THE THYMUS 327 



arises, as in the lower groups of vertebrates, from the dorsal 

 gill-pockets, in the lizard from II. and III., in snakes from 

 IV. and V. 



4. In birds, thymus buds have been described from third, 

 fourth, and fifth clefts. 



5. The thymus of mammals apparently differs very materi- 

 ally in many respects from that of all lower animals, inasmuch 

 as it is much more complex, and it is not the dorsal, but the 

 ventral, pockets of the gill-clefts which furnish its rudiment. 

 In most cases the third cleft is the most important, but some- 

 times the fourth, and occasionally, also, the second, plays 

 a part. This portion of the origin of the gland is entodermal 

 (see Fig. 90). 



Recent work has confirmed the observations of Kastschenko 

 as to the occurrence of an ectodermal component, derived 

 from the " ductus prsecervicalis," in the thymus of certain 

 mammals. This conception must have a far-reaching effect 

 upon our views as to the origin and nature of the mammalian 



EXPLANATION OF FIG. 90. 



(Diagrams A. and B.). 



A. illustrates the development of the branchial organs of mammals, B. shows 

 their actual relations in the adult. 



The different related rudiments of the same branchiomere are represented 

 by a similar direction of shading lines ; so also the corresponding organs. Thus 

 the rudiments from the third cleft are represented in A. by horizontal lines, 

 as also the organs thus arising in B. The rudiments and organs from the 

 fourth cleft are characterized by vertical lines. The post -branchial body is 

 shown in thick outline, the thyroid by crossed lines. 



The different kinds of tissue arising from one and the same branchiomere 

 are indicated by differences in the shading. The parathyroid tissue is shown 

 by lines, the thymus tissue by alternate continuous and interrupted lines. 

 The post- branchial body is represented in the developed condition as a hollow 

 space with several glandular nodules (shown in dark circles). 



A. shows the four internal gill slits (I. to IV.), the epithelial origins of the 

 parathyroids (p. thyr. III., p. thyr. IV.), the origin of the thymus (thym. III., 

 t]\ym. IV.), the rudiment of the thyroid (thyr.), and that of the post-branchial 

 body (p.b.b.). 



B. represents a schematic transverse section through the fully developed 

 thyroid (at about the level of the junction of the upper and middle third of 

 the thyroid lobe of a cat). The lettering corresponds to that in Diagram A. 

 The structures which arise from the third cleft become the " external " para- 

 thryoid and thymus nodule (separated fragment of thymus III.) ; these which 

 arise from the fourth cleft become the "internal " parathyroid and the thymus 

 nodule (p. thyr. IV., and thym. IV.). The post-branchial body is surrounded 

 by thyroid tissue. 



(A. is after Groschuff in ruminants ; B. from Kohn in the cat.) 



