EMBRYOLOGY 393 



becomes differentiated into three enlargements, the fore, mid and 

 hind brain vesicles clearly visible in longitudinal section at a slightly 

 later stage. Very shortly after, a striking flexure occurs in the mid 

 brain and as a result the fore brain is bent down at right angles to 

 the rest of the central nervous system. The notochord extends 

 from the posterior end forward to the end of the mid brain where it 

 stops. Right in front of the fore brain a median tongue of cells, 

 the hypophyseal ingrowth, passes inwards from the ectoderm and at 

 a later stage a terminal group of cells leaves this and passes inwards 

 to take up its position in front of the end of the notochord, under the 

 floor of the hinder part of the fore brain. This group of cells is the 

 rudiment of the hypophysis ; indications of the olfactory organs 

 and ears as ectodermal thickenings by this time. 



When the neural tube has been completed and the flexure of the 

 brain has taken place, the embryo has elongated noticeably in the 

 antero-posterior direction. Certain changes have also occurred in 

 the enteron during this period of growth. With the separation of 

 the notochord and mesoderm the whole of the dorsal region and the 

 anterior end of the ventral portion is lined by a single layer of cells. 

 A large number of yolk-laden cells still remain, forming a yolk mass 

 in the posterior half of the ventral region of the enteric cavity. Their 

 presence enables three regions of the enteron to be clearly differen- 

 tiated. In front of them the enteric cavity expands to form the 

 fore gut, a wide deep cavity lined by a single layered epithelium. 

 This region will give rise first to the pharynx, and later to the 

 oesophagus and stomach and at its antero-ventral end in the middle 

 line is a short oral evagination that will later be met by an ectodermal 

 invagination, the stomodceum, which is to form the mouth and 

 buccal cavity. The hinder end of the enteron is characterised by 

 being in communication with the cavity of the central nervous 

 system, via the neurenteric canal, and also by an evagination that will 

 later produce the rectum. This region, also lined by a single layer of 

 cells, is designated the hind gut. Between them the dorsal wall 

 is single layered, but the ventral wall many layered, owing to the 

 presence of the yolk cells which also reduce the enteron to a narrow 

 dorsal cleft. To this region the name of mid gut is given, and it 

 will later give rise to the intestine of the adult. 



Finally, we have to consider certain changes in the mesoderm. 

 It has already been noted that the mesoderm extends laterally from 

 the sides of the notochord and neural tube as a solid sheet of cells, 

 separating the ecto- and entoderm, save that at the primitive streak 

 region it is confluent with the other layers. With the development 

 and flexure of the brain the mesoderm cells extend up into this head 

 region. Here, however, they do not form a definite compact sheet, 



