554 



est idea whence they came. Koelliker's results on the thymus, proving 

 that it produced leucocytes, were remembered, as was also my own 

 confirmation of them. The comparatively early phase ^) at which leuco- 

 cytes appeared, did not lend colour to the idea, that in the skate and 

 dogfish the thymus could have much to do with the matter. For it 

 was recalled, that in such embryos the thymus -elements were then 

 nothing more than small pieces of modified epithelium. Although the 

 thymus was not a priori a very promising subject for new research, 

 there seemed to be none more likely. At such phases the spleen had 

 no existence, there was no rectal gland or coecum, and, in fact, lym- 

 phoid structures of any and every sort were entirely wanting. For 

 reasons connected with the illustration of the projected memoir on 

 Scy Ilium-), this form was now for a time placed aside, although 

 later on the embryos of it were more than once studied. In the pre- 

 sent communication it is proposed to confine all further remarks to 

 Raja batis, not because the facts made out in Scy Hi um are at 

 all opposed to those to be detailed concerning Raja, but because the 

 embryos of the latter form present pictures of a more beautiful de- 

 scription, easier to understand, and much better suited for demon- 

 stration, than such from the former. In Raja, and, so far as I have 

 studied the matter, this holds for the species R. radiata and R. 

 clavata equally with R. batis, the numerous preparations in my 

 possession are so clear and decisive, that they will bear exhibition 

 alongside any preparations, which can be produced, showing the de- 

 velopment of the thymus and the first origin of leucocytes in the 

 vertebrate body. 



As it is intended that this communication should be followed, 

 without undue delay, by the publication of a more extended memoir, 

 with numerous figures, it may not be necessary to discuss the litera- 

 ture more fully at this stage. And, moreover, there is not very much 

 of it to call for extended quotation; for, beyond the researches of 

 those already mentioned, there exists hardly any certain knowledge 

 concerning the first origin of leucocytes and the histogenesis of the 

 thymus. 



1) Early only as regarded from the point of view of my work, for 

 this extends to periods not usually dealt with in embryological in- 

 vestigation. 



2) It had been necessary to set aside a complete set of the em- 

 bryos for delineation by a skilled artist; and, until the figures were 

 finished, it was out of question to make the series of sections of Scyl- 

 l i u m quite complete. 



