CHAPTER VI 



FROM TWELVE TO THIRTY-SIX SOMITES. THIRTY- 

 FOUR TO SEVENTY-TWO HOURS 



I. Development of the External Form, and Turning of 



THE Embryo 



In the embryo of twelve somites only the head is distinctly 

 separated from the blastoderm; and there is no sharp boundary 

 between the embryonic and extra-embryonic portions of the 

 blastoderm in the region of the trunk; but this changes very 

 rapidly. The progress of the developmental processes, that have 

 marked out an embryonic axis in the blastoderm, produces in 

 the course of about eighteen hours a sharp distinction everywhere 

 between embryo and extra-embryonic blastoderm. The latter, 

 together with an outgrowth of the embryonic hind-gut (allantois), 

 then constitute the so-called evibryonic membranes, which become 

 very complicated, and which provide for the protection, respira- 

 tion, and nutrition of the embryo. We shall consider the forma- 

 tion of the embryonic membranes separately in order not to 

 confuse the account of the development of the external form of 

 the embryo. 



In considering the development of the external form of the 

 embryo, we must distinguish between those processes that sepa- 

 rate it from the extra-embryonic blastoderm, and those that occur 

 within its own substance leading to various characteristic bend- 

 ings and flexures; we may consider them separately, although 

 they are going on at the same time. 



Separation of the Embryo from the Blastoderm. The separa- 

 tion of the embryo from the blastoderm takes place by the 

 formation of certain folds or sulci that may be named: (1) the 

 head-fold or anterior limiting sulcus; (2) the lateral limiting sulci, 

 appearing as prolongations of the head-fold along the sides of the 

 embryonic axis; and (3) the tail-fold or posterior limiting sulcus. 



The head-fold has been described in detail in the preceding 



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