50 FRANK BLAIR HANSON 



Several human embryos from the Johns Hopkins Embryo- 

 logical Collection, ranging in size from 17.2 to 10.5 mm. are de- 

 scribed in the paper now under review. The 13-mm. human 

 embryo corresponds essentially to the 18-mm. pig and 13-mm. 

 cat. The pericardial cavity extends far forward; the sternal 

 bands are traceable to the level of the ventral extremities of the 

 fifth ribs; the ventral tips of the ribs are non-cartilaginous, and 

 any possible continuity between them and the sternal bands is 

 much too slight to suggest that the latter is a derivative of the 

 former; neither median sternal rudiment nor clavicles were de- 

 tected, though this was probably due to the loss of an entire 

 slide of sections from the very region in which one would 

 expect such a structure to occur, if present. 



In a 10.5-mm. human embryo, the sternal bands reach pos- 

 teriorly only as far as the fourth rib ; and although the cells com- 

 posing the bands are not sharply differentiated from the sur- 

 rounding tissue, they are still recognizable and ' ' it is evident that 

 they are not continuous with the tips of the ribs, but are con- 

 nected with them only by loose mesenchymal cells." 



In these stages which are far earlier than any Ruge describes, 

 there is a history quite different from that which later stages 

 had led us to expect. In the first place, there was no indication 

 of segmentation in the sternal bands, which might be expected 

 were these bands due to proliferated cells from the tips of the 

 costal cartilages, and, second, in the pig the first two ribs in the 

 earliest stages studied did not reach the sternal bands at all; 

 three did not do so in the cat, while in the 10.5-mm. human em- 

 bryo none of the ribs reached the band. 



These results also contradict the statement of Paterson that 

 the median rudiment is a part of the scapular arch, and that the 

 sternal bands are derivatives of the single median blastema. 

 Whitehead and Waddell do not mention the condition or extent 

 of the girdles in any of the forms studied by them, merely stating 

 that at no point was there any connection discernible between 

 sternum and girdles; and that in every case "the appearance of 

 the paired portion of the rudiment, the sternal bands, antedates 

 that of the median portion." 



