PART II 



THE FOURTH DAY TO HATCHING 

 ORGANOGENY, DEVELOPMENT OF THE ORGANS 



CHAPTER Vn 



THE EXTERNAL FORM OF THE EMBRYO AND THE 



EMBRYONIC ME:\IBRANES 



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 



211 



