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 embryonic 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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