OUTLINES OF CHORDATE DEVELOPMENT 



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III. THE EARLY DEVELOPMENT 

 OF THE EMBRYO 



||| 1. The Head-fold 



!j In the stage we have just been des- 

 J-c cribing, only the posterior limit of the 

 q ^ ^ embryo is definitely marked; else- 

 <|t| where embryonic and extra-embry- 

 (|d onic regions are directly continuous, 

 > ^ and no boundaries are indicated. 

 3 1 This remains the condition of affairs 

 J-+ * for a long time in the lateral directions, 

 g ;| but anteriorly the limit of the embryo 

 g I becomes sharply marked out very 

 3 ^| j early. This is accomplished about the 

 j * twenty-second hour after fertilization, 

 . ^ by the formation of what is called the 

 I J 'a head-fold, a transverse, crescentic fold 

 Ig-J of both ectoderm and endoderm, ex- 

 l^| tending nearly across the area pellu- 

 cida, a short distance in advance of 

 3 4 the head process (Fig. 97) . At first a 

 | ^ shallow depression, bordered posteri- 

 J orly by an elevation of the blastoderm, 

 fl this soon becomes a deep groove, as if 

 |~g the blastoderm were being tucked in 

 <> g under the anterior end of the med- 

 S ^ ullary plate. In longitudinal section 

 through the medullary plate and head 

 rttq ^ fold, the anterior part of the blasto- 

 |^*| derm has the appearance of a letter S 

 f a'g, ( Fi g- 98). This folding is really due 

 H | s r chiefly, or wholly, to the rapid for- 

 x . r ^ ward growth of the medullary plate, 

 .2 ej a which carries with it the underlying 

 ' J ^"a endoderm. Thus two cavities are 

 i g marked out; one occupies the dorsal 

 *o o, Hmbof the fold, is lined with endoderm, 



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