374 BRADLEY M. PATTEN 
point of view by Reagen (’17), it seems unnecessary to go over 
the ground again here. The abundant information available 
concerning the establishment of the primordial heart tube places 
us in a position to take up without preliminaries the processes 
involved in cardiac-loop formation. 
The observations recorded here are confined to the period ex- 
tending from the establishment of the heart as a nearly straight, 
double-walled tube to the period at which the process of loop 
formation has been completed and the main regional divisions 
of the heart have been definitely established. 
The later development of the chick heart, involving the for- 
mation of the septa and valves which develop during the division 
of the heart into chambers, is dealt with in papers by Lindes 
(65), Tonge (’69), Masius (’89), Langer (’94), Griel (’06), and 
Hochstetter (06). 
MATERIAL AND METHODS 
At the outset of the work it became obvious that there was 
considerable individual variability as to heart configuration even 
among embryos having the same number of somites. The first 
consideration was, therefore, the selection of a series of embryos 
which would show as nearly as possible the normal sequence of 
shape changes undergone by the heart. This phase of the work 
was. greatly facilitated by the availability of some 2000 chick 
whole-mounts in our laboratory collection. By studying the 
heart in a large group of embryos having the same number of 
somites, it was not difficult to determine the characteristic heart 
configuration for a particular stage of development. 
Twelve embryos ranging between 29 and 100 hours of incu-. 
bation were selected as showing the characteristic steps in the 
formation of the cardiac loop and the early regional differentia- 
tion of the heart. Each of the twelve embryos belonging to the 
initial series was then carefully matched so that three or four 
embryos of each stage, exactly like one another as far as could 
be determined, were available for the work. One embryo in 
each of these sets was reserved for study as a cleared and stained 
entire mount, the remaining embryos were used for dissection 
and serial sectioning. 
a inet 
