PART II 



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 



A B 



Fig. 121. — A. Embryo of 3 days' and 16 hours' incubation, x 5. 



B. Embryo of 5 days' incubation, x 5. (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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