THE DEVELOPMENT OF LEPIDOSIREN PARADOXA. 453 



6. The tootli-plates of Lepidosiren are not at any stage 

 represented by numerous separate denticles as is the case 

 with Ceratodus. 



7. The tooth germs appear while the mouth region is still 

 without a lumen. They develop according to the " Placoid" 

 type. 



8. There is a well-developed enamel organ^ and a layer of 

 what appears to be a peculiar type of enamel is the first of 

 the hard parts of the tooth to be laid down. 



9. The dentine appears to arise by metamorphosis of the 

 peripheral portions of the cell-substance of the odonto- 

 blasts, 



10. The brain of the adult Lepidosiren closely resembles 

 that of Protopterus, differing only in details. The fourth 

 and sixth cranial nerves are present though extremely thin. 



11. The thalamencephalon and mesencephalon do not 

 become marked off from one another until relatively late. 



12. The cerebral hemispheres arise as two separate lateral 

 bulgings of the wall of the thalamencephalon. 



13. The choroid plexuses of the lateral ventricles are simi- 

 larly paired originally; the plexus of the third ventricle 

 arises from them and is also paired. There is no velum in the 

 middle line. 



14. The pineal organ is simple^ without any trace of a sepa- 

 ration of a part of it to form a " parietal organ." 



15. There is a well -developed paraphysis closely resembling 

 that of Urodele Amphibians. 



16. The paraphysis is a product of the thalamencejihalon^ 

 not of the secondary fore-brain. 



17. The point in the brain of the adult which corresponds 

 to the anterior end of the floor of the neural rudiment of the 

 embryo^ lies well up on the anterior wall of the thalamen- 

 cephalon just below the root of the paraphysis. 



18. The rudiments of the eyes, like those of nose and car, 

 are at first solid, the cavity of the optic vesicle arising 

 secondarily, rather before the ventricle of the brain at its 

 level has appeared. 



