430 JOHN BEARD, 



VII. The Thymus-Placodes in Embryos of 38—36 mm. 



In the description of this epoch of the development eight embryos 

 will be made use of. This number, once more, is but a selection out 

 of those between 28 and 36 ram in the collection. The topographical 

 conditions are as shown in text-figure G. In the plates there are 

 figures from all eight embryos, but in certain cases only of one or 

 two sections from a particular embryo. Owing to the now much 

 larger sizes of the placodes the number of sections through each one 

 is pretty numerous, indeed, on the average this will not be less than 

 ten through each of the anterior placodes. 



Twenty of the figures belong to this period; and, as they are 

 really only samples of about 800 similar sections, their number may 

 not appear too great. But in order to restrict somewhat the total of 

 the figures, one plate of drawings, chiefly of consecutive sections, has 

 been rejected. Only of two of the eight embryos, Nos. 206 (2S mm) 

 and 209 (34 mm) will the appearances seen be described at all in 

 detailed fashion. 



An embryo of 28 mm does not present much advance on one of 

 25 mm in the sum-total of its characters. The external gills are still 

 very short, the lateral line reaches perhaps half way along the pectoral 

 fin, and the unpaired fins contain mesoderm. 



The figures from R. hatis No. 206 are Figs. 18, 25, 27, 28, 29, 

 and 30. Except Fig. 25 all are from the left side of the body. To 

 the various thymus-placodes belong: to 1 Fig. 28, to 2 Fig. 29, to 3 

 Figs. 18 and 30, to 4 (right) Fig. 25, and to placode 5 Fig. 27. The 

 positions of the sections figured may be gathered from the description 

 of plates. Some at any rate of these figures will recall to the reader 

 things already witnessed in embryo No. 443. Of these may be 

 specially named Figs. 28 and 30. These two figures, of sections of 

 the first and third placodes respectively, both show nests of leuco- 

 cytes. The nests lie well towards the central portions of the placodes, 

 and their dorsal and ventral ends, if not free from leucocytes or cells 

 becoming such, are still largely made up of epithelium. Single leuco- 

 cytes in emigration are still seen, thus in Fig. 28, but now this 

 process is rapidly making place for the emergence of leucocytes in 

 crowds. This is well seen in Fig. 30, where there are two such breaks 

 in the inner contour of the placode, and by these the leucocytes of 

 two or more nests are being passed out into the surrounding mesoderm. 

 Here and there, as in Fig. 30, cells taking on leucocytic characters 



