428 JOHN BEARD, 



Were there a loophole for doubt as to the conversion of the epi- 

 thelium of the placodes into leucocytes, this embryo alone would suf- 

 fice to close such completely. The sections of its placodes are of 

 such a convincing character, that no apology need be offered for the 

 inclusion of figures of several of these in the plates. Had the number 

 of these figures been smaller, it might have been deemed advisable to 

 reinforce the story they tell by the inclusion of drawings through the 

 placodes of several R. radiata embryos. However, it has been thought 

 better to give several views from this one embryo, rather than many 

 single drawings from several embryos. 



The conditions seen in embryo No. 443 are not so characteristic 

 of B,. hatis as of R. radiata. I have several embryos of the latter — 

 and of these with the exception of Fig. 50 there are no figures in 

 the plates — which reveal in their thymus-placodes conditions exactly 

 comparable to those about to be described in embryo No. 443. 



The figures from R. hatis No. 443 are Figs. 31—34, 35a and b, 

 and 36. The positions of the various sections figured will be found 

 in the description of plates. Here it may suffice to say, that Figs. 31 

 and 32 are from the first. Figs. 33 and 36 from the second. Fig. 34 

 from the fourth, and Figs. 35a and b from the fifth placode. Of 

 course. Figs. 31, 32, and 33 are drawn under somewhat lower magni- 

 fication than the other four. All the undrawn sections of the various 

 placodes of this embryo exhibit conditions comparable to those seen 

 in the sections illustrated. The chief feature of these figures, and it 

 is one which especially appeals to the histologist, is the sudden con- 

 version of masses of the epithelial cells en bloc into leucocytes, in 

 such a way as to mark them off sharply from the remains of the 

 original epithelium. The result has been, on the one hand, to restrict 

 the epithelium to the dorsal and ventral portions of the placode, and 

 on the other, it is in evidence in the form of small cell-bridges (Fig. 34), 

 cutting up the nests of leucocytes. These things, and the nests of 

 leucocytes themselves, are especially well seen in Figs. 33, 34, and 

 36. Fig. 33 also reveals a break in the placode, out of which leuco- 

 cytes are emigrating. The figures through the fifth placode (Figs. 35a 

 and b) show this to be still in the form of a simple high columnar 

 epithelium. The number of contained leucocytes is here not large, but 

 there are a few epithelial cells taking on the characters of such. In 

 the like degree the conversion of epithelial cells into leucocytes has 

 been seen in no other embryo of R. hatis of this epoch, but, as al- 

 ready stated, this mode would appear to be very characteristic of the 



