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



presented a more advanced stage of sternal formation than the 

 preceding one. In an embrj^o of 23 mm. the sternal bands had 

 fused with each other at their cranial ends in the region of the 

 first pair of ribs. The anlage of the ensiform process was seen 

 here for the first time as a caudal prologation of the sternal band 

 on each side. Histologically the bands now showed chondrifica- 

 tion from the level of the first to the fifth rib inclusive, and there 

 was distinct continuity of tissue between the ends of the ribs 

 and the bands, cartilaginous in the more cranial portion, precar- 

 tilaginous in the caudal part. Her study of the later stages con- 

 firms Ruge's account, and need not be reviewed here. From her 

 observations Mueller concludes that the sternal bands originate 

 directly from the ventral ends of the first seven ribs; andshehomol- 

 ogizes the bridge of mesenchymal cells which connects the cran- 

 ial extremities of the bands in the early stages with the epi- 

 sternum of other forms. ^ 



From this review of the literature it is seen that the earliest 

 stages of the sternum as observed by Paterson, Kravetz, and' 

 Mueller, while not identical, were still very similar; and yet these 

 authors give very different interpretations of their observations. 

 The median portion of the anlage, which Mueller regards as the 

 homologue of the episternum, Paterson considers to be the earliest 

 part of the entire anlage, and thinks that the two sternal bands 

 grow caudalward from it ; this median portion he thinks is derived 

 in its turn from the shoulder-girdle. Kravetz, on the other hand, 

 apparently attaches no special morphological significance to it 

 in the pig. Again, Mueller believes that the sternal bands are 

 derived directly from the ventral ends of the ribs, while Kravetz 

 and Paterson hold that the connection of the bands with the ribs 

 is purely secondary. It has seemed clear to us that none of these 

 investigators has had the earliest stages of the sternum before 

 him, and that justifiable conclusions could be reached only 

 through a study of such stages. Accordingly we have under- 

 taken to find such stages in several manmials. 



