478 JOHN BEARD, 



at the upper end the sensory epithelium s. e. of a portion of the vagus. 

 No. 203, VII, tenth section. 



Figs. 40 and 41. Represent respectively the fifth of ten sections 

 through the 1. left placode and the eighth of eleven through the 2. left 

 placode of an embrj^o of over SO mm. They are drawn under lower 

 power to demonstrate the emigration of leucocytes en masse from the 

 placode and their passage into the blood. In both epithelial cells are 

 almost entirely restricted to the base of the thickened placode. The 

 conditions here resemble those in text-figure G. No. 208, V, rows 2 

 and 5, sections 7 and 5. 



Plate 8. 



Fig. 42. From a frontal (horizontal) series of a 25 mm embi'yo. 

 The section passes through the 3. right placode t.p, and adjoining its 

 upper portion and abutting upon this is the extension of the sensory 

 epithelium s.e of part of the vagus. In the placode t.2J there are not 

 many fully formed leucocytes, but as forerunners of such epithelial 

 cells taking on leucocytic characters. In the sensor}' "placode" s. e 

 there are very similar cells, which, however, are becoming nerve- or 

 ganglion-cells. No. 203, VIII, third section. 



Fig. 43. The fourth section of eight through the 2. right placode 

 of an embryo of 23 mm. The placode is still little more than a single 

 layer of epithelial cells, but there are some leucocytes within it, one of 

 which is in emigration. Especially in its upper portion there are man}»- 

 cells taking on leucocytes characters. No. 619, II, 1, fifth section from 

 the bottom. 



Fig. 44. The seventh section of thirteen through the 1. left placode 

 of an embryo of 27 mm. The epithelial cells of the placode now form 

 more than one layer, the placode contains leucocytes and cells becoming 

 such, and at one point there is a break, at which leucocytes are 

 wandering out. No. G29, II, 5, 7. 



Figs. 45 — 46. Represent under different magnifications a section 

 through the 2. left placode of an embryo of 26.5 mm. While in Fig. 46 

 the thymus placode is depicted, in Fig. 45 the topographical relation- 

 ships of the thymus-placode and sensory epithelium of a branch of the 

 vagus are shown. The ending of the cellular nerve in the sensory 

 epithelium s. e is seen, and also the course of the latter, until its some- 

 what pointed end abuts upon the thymus-placode, can be followed. In 

 both figures in the thymus-placode there are numerous leucocytes, some 

 of which are in emigration. Compare also text-figure F. No. 616, II, 

 last row, 11. 



Fig. 47. From the 2. left placode of an embryo of 25 mm. To 

 show a nest of leucocytes, and the aperture of emigration of these. 

 No. 201, III, 4, 9. 



Fig. 48. The median section of the 4. right placode of an embryo 

 of 34 mm. Epithelial cells are still largely represented, but there are 

 many leucocytes and cells becoming such, and in one part of the 



