557 



this stain does not at all assist the observer in picking out the leuco- 

 cytes. Picrocarmine, on the other hand, possesses the peculiarity of 

 staining the protoplasm of the coloured blood -corpuscles yellow, and 

 very often (i. e. with picric acid-preparation) it does not stain their 

 nuclei at all; with it the leucocytes, wherever they may be, are ren- 

 dered very conspicuous and are characteristically stained. Their nuclei 

 are a brilliant red, and the little protoplasm they usually possess is of 

 a yellowish-brown hue^). In preparations of embryonic blood, stained 

 with picrocarmine, leucocytes, if present, are very easily detected, even 

 with low powers, and this is owing to their brilliant colouring as com- 

 pared with the dull stain taken on by the coloured corpuscles. Even 

 in the mesoderm they are not readily overlooked, if present. 



In studying the sections the two lenses most used have been a 

 Zeiss 2 mm apochromatic and a Leitz V12 oil-immersion. All the 

 sections from embryos of less than 30 ram have been carefully ex- 

 amined under both lenses, and to their use some of the more inter- 

 esting results are mainly due. 



The Origin of the Thymus-epithelium or Placode. 



As already stated, the term "placode", introduced by von Kupffer 

 to define a small piece of modified epithelium, will be used in the 

 description of each bit of epithehum, from which a thymus-element 

 takes its origin 2), In an embryo R. batis of 6 mm (No. 135) of the 

 six gill-pouches four are formed, but not a single one of them has yet 

 an opening to the exterior. There are 60 protoyertebrae or meso- 

 blastic somites present. 



Looking at a transverse section passing through the gill -pouch 

 of the first branchial region it is seen that the dorsal wall of the 

 pouch is mainly made up of a well-defined piece of high columnar 

 epithelium or placode. At the outer side of the pouch this epithelium 

 is sharply marked off from a number of cells of the pouch, destined 

 to be sacrificed when the latter breaks through. On the inner side 



1) This is to be ascribed to the platinum-chloride in Rabl's fluid. 

 After using various preserving fluids for Elasmobranch embryos in the 

 course of the last ten years, and after the examination of hundreds of 

 embryos of Raja and Scyllium, the immense advantages of picro-cor- 

 rosive sublimate and of Rabl's fluid over all other mixtures have clearly 

 asserted themselves. 



2) C. VON KupFFEK, Studien zur Entwicklangsgeschichte des Kopfes 

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



