I. The early History of the Thymus-Placodes. 



As will be evident from the following account of the early devel- 

 pment, each thymus-element is at first represented by a small well- 

 defined flattened plate, restricted in area, and made up of one layer 

 of epithelial (hypoblastic) cells. For the description of such a plate 

 of cells, destined to give rise to a specific tissue or organ, a term 

 has long been needed: by the introduction of the word "placode" this 

 want has been supplied by v. Kupffer^), Although he used this 

 name to describe the sensory epithelium, forming the basis of a 

 portion of a cranial ganglion and also of the connected sense organs, 

 it does not appear from his account, that the Munich Professor desired 

 to confine the use of the term to a plate of neuro-epithelium. The 

 term is too convenient for any such restricted application. 



It should be stated, that the details concerning the origin of the 

 thymus-placodes in Raja have only been worked out in full for one 

 cleft, the first branchial. This was chosen owing to its proximity to 

 the auditory organ, a circumstance which enables one to find it in 

 any series with speed and accuracy. But, it may be added, for the 

 remaining four clefts, and also for the spiracle, the development of 

 the placode in each case is the same; sufficient attention was paid 

 to all of them to render this certain. Indeed, it was the circumstance, 

 that the early history of the spiracular thymus-placode so closely 

 resembled that of a true thymus-placode, which finally brought to 

 light the facts concerning the spiracular thymus. 



1) V. KuPFFER, C, Studien zur Eutwicklungsgeschichte des Kopfes 

 der Kranioten, 1894, Heft 2, p. 64—65. 



