28 LABORATORY OUTLINES FOR EMBRYOLOGY 



the nuchal or cervical flexure. Now the region of the 

 hind-brain is the most anterior part of the embryo. 

 The lateral folds have been completed and the tail fold 

 is well formed. In the tail region the digestive tract 

 has begun to fold off from the yolk sac. The posterior 

 part of the digestive tract is called the hind-gut. Ante- 

 rior to the tail-fold on the ventral side of the hind- 

 gut is a bladder-like evagination. This is the allantois. 

 The amnion has completely closed, dorsally. The heart 

 is now within the body. Can you trace the twists of the 

 heart? What changes have taken place in the eye? 

 The ear? How many visceral clefts are there? The 

 fore-brain is now composed of two parts: an anterior 

 part composed of two lobes, the telencephalon, and a part 

 immediately back of this, the thalamencephalon, some- 

 times called the diencephalon. The telencephalon is the 

 rudiment of the cerebral hemispheres. Toward the ventral 

 surface of the telencephalon and anterior to the eye is an 

 invagination from the outside ectoderm, the olfactory pit. 

 There is a short diverticulum in the roof of the thala- 

 mencephalon. This is the epiphysis. The mid-brain 

 forms the apex of the cephalic flexure. The mid-brain is 

 the mesencephalon. Between the mid-brain and the 

 hind-brain, the central nervous system is considerably 

 narrowed. This narrow part is the isthmus. The hind- 

 brain is composed of two parts, the metencephalon and 

 the myelencephalon. The metencephalon is the most 

 anterior part of the hind-brain and is the rudiment of 

 the cerebellum. The myelencephalon has a thin roof 

 and sides. It is the rudiment of the medulla oblongata. 



