152 IVAN E. WALLIN 



The activities within the placodes of the full grown larva (120 

 mm.) have diminished and are apparently approaching a condi- 

 tion of cessation. The number of epithelial nuclei has been 

 reduced considerably. Transitional stages may be found, but 

 are quite scarce. Mature lymphocyte-like cells are also pres- 

 ent, but not in great numbers. A definite basement membrane 

 is now present at the connective tissue border of the placode. 

 Figure 9 represents a portion of a transverse section of the 

 placode and the tissue above it in a 120 mm. larva. The sec- 

 tion is taken near the cephalic end of the placode. In such a 

 region a peculiar formation has occurred in the connective 

 tissue, the significance of which I am quite unable to explain. 

 This formation consists of what appears to be red blood cells 

 held in the meshes .of the connective tissue {x, fig. 9). The cells 

 have the morphological characters of the red blood cells. The 

 cytoplasm has a decided yellow tint, while the pale nuclei have a 

 green tint. In some cases what appears to be the nuclei have 

 morphological characters similar to the lymphocyte-like cells. 

 These formations are present in the periphery of the whole con- 

 nective tissue. A section through the central part of the plac- 

 ode would show the same character that was indicated in the 63 

 mm. larva, that is, a central core of connective tissue in which there 

 are no 'vascular channels' surrounded by a 'vascular area.' 

 The tissue between the 'vascular channels' in the 120 mm. larva 

 consists entirely of the peculiar tissue just described. 



Development of lymphocytes 



A brief description of the general development of lympho- 

 cytes in the petromyzon larva is here given since the nature of 

 this formation in the advanced larvae has a. direct bearing on the 

 interpretation of the histogenetic processes in the above de- 

 scribed placodes. My observations do not include the first 

 appearance and development of the blood in the embryo, but 

 begin with the development in the 5 mm. larva. The nature 

 of the blood formation in this stage of larval development need 

 not be described here for the reason that it occurs at a time when 

 the placodes have not begun to form. However, in larvae 



