CELL CLUSTERS IN MA^LMALIAN EMBRYOS 405 



of an absence of attachment, Finally with the gradual disap- 

 }:)earance of the clusters in older embryos an intimate histologi- 

 cal relationship with the aortic wall luay become less and less 

 evident. 



3. Evidence as to their origin from the vascular endothelium. 

 The preceding considerations rendered necessary- a more de- 

 tailed investigation of the cytological relations and origin of 

 the aortic clusters. On the basis of the following results the 

 conclusion is drawn that they are endothelial derivations and, 

 as will be subsequently more fully elaborated, arise in relation 

 to certain vascular conditions in the ventral portion of the aorta. 



The first notable feature to which attention may be directed 

 is the absence in the majority of cases of a definite continuity 

 of the vascular endothelium in the region of contact between 

 the clusters and the aortic wall. Cell boundries are not clearly 

 defined and the cells of the endothelium are in evidently syn- 

 cytial relation (illustrated in figure 9 but not clearly evident 

 in the low power drawings for figures 2 and 3). The cluster 

 in figure 9 cannot be said to be resting upon a contmuous sheet 

 of typical flattened endothelium. On the contrary it can in the 

 second place be shown that endothelium at the base of the 

 cluster presents marked cytological modifications. In the 

 vicinity of the cell masses the endothelial cells can be observed 

 usually closer together than normally and the rounded nuclei 

 frequently at one side present a more or less marked indentation 

 or concavity giving -it a kidney shaped appearance, (fig. 9). 

 It is also to be noted that such endothelial conditions are most 

 evident in the case of the clusters presenting the more intimate 

 relationship with the vascular wall. Third, w^hat appears to 

 be transitional cytological changes can be traced from the 

 somewhat lighter stained endothelial cells at the base of the 

 cluster to the more sharply outlined and more deeply basophilic 

 cells at the periphery of the mass (figs. 9, 6). It may also be 

 observed in the same figures that many of the more peripheral 

 cells still present a clearly defined cytoplasmic elongation or 

 pedicle attaching them to the more central regions of the clus- 

 ter. Fourth, that these conditi(nis represent active cellular 



