EMBRYOLOGY 403 



they transform the dorsal groove into a much wider and deeper 

 medullary groove. At first the folds are lateral, but later they bend 

 round in front to meet in the middle line, just behind the anterior 

 border of the embryonal shield, and thus mark the limit of the 

 medullary plate. Thus we have laid down the rudiment of the central 

 nervous system. 



A very important change occurs after the formation of the 

 transverse neural fold, and that is the growth in this region becomes 

 unequal. The fold itself rises up considerably above the level 

 of the surrounding ectoderm, and then its dorsal region grows much 

 more rapidly than its ventral part or than the front margin of the 

 embryonal shield. It follows as a mechanical result from this that the 

 transverse fold region begins to bend over and grow forward inde- 

 pendently of and over the rest of the blastoderm. This overgrowth 

 leads to the formation of a rounded process overlying the anterior 

 end of the blastoderm in front of the notochordal process, a region 

 which, it will be recalled, is the proamniotic area and contains no 

 mesoderm. The folding produced in this way is known as the 

 head fold, and it is the beginning of an important transformation. 

 Up to the present, while there have been certain alterations in the 

 thickness of its various parts, etc., the blastoderm has grown as a 

 whole. Now, however, we find that a portion of it is becoming 

 folded off from the rest and very sharply marked off from it. This 

 process is known as delimitation, and we find later that the portion 

 so folded off is destined to form the embryo, while the remaining 

 part of the blastoderm takes no direct part in its formation, and is, in 

 consequence, termed extra-embryonic. The growth of the head 

 fold soon comes to involve both layers of the hinder part of the pro- 

 amniotic area, and so in viewing the blastoderm from the surface, 

 two rounded bay like formations facing in opposite directions can 

 be distinguished. The uppermost of these is superficial, and lies 

 between the shield ectoderm on the under side of the head fold and 

 the extra embryonic ectoderm of the proamniotic area. It is known 

 as the head fold bay and of course opens forwards on to the surface 

 of the blastoderm. The other is a deeper-lying structure and is 

 brought into being by the pulling forward and folding of the ento- 

 derm of this region with the extension of the head fold. This bay 

 is enclosed by entoderm and opens backwards into the sub-germinal 

 cavity. It is at first quite short, but nevertheless, an important 

 structure, since it represents the fore-gut of the embryo. The rela- 

 tions of all these structures can be readily seen in a sagittal section. 



When the head fold is commencing the mesoderm, as we 

 have seen, covers a fairly extensive area, spreading out sideways 

 from the primitive streak and noto'chordal process and it sweeps 



