PART Il 
THE FOURTH DAY TO HATCHING 
ORGANOGENY, DEVELOPMENT OF THE ORGANS 
CHAPTER VII 
THE EXTERNAL FORM OF THE EMBRYO AND THE 
EMBRYONIC MEMBRANES 
I. THe EXTERNAL FoRM 
General. The development of the external form of the em- 
bryo is conditioned by the order of development of the organs. 
The early form is thus given by the nervous system, somites 
ae et / 
A B 
Fig. 121.— A. Embryo of 3 days’ and 16 hours’ incubation. x 5. 
B. Embryo of 5 days’ incubation. x5. (After Keibel and Abra- 
ham.) 
and viscera. The development of muscles, bones, limbs, etc., that 
define the form of the fowl, begins relatively late, and only gradu- 
ally conceals the outlines of the internal parts. 
Figs. 121 to 124 illustrate the development of the external 
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