portion of the thymus, and 

 in the embryo of the sheep is largest on the left side, corres- 

 ponding in this respect to the large size of the left vena azygos 

 and left vena cava at this period. 



The cervical portion of the blastema now begins to exhibit 

 a separation into the thyroid and cervical portion of the 

 thymus. This is effected by the absorption of a portion of the 

 blastema, of a triangular form, a little behind the larynx, the 

 apex looking backwards, the concave base forwards, so that 

 the future thyroid presents a crescentic form, its sides being 

 as yet united to the anterior horns of the thymus, which pass 

 along the jugular veins. 



The thyroid now separates more completely from the 

 thymus, by the prolongation forwards of the absorption pre- 

 viously mentioned from the anterior angles of the triangular 

 portion, so as to separate the thyroid from the anterior horns 

 of the thymus ; at the same time the posterior angle of the 



