34 SEPARATION OF THE EMBRYO. 



described under the name of the notochordal canal, and corresponds with the hypo- 

 blast ic invagination, which in Amphioxus (fig. 29, 1., cli) also forms the first stage in 

 the development of the notochord. 



A flattening out and even eventually a duplication of this canal occurs in the chick and in 

 various mammals at a somewhat later stage than that given in fig. 34. According to Spee, 

 its cleft-like cavity may pass laterally into the commencing mesoblastic cleavage (coelom- 

 invagination ?). 



The notochord is essentially an embryonic structure in mammals, although it does 

 not completely disappear, for traces of it are to be found throughout life in the 

 centre of the intervertebral discs. When fully developed it is a cylindrical rod 

 composed of clear epithelium-like cells, enclosed within a special sheath of homo- 

 geneous substance. These cells, although they may become considerably enlarged 

 and vacuolated, undergo no marked histogenetic change and take no part in the 

 formation of any tissue or organ of the adult. 



Separation of the embryo from the blastoderm. The embryonic rudiment 

 which thus first makes its appearance in the blastoderm, and which consists 



cum* 



Fig. 35. MESIAL SAGITTAL SECTION THROUGH THE ANTERIOR END OP AN EARLY SHEEP EMBRYO 



SHOWING THE COMMENCING FORMATION OP THE FORE-GUT. (Bonnet.) 



cp, epiblast ; hy, hypoblast ; /.#., foregut, formed by folding over of layers ; am, amirion (head fold). 

 Below the foregut the cephalic coelom is becoming formed as clefts in the mesoblast. 



essentially of neural groove, mesoblastic thickenings, and notochord, becomes 

 at a very early period marked off in front by a dipping down of the blastodermic 

 layers immediately in front of the anterior end of the neural groove, so as to form a 

 transverse curvilinear sulcus the anterior limiting sulcus. This is at first wide 

 and shallow, but soon deepens and narrows, and takes at the same time an oblique 

 direction, curving downwards and backwards under the front end of the neural 

 groove. The sulciis is really due to a growth forwards of the anterior end of the 

 embryonic rudiment, over the part of the blastoderm immediately in front of it, so 

 that this anterior end now projects as a distinct head (fig. 35). 



All the three layers are involved in the forward growth and overfolding which 

 produces the head, so that a prolongation from the blastodermic cavity, which is of 

 course lined by hypoblast, becomes included in the head, and the anterior part of 

 the primitive alimentary canal, or 'fore-gut (/.#.) is thereby produced. Formed in this 

 -way its front end is necessarily blind, and for a long while there is no mouth nor any 

 communication between the fore-gut and the exterior of the embryo. The mouth 

 becomes formed later by invagination from the exterior. 



In the rabbit, and also in the chick, the blastoderm at this time is still bilaminar in and 

 near the middle line in front of the embryo, for the growth of the mesoblast has not yet 



