260 OUTLINES OF CHORDATE DEVELOPMENT 



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III. THE EARLY DEVELOPMENT 

 OF THE EMBRYO 



1. The Head-fold 



In the stage we have just been des- 

 cribing, only the posterior limit of the 

 embryo is definitely marked; else- 

 where embryonic and extra-embry- 

 onic regions are directly continuous, 

 and no boundaries are indicated. 

 This remains the condition of affairs 

 for a long time in the lateral directions, 

 but anteriorly the limit of the embryo 

 becomes sharply marked out very 

 early. This is accomplished about the 

 twenty-second hour after fertilization, 

 by the formation of what is called the 

 head-fold, a transverse, crescentic fold 

 of both ectoderm and endoderm, ex- 

 tending nearly across the area pellu- 

 cida, a short distance in advance of 

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

 shallow depression, bordered posteri- 

 orly by an elevation of the blastoderm, 

 this soon becomes a deep groove, as if 

 the blastoderm were being tucked in 

 under the anterior end of the med- 

 ullary plate. In longitudinal section 

 through the medullary plate and head 

 fold, the anterior part of the blasto- 

 derm has the appearance of a letter S 

 (Fig. 98). This folding is really due 

 chiefly, or wholly, to the rapid for- 

 ward growth of the medullary plate, 

 which carries with it the underlying 

 endoderm. Thus two cavities are 

 marked out; one occupies the dorsal 

 limb of the fold, is lined with endoderm, 



