474 JOHN BEARD, 



Description of Plates. 



The figures are orientated in the following manner. The transverse 

 sections are arranged with their dorsal aspects nearest the top of the 

 plate, the frontal ones with the anterior portion in this position. The 

 asterisk, attached to each figure, denotes its outer side. 



All the figures have been reduced by the lithographer to two- 

 thirds of the following dimensions , which are those of the original 

 drawings. 



The original magnifications are : 

 240 diameters Fig. 57. 



-49, 50—53, 



All the figures were drawn under the Zeiss-Abbe camera No. 44a. 

 Unless otherwise stated, they are of transverse sections. 



The numbers refer to series, row of sections, and number of 

 section in row. 



Descriptive letters. 



c. m closing membrane of pouch s. e sensory epithelium 



I leucocyte S.^9 spiracular thymus-placode 



t.p thymus-placode v blood-capillary 



t trabeculae 



Plate 5. 



Fig. 1. The epithelium of the left branchial pouch of text-figure A. 

 To show the thymus-placode t.p prior to the rupture of the pouch. 

 No. 135 (6 mm), I, 4, 17. 



Fig. 2. The structures enclosed within the quadrilateral to the 

 right of text-figure B. To show the thymus-placode tp etc. just after 

 rupture of the pouch. No. 143 (8 mm), II, 1, 3rd section from the 

 bottom. 



Fig. 3. The thymus-placode of one gill-pouch of a Raja clavata 

 embryo of 14 mm. It consists of a layer of epithelial cells and as yet 

 there are no leucocytes within it. From the second placode of the 

 right side. No. 7, II, 1, 10. 



Fig. 4. A section of the 1. left thymus-placode of an embryo of 

 about 18 mm (No. 345, 130 somites), showing leucocytes and cells in 

 the act of becoming such. No. 345, II, 1, 5th section from the bottom. 



Fig. 5. A section from about the centre of the 2. right placode, 

 showing numerous leucocytes within and to the inner side of the 

 placode, one also in the blood, and epithelial cells of the placode tak- 

 ing on leucocytic characters. No. 189 (circa 20 — 21 mm), II, 6, 1. 



