92 



THE DEVELOPMENT OF THE CHICK 



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growth of the medullary plate, 

 which causes the latter to extend 

 forward above the thinner and more 

 pliable membrane in front. The 

 entoderm is attached to the inner 

 surface of the anterior end of the 

 medullary plate (Fig. 47), and is 

 apparently carried forward with the 

 latter to form the anterior portion 

 of the fore-gut. The actual form of 

 the fold depends upon the mechani- 

 cal properties of the membranes 

 concerned, especially the unequal 

 thickness of their parts produced 

 by unequal growth. 



Although the head-fold thus ap- 

 pears to be a single fold involving 

 the two primary layers, it is con- 

 venient, for purposes of description, 

 to consider it as two separate folds, 

 ectodermal and entodermal. The 

 deepening of these folds takes place 

 at the same rate up to the time 

 when four somites are formed (Fig. 

 49). At about this time the paired 

 primordia of the parietal cavity 

 (amnio-cardiac vesicles), which ap- 

 pear in the mesoblast in the lateral 

 extensions of the head-fold (Fig. 

 50), push in towards the mid- 

 dle line so as to separate the ecto- 

 dermal and entodermal limbs (Figs. 

 52 and 58). When six somites 

 are formed, these cavities fuse in 

 the middle line, thus effecting a 

 complete separation of the two 

 limbs. The further progression of 

 the head-fold, after this union, 

 takes place separately in the two 

 limbs. 



