SUBSEQUENT DEVELOPMENT 



227 



POSTERIOR LIMB BUD 



EMBRYONIC COELOM 



Fig. 154. The hindgut and allantois. The hindgut has enlarged since 48 

 hours, and a large ventral outgrowth is present. This is the allantois, which, 

 together with mesoderm, grows out to the inner surface of the chorion. The 

 large swellings of mesoderm on either side are the posterior limb buds. The 

 two small cavities on either side of the hindgut are the embryonic coelom. 



A number of new structures develop in the trunk. Just posterior to the 

 heart are the paired wing buds. These are dense aggregations of mesenchyme 

 which develop underneath the epidermis. More posteriorly a similar pair of 

 lateral swellings indicate the hindlimb buds. At 72 hours' incubation time, 

 the limb buds, if transplanted to the chorioallantoic membrane, undergo 

 self-differentiation. 



The tail fold described for the 48-hour chick results in the formation of 

 a hindgut. From it a ventral outgrowth develops into the allantois. A tail 

 develops and curves ventrally. Figures 146 to 154 show further details of 

 structure in the 72-hour chick. 



Subsequent development 



Further development from 72 hours to hatching is mainly a matter of 

 increased differentiation within the structures of the embryo. Chapters 11, 12, 

 13, and 14 have dealt with much of this later differentiation in the chick. In 

 Chapter 19 we shall discuss the form changes and the development of func- 

 tion in the human embryo. This will complete our descriptive study which 

 began with early development of the frog egg (Chapter 9), continued with 

 somewhat later development in the chick (this chapter), and concludes with 

 the final phases of development in the human. 



Thus far we have discussed the organization of the egg, the formation of 

 fields in the early embryo, the differentiation within fields in the later embryo, 

 and the interrelationships between developing structures in very late stages 

 of development. We have seen that labile organization gives way to more 

 rigid organization, that cells become more restricted in their potencies and 



