7:m) 



cleft, with the other reaches to tlie body of the thymus without 

 entering it. 



Fig. 1. Cross section 

 through 2'"^ branchial 

 cleft of a 63 m.m. long 

 embryo of Heptanchus 

 cinereus. In this and in 

 the following figs, the 

 cartilage (stained blue in 

 section) is striated hori- 

 zontally. 



The wall of the duct is 2 cells thick, and is conslituted of a 

 double layer of fairly tiat epithelium cells, amongst which nol a 

 single round cell is to be found. 



The excretory duct of each of the lemaining thymomeres shows 

 a similar structure, viz. a double layered epithelial wall, encircling 

 a lumen, which opens into its respective gill cleft. 



These ducts from the 2""^ caudally, gradually shorten ; the last 

 (6''') forming a rather unimportant, yet distinct attachment to the 

 7''' branchial cleft. 



The excretory ducts are not permanent. They later on lose their 

 epithelial structure and lumen. This e. g. happened in the 225 m.m. 

 long embryo. Here, in (he place of the excretory duct of the first 

 (anterior) thymomere, one finds a long pedicle, which appears as 

 an outgrowth of the thymus. The pedicle runs over the top of the 

 I'** epibranchial and reaches the wall of the branchial cleft. 



It shows ilself as a chord, which appears entirely (o consist of 

 lymphocyt-like round cells. No traces are left of the original epithe- 

 lial structure and lumen. I however do not wish to deny the 

 presence of a reticulum. It would also be possible to make it 

 clear in the pedicle by appropriate methods. 



For completeness the so called epithelial bodies and the supia- 

 pericardial organ should also be mentioned. In the 63 m. m. 

 embryo an epithelial body is found, immediately above the opening 

 of the I'*' and 2'"' thymomere. Each little body is a round isolated 

 celimass, which resembles an acinus of the thymus in form and 



