The origin and histogenesis of the thymus in Raja batis. 



441 



thymus-placode. He blamed the writer for confusing together branchial 

 and lateral sense organs, but he himself failed to distinguish between 

 sensory placode and thymus-placode. 



Why Froriep should have commenced his description with an 

 embryo of 12 mm, for Torpedo an advanced period, is not easy to 

 understand. Had he from this point traced the developmental history 

 backwards to earlier phases, his conclusions would have been different. 

 In text-figures A — F of the present writing the sensory placode is 

 not shown, either because in the particular section outhned it is not 

 a prominent structure, or because the section contains little or nothing 

 of it. When the text-figures were originally drawn, the sole object 

 in view was the thymus-placode, and there existed no intention of writing 

 anything about the sensory ones. 



In text-figure H a schematic 

 section through a gill-pouch is 

 given. The hypoblast of the gut, 

 that of the thymus-placode of 

 each side, and that of the closing 

 membrane will be clearly made 

 out. Below the latter is another 

 modified hypoblastic placode, not 

 hitherto referred to anywhere in 

 the text. Some time back the 

 history of this placode was partly 

 followed, and only the cropping- 

 up of new problems in other 

 directions prevented the com- 

 pletion of this quest. It may be 

 termed the gill-placode {g.p). For 

 whether it be also concerned in 



the formation of the internal gills — a point not yet determined, as 

 these develop very late — it is certainly the structure, from which 

 the external gills are derived. In the other text-figures its limits are 

 not shown ; but, like the thymus-placode, the gill-placode on rupture 

 of the pouch in part wanders outwards and downwards (by extension), 

 and thus comes to take up somewhat of an "epiblastic" position on the 

 gill-arch. At a later period the long external gills of Raja are pro- 

 cesses of it. As already remarked its full history has not been traced. 



In the text-figure H another placode is shown above the pouch: 

 this is the sensory placode s.p^ or original branchial sense organ. It is 



