FROM TWELVE TO THIRTY-SIX SOMITES 133 



cavity being the body-cavity. The unclosed part of the body- 

 wall is continuous with the extra-embryonic somatopleure, more 

 specifically the amnion (see below), and this connection is known 

 as the somatic stalk or umbilicus. The yolk-stalk and neck of the 

 allantois pass out of the body-cavity through the somatic stalk, 

 which therefore remains open until near the end of incubation. 



The Turning of the Embryo and the Embryonic Flexures. 

 We have described the separation of the embryo from the extra- 

 embryonic blastoderm without reference to its turning from a 

 prone to a lateral position or to the formation of the flexures 

 of the entire head and body that are so characteristic of amniote 

 embryos generally. These changes begin about the 14 s stage 

 and are first indicated by a slight transverse bending of the origi- 

 nally straight axis of the head in the region of the mid-brain 

 (Fig. 67). By means of this bending, known as the cranial flex- 

 ure, the fore-brain is directed toward the yolk; but almost simul- 

 taneously another tendency manifests itself, viz., rotation of the 

 head on its side, at first affecting only the extreme end. (See 

 Figs. 71, 73, 99, etc.) By the 27 s stage these two processes 

 have resulted in the conditions shown in Fig. 105: by the cranial 

 flexure the fore-brain is bent at right angles to the axis of the 

 embryo, and owing to the rotation the head of the eml^ryo lies 

 on its left side. But inasmuch as the trunk is still prone on the 

 surface of the volk the axis of the embrvo is twisted in the inter- 

 mediate region. This twist is transferred farther and farther 

 backwards as the turning of the head gradually involves the 

 trunk, until finally, at about ninety-six hours, the embryo lies 

 entirely on its left side. 



Exceptionally the rotation may be in the inverse direction 

 (heterotaxia) ; in such cases it is often associated with situs in- 

 versus viscerum. Heterotaxia has been produced experimentally 

 (Fol and Warynsky). 



After the appearance of the cranial flexure a second trans- 

 verse flexure appears in the embryo, this time at about the 

 junction of head and trunk, hence known as the cervical flexure 

 (Figs. 73, 99, etc.). This flexure gradually increases in extent 

 until the head forms a right, or even smaller, angle with the 

 trunk; thus the fore-brain is turned to such an extent that its 

 anterior end points backwards and its ventral surface is opposed 

 to the ventral surface of the throat (Fig. 117). 



