The origin and histogenesis of the thymus in Eaja batis, 421 



14 — 17 mm. The general impression of the whole course of the de- 

 velopment, gathered by the writer, is, that of an orderly well-regu- 

 lated series of events, as in similar words E. B. Wilson has already 

 remarked. Nothing happens by chance : all is a pre-established or pre- 

 determined harmony. 



In nearly everyone of my publications issued since 1894 I believe 

 this to have been recognised in some form or other. The whole 

 history of the transient nervous system bears this character, and it 

 is again presented in the descriptions of the critical period, of the span 

 of gestation, the formation of the primary germ-cells, the determin- 

 ation of sex, and the numerical law of the germ- cells. 



The sizes of embryos are misleading, quite apart from the pos- 

 sible existence in the skate of differently sized embryos corresponding 

 to the future males and females. 



The correlation of phenomena in the development is really 

 remarkable. Did the present state of other problems, occupying all 

 the writer's spare time and attention, admit of it, the demonstration 

 of this for Raja and Scyllium would be interesting and instructive. 

 My experiences of Elasmobranch development — and they are now 

 neither new nor narrow — go emphatically to support Keibel's atti- 

 tude in this matter, as against the assertions of Mehnert. If the 

 variations described by the latter obtain, then he is dealing with ab- 

 normalities, induced by the mode of cultivation employed. Under 

 normal circumstances the individual variation is, to my mind, an in- 

 significant factor. 



When in the embryos of Raja batis the connection of commencing 

 histogenesis in the thymus-placodes is sought for, it may perhaps be 

 found in the formation of the last gill-cleft, and with this the practical 

 completion of the laying-down of mesoblastic somites. Some of these 

 latter may not as yet be segmented off, but practically the proliferation 

 of "mesoderm" to form them is about finished. With embryos, there- 

 fore, of 125—140 somites and six gill-clefts the histogenesis of the 

 thymus-placodes may be said to be initiated. 



From embryos of 6 mm up to such of this period there are in 

 the plates figures from six, these being Figs. 1, 2, 20, 3, 4, and 6, 

 The first four of these relate to the formation and characters of the 

 placodes in early embryos. In them the epithelium is simple and 

 contains no leucocytes. In Figs. 4 and 6 the histogenesis of the 

 thymus -placode has already started, in the latter there is one 

 leucocyte, while in the former three fully formed leucocytes and two 



