DEVELOPMENT OP^ THE THYMUS 451 



eating a transformation of epithelial cells into lymphocytes, 

 and the evidence indicating an infiltration of the epithelial 

 thymus by Ijinphocytes from the mesenchyme surrounding it or 

 from the blood, were carefully followed out. The transformation 

 theory will be first considered. 



In describing the structure of the thymus of a 23 mm. embryo 

 it was stated that not all of the epithelial nuclei stain with the 

 same intensity. In the enlarged portions of the thymus — super- 

 ficial thjnnus, thymus head and mid-cervical and thoracic seg- 

 ments — are found normally shaped nuclei (spherical and ellip- 

 soidal forms; fig. 1, D.e.N.), of both the larger and smaller 

 types, the nucleoplasm of which stains much more intensely 

 than it does in the great majority of epithelial nuclei present. 

 The nucleoplasm, however, does not stain with the same degree 

 of intensity in all of the darkly stained nuclei for, in some the 

 deeply stained nucleoplasm almost completely masks the chro- 

 matin fibrils and granules while in others the chromatin is seen 

 some what more clearly. Thus the transition forms, observed 

 by Bell, occur between the more usual clear type of nuclei (which 

 I regard as the normal nuclei) and the more deeply stained ones. 

 In the stage (30 mm.) being described they are less numerous in 

 the intermediary and cervico-thoracic cords than in the en- 

 larged segments of the thymus. In a single section through 

 the thoracic segment nineteen were found. In a 23 mm. em- 

 bryo only an occasionally one was found in the intermediary 

 and cervico-thoracic cords, while in a section through the super- 

 ficial thymus ten were counted, thus indicating that they appear 

 first in those portions of the organ that develop, most rapidly. 

 In a 17 mm. embryo none of the intensely deeply stained nuclei 

 were present although a few were found that were stained some- 

 what more deeply than the great majority of normal nuclei 

 present. 



A consideration of these darkly stained nuclei is of the great- 

 est importance, for Prenant ('94), Bell ('06), and others appar- 

 ently have taken these cells to be the forerunners of the first 

 formed lymphocytes in the thymus. A close examination of 

 them, therefore, is necessary in order to reveal their fate. Be- 



