PLATE 2 



EXPLANATION OF FIGURES 



5 Epipharyngeal ridge containing primitive thymus p'acode from a 15 mm. 

 larva, d.a., dorsal aorta; tnes., mesenchyma containing vascular spaces; pi., 

 primitive thymus placode; 6.m., basement membrane separating placode from 

 mesenchyma. dV oil immer. obj., ocular No. 3.) 



6 Primitive thymus placode in 15 mm. larva. Im., lymphocyte; 7nes., 

 mesenchyma; v., vacuoles containing protoplasmic bodies, (jo oil immer. obj., 

 comp. ocular No. 12.) 



7 Primitive thymus placode in a 31 mm. larva (ventral surface to the left). 

 ep., nuclei derived from the placode; c.n., connective tissue nuclei; bju.. remains 

 of basement membrane. dV oil immer. obj., ocular No. 3.) 



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