The origin and liistogenesis of the thymus in Eaja batis. 



411 



The earliest embryo, to which the thymus-placode has yet been 

 traced, is one of about 6 mm. It is possible, that in some one or 

 more of the early embryos recently sectioned, i. e., within the last 

 two years, it might be traced to still earlier phases. These newer 

 embryos, of which a large number has been prepared for other pur- 

 poses, have not been examined as to the thymus-placodes, and, like 

 most of the results of the present writing, the description of the early 

 history of the placodes is given as made out in embryos of older date 

 than 1899 onwards. 



In Eaja hatis No. 135 (6 mm) of the six gill-pouches four are 

 formed, but not a single one of them has yet an opening to the ex- 

 terior. 60 mesoblastic somites were counted posterior to the last 

 formed gill-pouch i). 



The figures from this embryo are text-figure A and Fig. 1, PI. 5, 



Looking at a transverse section (text- figure A), passing through 

 the gill-pouch of the first branchial region, it is seen, that the dorsal 

 wall of the pouch is mainly made up of a well-defined piece of high 

 columnar epithelium or placode, tp. The placode and adjacent 

 structures of the left side of text-figure A are accurately represented 



1) In all my embryos, where the approximate number of somites 

 is given, this is usually based upon the results of several counts, and 

 it is always the total posterior to the last well-defined gill-pouch or 

 cleft. The final number of somites is very large in R. haiis, as many 

 as 140—150. 



