THYMUS-LIKE STRUCTURES IN AMMOCOETES 145 



general epithelium of the branchial cavity has also acquired a 

 new character in this stage. Beneath the placodes and in 

 direct contact with them the peribranchial blood channels are 

 forming. It is difficult to distinguish an endothelial lining of 

 these channels in all cases. 



The degeneration of the placode has progressed farther in a 

 15 mm. larva (fig. 4). In places the cytoplasm has taken the 

 stain very faintly.. Scattered about in the placode are streaks of 

 cytoplasm which are very deeply stained. Vacuoles are also 

 present. The nuclei appear to be fewer in number than in the 

 earlier stages. They appear more constant in their general 

 appearance and chromatin content. The amoeboid character 

 of the nuclei has also become more prominent. Some nuclei 

 have taken up positions at the surface of the placode and the 

 cytoplasm appears to be cutting off a layer of flattened cell (fig. 

 4, s.L). The formation of a layer of flat cells at the surface of 

 the general epithelium was begun in a much earlier stage of 

 development. 



The changes which occur in these placodes in older larvae 

 approximate the character represented in the 63 mm. larva 

 (fig. 1). It is a significant fact that cells in mitosis have not 

 been seen in any stage of the development of these placodes. 

 Furthermore, patches of epithelium giving the appearance of a 

 degeneration are present in various other parts of the branchial 

 lining, especially at the lateral attachments of the gill septa. 

 Lymphocytes are present in the placodes in the older larvae, 

 but they are also present in the general branchial epithelium. 

 They do, however, occur in greater numbers at these placodes. 



From these brief descriptions it is apparent that these placodes 

 do not represent active anlagen of a future structure. Their 

 development and structure do not suggest anything which might 

 indicate their significance. 



The lymphocyte accumulations in relation to the above de- 

 scribed placodes are contained within vascular channels. These 

 vascular channels contain red blood cells, and as has been shown 

 by Mozejko ('10) and others they are in communication with 



THE AMERICAN JOURNAL OF ANATOMY, VOL. 22, NO. 1 



