PLATE 4 



EXPLANATION OF FIGURES 



10 A small portion of a primitive thymus placode and the connective tissue 

 in relation to it in a 31 mm. larva. The lettering a to j shows the line of transi- 

 tion from the typical epithelial nucleus (a) to the completed lymphocyte-like 

 cell (j). (fW oil immer. obj., comp. ocular No. 12.) 



11 Transforming cells from primitive thymus placode of a 31 mm. larva. 

 Cells g and h (not represented in fig. 10), complete the series of transforming 

 cells shown in figure 10. c.t.c, connective tissue nucleus, {j-i oil immer. obj., 

 comp. ocular No. 3.) 



12 Portion of the epithelium of a gill in a 31 mm. larva showing the migra- 

 tion of an epithelial cell («) into a blood vessel. Cells in the left part of the 

 figure (a to /;) show various stages of transformation of the epithelial cell to 

 lymphocyte, ei-y., red blood cells; i, blood cell in vessel in which the nucleus 

 appears to be dividing; j., blood cell in vessel in which there are two nuclei. 

 (xV oil immer. obj., comp. ocular No. 12.) 



13 Epithelial cell beginning migration into blood vessel. dV oil inmier. obj., 

 comp. ocular No. 12.) 



14 Nucleus in primitive thymus placode showing phagocytic properties. 

 (iV oil immer. obj., comp. ocular No. 12.) 



15 Nucleus in primitive thymus placode dividing by simple fission. dV oil 

 immer. obj., comp. ocular No. 12.) 



16 Epithelial cell found in a connective tissue space in dorsal part of epi- 

 pharyngeal ridge. Nucleus beginning to divide. dV oil immer. obj., comp. 

 ocular No. 12.) 



17 Cell found in a connective tissue space in dorsal part of epipharyngeal 

 ridge. Two nuclei in the cell which still retains epithelial characters. (jV oil 

 immer. obj., comp. ocular No. 12.) 



18 Connective tissue nucleus dividing by simple fission. (iV oil immer. 

 obj., comp. ocular No. 12.) 



1G6 



