106 R. H. WHITEHEAD AND J. A. WADDELL 



As to the derivation of the median portion of the sternal anlage 

 the evidence is not so conclusive. The fact that in both pi^ and 

 cat embryos this portion of the anlage originates independently 

 of the clavicles enables us to conclude at once that it is not derived 

 from the medial ends of the clavicles. Either of two other inter- 

 pretations, however, seems possible so far as our observations 

 go: (1) it maj^ be formed in situ, or (2) it may be derived from 

 the anterior ends of the two sternal bands by each of them send- 

 ing a prolongation medialward to join its fellow in the median 

 plane. The fact that we never found this anlage in a paired 

 condition, but always as a single band of cells uniting the anterior 

 ends of the sternal bands leads us to belie \^e that the first interpre- 

 tation is the more probable. 



In conclusion, the conception which we have gained of the 

 early development of the mammalian sternum may be stated as 

 follows: The sternum is developed from three anlagen. First 

 to arise are the lateral portions, the two sternal bands, which 

 make their appearance one on each side of the body wall as a 

 longitudinal blastema dorsal to the junction of the axilla with 

 the lateral wall of the thorax and ventral to the tips of the ribs. 

 The vencral extremities of a certain number of the ribs soon reach 

 this region and fuse with the sternal band. A little later, as the 

 heart begins to sink into the thorax, the thix-d or median anlage 

 makes its appearance as a transverse bridge of cells extending 

 bet ween the anterior extremities of the two lateral anlagen, and the 

 entire blastema now presents the shape of a two-pronged fork. 

 As the heart sinks still more into tne thorax, the anterior ends of 

 the lateral anlagen are allowed to approach each other and soon 

 fuse, the median anlage being incorporated with them. 



With respect to the morphological significance of the median 

 anlage, we incline to the view that it is the homologue of the epi- 

 sternum of certain of the lower vertebrates, or, perhaps we should 

 rather say, of the prosternum of the monotremes. 



