A DESCRIPTIVE SKETCH 31 



becomes separated off from the epidermis to occupy a position in 

 the mouth of the optic cup. The hypophysis grows in towards the 

 ventral surface of the brain from the epidermis of the front of the 

 head. On the under-side of the head, folds of epidermis give rise 

 in Anura to the ventral sucker, while in many Urodela a finger- 

 shaped outgrowth beneath the eye forms the so-called balancer. 



On either side of the brain, behind the eyes, epidermal pits sink 

 in to form the ear-vesicles. These pits arise from the deeper layers 

 of the epidermis, and so the invagination may or may not be covered 

 over by the superficial epidermal layer. At all events, the ear- 

 vesicles soon become closed if they were open, and their original 

 connexion with the epidermis and the exterior is reflected in the 

 endolymphatic duct (open to the exterior throughout life in the 

 Selachii). Another pair of pits, on the snout, gives rise to the nasal 

 sacs and nostrils, and a larger median depression beneath them 

 sinks in and breaks through into the anterior end of the endodermal 

 gut. This anterior ectodermal portion of the gut is known as the 

 stomodaeum, and its aperture of course constitutes the mouth- 

 opening. A posterior ectodermal portion of the gut, or procto- 

 daeum, is formed in a similar manner, close to the point at which 

 the blastopore closed. Its aperture constitutes the anus, and in- 

 ternally it fuses with and breaks through into the hinder end of the 

 endodermal gut. 



The fusion of the neural folds has not only resulted in the 

 formation of the neural tube, but it has also led to the inclusion 

 beneath the epidermis of narrow strips of cells, situated along the 

 dorso-lateral sides of the neural tube, which constitute the neural 

 crests. From the neural crests arise the nerve-cells or neurons 

 which make up the ganglia or aggregations of nerve-cells situated 

 on the dorsal roots of the segmented cranial and spinal nerves. 

 Other cells derived from the neural crests give rise to the sheaths 

 in which various nerves are enclosed. In the head region, it appears 

 that the neural crests also give rise to parts of the visceral carti- 

 laginous skeleton. In various places on the surface of the head, 

 thickenings of the epidermis give rise to placodes, which form the 

 sense-organs of the lateral-line system, and also contribute some 

 nerve-cells to the ganglia of some of the cranial nerves. Outgrowths 

 from the sides of the head form the rudiments of the external gills, 



