426 JOHN BEARD, 



As Raja batis No. 201a is perhaps slightly the youngest of the 

 series, the conditions here may be described first of all. In the 

 plates are four figures, one through each of the four anterior pla- 

 codes of the right side, the fifth placode is still entirely epithelial. 



The order of these is as follows: Fig. 23, placode 1; Fig. 15 2; 

 Fig. 22 3 ; Fig. 24 4. The comparison of the four figures is interest- 

 ing; for, while in those of placodes 1 and 2 the original single epi- 

 thelium has become much thicker, and many of its cells are now 

 leucocytes, in placodes 3 and 4 there are few leucocytes, more especi- 

 ally in the latter, and the epithelium of the placode has departed 

 little from its original simple form. In placodes 1 and 2 cells in the 

 act of becoming leucocytes are outnumbered by fully formed leucocytes : 

 in 3 and 4, on the other hand, there are many epithelial cells, 

 engaged in conversion into leucocytes. And, whereas in the latter two 

 placodes leucocytes, if in emigration, are met with only singly, as in 

 Fig. 24, in the two former, while one may still meet with single leuco- 

 cytes caught in the act of emigration, the more usual process is the 

 passage of leucocytes en masse from the placode, thus causing "breaks" 

 of greater or less extent in its contour. 



As shown in the figures under discussion, and in those of other 

 embryos to be afterwards referred to, the placodes are still largely 

 epithelial, more especially in their ventral portions. 



Conditions similar to those described in the foregoing embryo are 

 seen in R. batis No. 633 (24.5 mm). Fig. 49 represents a section 

 through the first right placode of this embryo. At one point in the 

 placode there is a nest of leucocytes, in other portions of the struc- 

 ture there are single leucocytes, and also cells becoming such, and one 

 has been caught in the act of emigration. 



The figures from another embryo of 25 mm (No. 202a) are Figs. 11 

 and 12. These respectively depict the median section through the 

 first left and the first right placode. The degree of development 

 here is about that already seen in embryo No. 201a. The placodes 

 are still largely epitheUal, fully formed leucocytes are not numerous, 

 and there are many epithelial cells taking on the characters of 

 leucocytes. 



A contrast to the figures from the preceding embryos is afforded 

 by several through the placodes of two similar embryos, cut in 

 frontal (horizontal) sections. These are Nos. 202 and 203. The figures 

 from the former are Figs. 14, 37, and 38, and from the latter Figs. 16, 

 39, and 42. 



