567 



break in each section, and in some of them single leucocytes are 

 emigrating here and there. 



It would unduly increase the size of this communication to give 

 an account of the emigration of leucocytes from the thymus-elements 

 of all the specimens, for the results are based on the examination of 

 about twenty embryos of sizes from 25 to 42 mm. In almost every 

 section through every thymus-placode of all the embryos there are 

 breaks, and it is quite out of question to suppose for a moment that 

 they are artefacts. The mere recital of all the breaks, apart from 

 other details, would itself occupy much space, and, moreover, they 

 are not yet fully recorded in my notes. 



But the evidences of the emigration of leucocytes from the 

 thymus-elements of Raja, as afforded by my sections, are quite 

 overwhelming. In a working experience of over eighteen years in 

 vertebrate embryology I can recall no fact more clearly and convin- 

 cingly demonstrable than this. 



Many years ago it fell to me to describe for the first time the 

 origin of elements of the cranial ganglia of vertebrates from the 

 neuroepithelia of the branchial or lateral sense organs. That dis- 

 covery is now generally accepted. The proliferation of such ganglionic 

 elements can be observed in many of the very sections of Raja 

 embryos, in which the emigration of leucocytes from the thymus-ele- 

 ments obtains. But — and this is why the origin of lateral ganglionic 

 elements is recalled — it is a matter of ease to produce far more 

 sections exhibiting this emigration of leucocytes than such showing 

 the prohferation of ganglionic elements. The reason is simple. The 

 thymus elements of Raja are nearly as numerous as its ganglia, and 

 the process goes on in them for a much longer period than it does 

 in the neuroepithelia. 



Later History of the Thymus. 



This part of the subject will be treated of here only in so far 

 as it relates to embryos of 33 to 71 mm. In the collection there are 

 now numerous preparations from the thymus-elements of specimens 

 above this latter size up to those of 19 centimetres; that is to say 

 up to young skate about to leave the egg-capsule. But these latter 

 do not call for detailed treatment here, for the thymus of an embryo 

 of 71 mm has practically reached the adult condition. 



Moreover, in previous publications it has been put on record i). 



1) J. Beard, History of a Transient Nervous Apparatus etc. Zool. 

 Jahrb., Anat. Abteil, Bd. 13, 1895. 



