412 



JOHN BEARD, 



in Fig. 1, PI. 5, under high magnification. At the outer side of the 

 pouch the epithelium of the placode is sharply marked oti' from a 

 number of cells of the pouch and of epiblast, cm, destined to be 

 sacrificed, when an actual cleft is formed by rupture. On the inner 

 side the placode of the dorsal wall is as well marked otî from the 

 much flatter epithelium of the pharyngeal hypoblast. 



The size of the next embryo to be noted, B. batis No. 143, is 

 given in my list as 10 mm, but with 86 somites and 5 gill-pouches 

 it is classified among the embryos ranging from 8—10 mm. Text- 

 figure B and Fig. 2 relate to this embryo. On the right side of text- 

 figure B a quadrilateral has been inscribed around certain structures, 

 and these are depicted under higher magnification in Fig. 2. On both 

 sides of the body rupture of the pouch has now been eiîected, and in 

 Fig. 2 the closing membrane, cm, is shown still attached to the outer 

 end of the thymus-placode. By rupture of the former the placode 

 has become turned slightly upwards and outwards. Its epithelium is 

 as sharply marked off from neighbouring structures as before. 



Raja batis Nos. 163 and 164 are 

 similar in size and other characters. Of 

 No. 163 certain characters will be given, 

 and from No. 164 text-figure C is taken. 

 There are in the plates no figures from 

 either embryo, but the placode under 

 consideration in each embryo is practically 

 identical in minute structure with the one 

 of Fig. 2. 



R. batis No. 163 is about 10 mm in 

 length and 101 somites were counted. The 

 spiracle and the first two branchial clefts 

 are open, the remaining three being re- 

 presented by pouches. 



As shown in text-figure B, the same 

 piece of modified epithelium or placode, 

 t.j), is still obvious, just above the opening of the first branchial cleft. 

 It is now turned upwards and outwards, in such a way, that, as 

 compared with its former position before the opening of the cleft, its 

 upper and dorsal end has been raised through an angle of about 45 ^. 

 It has thus acquired a position, which makes it appear to be a 

 portion of the epiblast. And, be it added, disregarding its earlier and 

 later histories, from now until the embryo is 18—20 mm in length it 



c/n 



