402 V. E. EMMEL 



sections of the dorsal aorta. These figures illustrate the gen- 

 eral appearance, maximum size, and anatomical relations of 

 these structures. In consequence of their basophilic reaction 

 to Giemsa's stain they stand out in sharp contrast to the red 

 stained erythrocytes of the aortic circulation. 



Cytological details in the structure of these clusters are il- 

 lustrated in figures 6 and 9. The cells usually approximate 

 a spherical form. The nuclei may be round but they more 

 frequently present an indented or kidney shape and generally 

 occupy an eccentric position in the cell body (figs. 6 and 9, vh, mo). 

 Usually, the nucleoplasm takes a lighter stain than that of the 

 cytoplasm. Chromatin granules are rather evenly distributed 

 throughout the nucleus. The cytoplasm takes a basophihc 

 stain of varying intensity. Occasionally the cytoplasm is 

 also vacuolated and contains phagocytized cellular inclusions, 

 the latter being evidently chiefly of an erythrocytic nature. 

 In some instances a reddish tinted or what appears to be a cen- 

 trosphere is observed in the region of nuclear indentation (fig. 

 9, mi). Aside from certain structural variations to be subse- 

 quently considered, the nuclear and cytopla.smic characteristics 

 of the majority of these cells are comparable to the basophilic 

 and phagocytically active cells or macrophags (mesamoeboids?) 

 present in the circulating blood of the same embryos. That 

 the component cells of these clusters become gradually dis- 

 sociated and as detached cells contribute to the circulatory 

 elements of the blood is indicated by those relationships in 

 which certain cells are attached by only a slender cytoplasmic 

 pedicle (figs. 6 and 9, ?Wi) and others are apparently entirely 

 free (figs. 6 and 9, m2). 



These clusters were studied in pig, mouse and rabbit em- 

 bryos and at certain developmental stages found in each of 

 these mammals. 



II. THE QUESTION OF THEIR ORIGIN 



1. Statement of problem. In endeavoring to ascertain the 

 nature and origin of these cell clusters in the aorta certain ques- 



