The origin and histogenesis of the thymus in Raja batis. 431 



are seen, and quite apart from other facts and factors, the existence 

 of these is sufficient to negative any suggestion, that the leucocytes 

 are immigrants, and not emigrants. 



Figs. 18 and 29 reveal very similar conditions. The remaining 

 two figures. Figs. 25 and 27, passing through the fourth and fifth 

 placodes respectively, are of interest as demonstrating the lesser degree 

 of development of these. The latter are still almost entirely epithelial, 

 and, indeed, their degree of development is not greater than it is in 

 the first and second placodes of an embryo of 18—20 mm. Both 

 placodes contain leucocytes and cells becoming such, especially in the 

 dorsal portion of the placode, and in both single leucocytes in the act 

 of emigration are to be found. As will appear, this period marks a 

 turning-point in the history of the placode. 



Raja batis No. 209 measured 34 mm. The olfactory pit is still 

 widely open. In the eye there are no signs of eye-muscles, no pigment- 

 formation, no optic nerve. The auditory organ is still a simple sac. 

 Procartilage is forming on each side of the notochord in the head- 

 region. The lateral line reaches some little distance into the genital 

 region. The spiral valve is small, and there is a much-budded rectal 

 gland. 



The figures from this embryo are: from placode 1 Fig. 17, from 

 2 Fig. 51, from 3 Fig. 67, from 4 Fig. 48, while in Fig. 13 a small 

 portion of the 2. right placode is given under high magnification. As 

 revealed by the figures of the three anterior placodes, most of their 

 component cells are now leucocytes, and epithelial cells are largely 

 restricted to the basal portion of the knob-like structure. In the 4. 

 (Fig. 48) and 5. placodes epithelial cells are more in evidence. In 

 all large nests of leucocytes are the predominant feature of the placode, 

 and leucocytes are now seen, in practically every section through these 

 structures, emigrating in numbers by breaks upon the contour of the 

 placode. One such break has been depicted under high magnification 

 in Fig. 13. To this period belongs also Fig. 56 from embryo No. 214. 

 This reveals under very high magnification (2/3 of 2500) the emigration 

 of one leucocyte from the placode. 



The other figures from embryos of this epoch are Figs. 52, 53, 

 57, 68 and 69, and these again show phenomena in other embryos 

 like those already described. Figs. 68 and 69 are from haematoxylin 

 preparations, and interesting though they be, the reader may also 

 gather from them, that for the study of leucocytes this reagent is not 

 a particularly good one. 



