DEVELOPMENT OF THE THYMUS 329 



mediary cords is considerably greater than in the preceding 

 stages. Lobules are now present along their entire extent. They 

 are a little shorter in this stage than in the 63 mm. embryo while 

 the mid-cervical segment is somewhat longer. The cervico-tho- 

 racic cords are short and lie closely together. The thoracic seg- 

 ments are thin and flat and spread out over the pericardium to 

 the left of the median line. They extend only a short distance 

 to the right beyond the median line. 



Embryos of 105, I40, 170 and 280 mm., and pig 1 day post 

 partem (text figures A, B, C, D and E, respectively). In these 

 stages the structures covering the thymus were removed and 

 the entire organ on the left side, undisturbed, was exposed to 

 view. Diagrammatic drawings, representing accurately the out- 

 line of the lateral aspect of the different regions of the thymus, 

 were made. In all cases specimens were selected in which the 

 connecting band on both sides looped over the hypoglossal nerve 

 and connected the superficial thymus with the thymus head. 



By referring to the figures cited above it will be seen that the 

 comparative size of the superficial thymus {T.s.) and the thymus 

 head {C.t.) vary somewhat in different developmental stages. 

 In general, the proportional size of the former to the latter is 

 greater in earlier than in later developmental stages. From 

 numerous dissections that were made it was found that the 

 comparative sizes of the two structures vary considerably in 

 embryos of about the same developmental stage or even in those 

 of the same litter. Figure A represents about the average com- 

 parative size of the superficial thymus and the thymus head in 

 embryos of about 105 mm. in length, while the size of the super- 

 ficial thymus of an 140 mm. embryo as represented in figure B 

 is considerably larger than it ordinarily occurs in corresponding 

 developmental stages. Variations in size of the superficial thymi 

 in the same embryo also occur; e.g., the right one in a 170 mm. 

 embryo was an oblong flap that covered the anterior one-fourth 

 of the lateral surface of the thymus head, while the left one is 

 much smaller as represented in figure C. In all the embryos 

 examined the superficial thymus was always closely associated 

 with the thymus head but never fused with it. 



