CELL CLUSTERS IN MAMMALIAN EMBRYOS 403 



tions arise for consideration. Are they merely more or less 

 agglutinated masses of circulatory^ blood corpuscles incidentally 

 resting against the vascular surface or are they structures aris- 

 ing directly from the vascular wall? Furthermore, can their 

 occurrence be correlated with any special developmental pro- 

 cesses of the embryo? 



Of preceding investigators w^ho have recorded the observa- 

 tion of similar structures may be noted Maximow ('09. p. 157) 

 for rabbit and cat embryos, Dantschakopff. ('07) for the chick 

 embryo and Minot ('12, p. 525) for the human embryo. Maxi- 

 mow and Dantschakopff interpret these cell masses as endothe- 

 lial derivatives differentiating in situ from the vascular wall. 

 Minot on the other hand did not find the evidence sufficiently 

 convincing to justify the conclusion that they are of endothelial 

 origin. Beyond the immediate question of their origin no data 

 has been advanced as to why and under what conditions these 

 cell clusters occur in the aorta. The present status of the 

 problem and certain hematological questions associated with 

 the subject are consequent^ such as to justify a further exten- 

 sion of the investigation. 



Of the mouse, rabbit, and pig embryos studied in the present 

 work the cell structures in question were found most pronounced 

 and striking in the pig embryo. Consequently the following 

 account is based chiefly on the results derived from the latter 

 mammal, in which it appears that these clusters have not been 

 previously described. The pig material for this purpose con- 

 sisted of seventeen embryos varying in size from 6 mm. to 25 

 mm. fixed in Zenker-formalin (Helly's combination), embedded 

 in paraffin or celloidin and the serial sections stained with Giem- 

 sa's fluid. Several of the 12 mm. embryos were also stained 

 in toto with borax carmine and the sections counter stained 

 with Lyon's blue. 



2. Grounds for regarding these clusters as of greater signifi- 

 cance than merely incidental structures. Upon first impression 

 one may be inclined to discredit any special relationship between 

 these cell masses and the aortic wall. As the result of further 

 investigation it appears clear, however, that the phenomenon 



