144 IVAN E. WALLIN 



Schaffer ('94) and later described more in detail by Giacomini 

 ('00, 1 and 2) and Castellaneta ('13). The origin of these 

 lymphocytes as well as the cause for their accumulation in these 

 places has not been determined and consequently constitutes a 

 problem to be solved in the consideration of a possible thymus 

 structure in this animal. The descriptions in the literature 

 have been limited to single stages of development and are con- 

 sequently incomplete. The following descriptions are based on 

 my own material and includes the essential developmental 

 stages : 



The branchial cavity of a 5 mm. larva represents a very simple 

 condition. The gill pouches are present as mde evaginations 

 extending to the ectoderm in the mid-lateral plane leaving the 

 gills as simple projections of the lateral endodermal walls. 

 Loose mesenchyma cells fill up the space within the gill. The 

 aortic arches are forming near the free medial border of the 

 gills, but the vascularization of the body of the gill has not begun 

 as yet. Although the larva contains a great quantity of yolk 

 in this stage the branchial region is quite free from it. 



The epithelium lining the branchial cavity including the gills 

 consists of a single layer of columnar cells. Dorsal and ven- 

 tralto the mid-lateral plane of each gill pouch and correspond- 

 ing to the position of the future lymphoid accumulations of 

 Schaffer the endoderm shows a slight thickening (fig. 2). In 

 these places which may be called placodes the cells have lost 

 their columnar shape and their outlines have more or less dis- 

 appeared. The area appears to be taking on a syncytial char- 

 acter in which the nuclei do not have any definite grouping. 



Marked changes have occurred in these placodes in a 9.5 mm. 

 larva. In general, the placodes have become enlarged both in 

 thickness and area (fig. 3). Cell outlines are practically all 

 obliterated. The cytoplasm is streaky in appearance suggest- 

 ing a degeneration. The nuclei exhibit variations in character. 

 Some are very dark with a large chromatin content while others 

 are pale and contain a small amount of chromatin. Still others 

 show amoeboid characters. I have not been able to determine 

 whether all these nuclei are indigenous to the placode. The 



