388 BRADLEY M. PATTEN 
SUMMARY 
The early phases in the establishment of the chick heart and 
the later stages of its division into chambers have already been 
carefully investigated by many workers. This paper covers the 
somewhat familiar, but heretofore less completely described, 
intermediate processes of loop formation and early regional 
differentiation. 
The work is based on dissections from which plastic models 
were made and on wax-plate reconstructions from serial sections. — 
It deals with: 
1. The formation in the heart tube of the U-shaped bend to 
the right and some of the alleged causative factors in this process. 
2. The formation of the cardiac loop and the relation of tor- 
sion and flexion of the body of the embryo to loop formation in 
the heart. ; 
3. The regional differentiation of the heart into bulbus cordis, 
ventricle, atrium, and sinus venosus, and the early changes in 
each of these regions. 
Since these phases of heart development all involve complex 
changes in configuration and relations, the figures constitute a 
graphic summary much more satisfactory than a written résumé. 
The shortness of the intervals between the phases of development 
figured allows the continuity of the processes to be followed 
readily. 
