The origin and histogenesis of the thymus in Raja batis. 423 



entire ventral portion of the placode consists of a simple epithelium 

 of one layer of high columnar cells. 



From R. hatis No. 190 the figures are Figs. 9 and 10, representing 

 respectively median sections through the first right and left placodes. 

 The things revealed by them are very like those seen in Fig. 5. Under 

 higher power a section through the first right placode of an embryo 

 of 21 mm (No. 617) is given in Fig. 26. The greater portion of the 

 placode is made up of about two layers of high columnar epithelial 

 cells, among these being some taking on the characters of leucocytes, 

 and one or two fully formed leucocytes. Of the latter one has been 

 caught in the act of emigration. Quite similar conditions are seen in 

 Fig. 8 from an embryo of barely 22 mm (No. 192). The final figure 

 from embryos of this epoch is Fig. 43. This is from K hatis No. 619, 

 whose size was noted as 23 mm. The figure represents the fourth 

 section of eight through the second right placode. From the two 

 mitoses the cells of the placode are evidently in activity. For its 

 greater portion it cannot be said to consist of more than one layer 

 of epithelial cells, and this is especially true of the ventral part. 

 Leucocytes and cells becoming such are confined to the dorsal half. 

 At one point there is a leucocyte in the act of emigration, and at 

 another a space in the placode, out of which a leucocyte has evidently 

 recently wandered. 



Summing up, in embryos of 19—23 mm while the production of 

 leucocytes in the placodes is not very great, it is evidently rapidly 

 increasing. The epithelium is still comparatively simple, especially at 

 the ventral end, where it hardly consists of more than two layers. 

 Leucocytes, when fully formed, wander out singly, and many times a 

 single one has been caught in the act of emigration. 



V. The Mode of Formation of Leucocytes from epithelial Cells 



of the Placodes. 



The mode, in which the epithelial cells of the placodes become 

 converted into the first leucocytes, can be best observed — under 

 high magnifications — in the thymus-placodes of Raja hatis embryos 

 of 17 — 23 mm, as well as in somewhat older ones. As elsewhere 

 already recorded, the process is exactly comparable to that, by which 

 the original epithelial cells of the brain or spinal cord become changed 

 into ganglion-cells. Or again, it resembles the origin of ganglionic 

 elements of cranial ganglia from the sensory placodes of the head- 

 region. 



