732 



and this little group more atrophied. He Ihonght he saw an indica- 

 tion of an antiniere of the left vesicle on the right side of the body, 

 in the shape of a more caudal!} situated diverticnlun) of the 

 branchial gut. 



Let IIS however return to the thymus. The yenus Heptanclms is 

 indeed rightiy regarded as the niosl primitive of liie living Sela- 

 chians. The nninber of visceral pouches (i. e. 8) surpasses that of 

 all other fishes and higher animals. Only the anterior 5 are still 

 formed in mammals. 



Concerning the 63 m.m. long embryo of HeplanchiiS, we may 

 now assume, that al.so its thymus ap|)ears in a more primitive form 

 than in the development of higher animals. 



The original function of the tliymiis could then not have been 

 internal secretion only, bm it must also have removed products 

 through its excretory ducts. 



Originally each thymomere was a true gland, according to the 

 old notion, with an excretory dnct even as was the case with the 

 thyroid and the anterior lobe of the hypophysis. 



The presence of excretory ducts is also of importance for the 

 conception of the morphological significance of the gland. Since the 

 leseaiclies of Dohun, it is generally accepted that the thymus is a 

 hranchiomere organ, a division of which occurred on each branchial 

 cleft. 



Now Amphioxus has on each of its many branchial clefts a 

 glandular body, which opens with its excretory duct into the top 

 of the cleft. This bianchionepiiros functi(ms as an excretory organ, 

 and for many years I have presumed, that it would prove homo 

 logons to the thymus of higher animals. 



This presumption was strengthened, when in 1909 Goodrich found 

 that the branchionephros does not develop from the coelomic epi- 

 thelium, as one would rather be inclined to assume for an excretory 

 organ in chordates. 



But he does not state that it develops from the branchial epithe- 

 lium. His drawings however give this impression. Might this 

 impression prove . to be correct by later investigations, then the 

 branchionephros develops from the same tissue as the thymus of 

 higher animals. Cells resendiling lymphocytes are never found in it. 

 Lymphocytes do not occur in the blood of Amphioxus, the blood of 

 which only consists of plasma, without any red or white blood 

 corpuscles, just as the blood in its earliest stage in craniates. ') 



M A few investigators profess to have found cells in the blood of Amphioxus. 

 I have never observed anv in mv numerous sections of larvae and adult animals. 



