422 JOHN BEARD, 



other cells becoming such are seen. With reference to Fig. 6 it naay 

 be here noted, that the formation of leucocytes is at first more 

 prominent in the dorsal than in the ventral portion of the placode. 

 In this period the formation of leucocytes is scanty, and but few fully 

 formed ones are ever seen in the placodes. In the mesoderm and 

 blood-vessels the number of such cells is at the close of this period 

 not a large one. Sometimes, and this is not shown in the figures, 

 sections of the placode, containing more cells taking on the characters 

 of leucocytes, may be met with. 



IV. The Thymus-Placodes in Embryos of 19—23 mm. 



Of the characters of embryos of this epoch the following may 

 be mentioned. The muscle-buds for the paired fins are in course of 

 differentiation. There are as yet no traces of unpaired fins. Until 

 the embryo is about 21 mm in length the neurenteric canal is some- 

 times open, afterwards it is closed. Externally, the pectoral fins are 

 slight, the pelvics very slight. The lateral line does not extend beyond 

 the gill-region. The external gills are but small. 



The condition of an anterior placode during this period is as 

 shown in text-figure E. It increases somewhat in thickness, but not 

 in length during this interval. Of embryos lying between 19 and 

 23 mm figures from five. Nos. 189, 190, 617, 192, and 619, will be 

 found in the plates. The figures are: Figs. 5, 19, 9, 10, 26, 8, 

 and 43. 



The figures from R. batis No, 189 are Figs. 5 and 19. Fig. 5 

 represents a section through about the centre of the second right 

 placode. In it there are some cells becoming leucocytes, as well as 

 fully formed ones. Of the latter one has been caught in the act of 

 emigration. There are a few leucocytes in the mesoderm of the 

 neighbourhood of the placode, and one in the section of the blood- 

 vessel to the inner side of the thymus. In this figure and in Fig. 19 

 the epithelium of the placode is still comparatively simple, consisting 

 of not more than two layers of epithelial cells. Fig. 19 depicts the 

 median section through the first left placode of the same embryo, 

 drawn under the 2 mm apochromatic, and the same section is outlined 

 in text-figure E. In the placode there are one or two leucocytes and 

 many cells taking on the characters of such. Of about this period is 

 Fig. 21, taken from a R. radiata embryo. It shows very clearly the 

 limits of the placode, within which to its dorsal side there are two 

 or three leucocytes and several cells taking on such characters. The 



