DEVELOPMENT OF THE THYMUS 469 



in the parenchyma of the thymus which, to anticipate, develop 

 from the same type of undifferentiated cells as those in the 

 mesenchyme. 



In early stages the development of the free blood-cells can be 

 demonstrated in most any portion of the mesenchyme in the 

 neck and upper thoracic region of young embryos. There are, 

 however, locahzed regions where this process is carried on even 

 in later developmental stages. The interlobular septa of the 

 thymus are particularly favorable places to study the develop- 

 ment of blood-cells in well advanced embryos. The thymus of 

 an embryo 125 mm. in length was selected for the cytomorphosis 

 of the erythrocytes, eosinophile cells, and phagocytes, although 

 somewhat later stages could have been used. The connective 

 tissue of the septa is loosely arranged and contains many transi- 

 tional forms leading from the connective tissue cells to the above 

 named elements. A few stellate-shaped connective tissue cells 

 are still found but the spindle shaped type is most numerous. 

 In figure 7 is a series of diagrams representing a suggested 'cell 

 lineage' between connective tissue cells and their derivatives, 

 i.e., erythrocytes (blood-plastids of Minot), phagocytes, and 

 granular cells. Diagram a represents a connective tissue cell. 

 The cytoplasm is finely granular and is only very slightly baso- 

 philic. In some small vacuoles occur. No cell membrane 

 could be demonstrated. The nucleus may be round or oval and 

 has a distinct nuclear membrane. The chromatin is in the 

 form of small irregularly shaped granules many of which are 

 joined together by very fine chromatin threads. The nuclei 

 vary in number from one to three. Diagram e represents a 

 transformed mesenchymal cell. Its protoplasmic processes are 

 retracted and now lie free in the septa. The cytoplasm has 

 slightly but appreciably increased in amount and has become 

 more basophilic and now is homogeneous. The nuclear changes 

 are represented apparently by a slight massing of the chromatin, 

 the granules becoming slightly coarser and less numerous. In 

 some connective tissue cells the cytoplasm becomes more baso- 

 philic and the nuclear changes occur even before its processes 

 have been retracted (diagram h). This type of connective 



THE AMERICAN JOURNAL OF ANATOMY, VOL. 17, NO. 4 



