PLATE 3 



EXPLANATION OF FIGURES 



8 Primitive thymus placode in a 44 mm. larva. Ventral surface to the 

 right. (,'3 oil immer. olij., ocular No. 3.) 



9 Portion of primitive thymus placode in a 120 mm. larva, b.m., base- 

 ment membrane; .r., cells which are apparently red blood cells held in the con- 

 nective tissue. (iV oil immer. obj., ocular No. 3.) 



164 



