72y 



be selfk'd hv the siiidy of rlie fi8 ni.iii. long erabiyo of Heptanohus, 

 hut a further qiieslion can be explained thereby, viz. whellier the 

 thyiims has to be considered as a gland whirh has lost its original 

 excietory duet and thns only has internal secretion left. It wonld 

 then find itself in a siniilar condition as the anterior lobe of the 

 hypo|)hysis and the thyroid gland, \vhich, however, in the embryo 

 of vertebrates, always have an excretory duct which is only lost 

 (inring the fnrtlier coarse of development. 



Tlie thymus does not sever itself from the epithelinm of the 

 branchial gnt in (Jyclostomes and most of the bony fishes. This is 

 however the case with the remaining vertebrates. But a true ex- 

 cretory duct, as a rnle, does not appear. This wonld be expected 

 in sharks, but P'ritsche (1910i says: "Ein Lumen nnd einen Aus- 

 führnngsgang habe icli bei Spinax ebensowenig aufïinden könneii 

 wie DoHRN bei seinen Haifischen." 



in u very early stage of rays (Torpedo), they however noticed 

 something which resembled an excretory duct. 



In some of ilie sharks examined up to now, the body of the 

 tliymus separates itself directly, without a pedicle, from the epithelium 

 of the l)ranchial gut; while in others it still remains connected 

 for some lime by a stalk to the epithelium. 



Tills stalk lacks the characteristics of an excretory duct, because 

 it not only has no lumen, but also shows the same structure as liie 

 body of the thymus and consists almost exclusively of the rounded 

 cells, which resemble lymphocytes. 



in our embryo of Heptanchus we on the contrary find an ex- 

 cretory duct in optima forma for each of the thymus divisions 

 (thyinomeresi which are found on both sides of the body, one for 

 each side from the second to the seventh branchial cleft. There are 

 8 gill clefts, but in the first (spiracle) and last the thymus is 

 absent. 



It is the largest in the second and third cleft and has the form 

 of a bunch of gi'apes. The bunch is smaller in the 4''' cleft, in the 

 ö'h still smaller, and in the 6''' tiie thymus no longer has the bunch 

 form, but is composed of a .single acinus, into which the excretory 

 duct opens. 



In the 7''' cleft every acinus is found missing from the short 

 excretory duct. 



In the figure of the .section we see the large thymus of the 2"*^ 

 branchial cleft. If runs over the top of the 1=*' epibranchial and 

 then continues as the fairly long excretory duct. This has an obvious 

 lumen, which with its one end opens at the top of the branchial 



48 



Proceedings Royal Acad. Amsterdam. Vol. XXVI 



