32 Charles Russell Bardeen 



of cells. The lateral walls of the midbrain are excessively thick 

 and the lumen of the aqueduct is narrow. The ventral w^ll of 

 the medulla is irregular in outline in places (section b). There 

 are many desquamated cells in the fourth ventricle. The spinal 

 cord is greatly deformed in most regions and in many cases shows 

 no central canal (sections d and e). In the midthoracic region it 

 is fi^irly normal in form. The spinal cord does not extend into 

 the rudimentary tail. 



Organs of Special Sense — The nasal fossae are patent. The 

 medial wall of each fossa is thin (section a). The optic stalks 

 are patent. The pigment layer of the retina is separated from 

 the sensory layer. The latter is not specifically differentiated. 

 A lens rests against the sensory layer (section a). The auditory 

 vesicles are simple sacs with dorsal diverticulae. 



Peripheral Nerves — The sensory ganglia of the fifth, seventh 

 and eighth, ninth and tenth nerves can be distinguished. The 

 nerves can be followed but a short distance. Spinal ganglia are 

 present in the midthoracic region where the spinal cord is fairly 

 normal in form, but are not present elsewhere. 



Ahmeiitary Canal — The mouth is patent. The jaws and lips 

 are partially differentiated. The gill slits are patent. The in- 

 ternal gills are rudimentary in form and contain no obvious blood 

 vessels. The operculum is only partially differentiated. There 

 is no opercular cavity in front of the heart. The tracheo-pulmon- 

 ary anlage is a short branched column of cells. The oesophagus 

 is composed of dense tissue in which several irregular spaces sug- 

 gest a coiled tube (section c). The stomach curves about the 

 left side of the anlage of the liver (see right side of section d). 

 The liver is highly differentiated (section c) and is connected by 

 ventral and dorsal mesenteries to the walls of the body cavity. 

 The large blood spaces in the liver contain traces of blood. The 

 bile duct is large. The pancreas is not specifically differentiated 

 but its anlage is marked by a mass of yolk cells. The gut curves 

 from left to right in front of the anlage of the pancreas and then 

 turns distally. Beyond the region of the pancreas the gut passes 

 nearly straight back to the anus. The ventral portion is thick 

 and filled with yolk cells as in young larvae. The anus opens on 



