xxxii Journal of Comparative Neurology. 



The Der el opiuent of the Head of Ammocoetes planeri. 1 



Professor Kupffer pursues the subjects treated in his previous pa- 

 pers and is obviously much influenced by the conceptions derived 

 from his studies in the embryology of the sturgeon. This author 

 never fails to prove suggestive even where his arguments may seem 

 unconvincing. 



The present paper begins upon the basis of the statement that, in 

 the sturgeon and chick, the axis of the brain tube terminates between 

 the olfactory tubers in an azygos plate of the epidermis which the 

 author homologizes with the olfactory organ of the Monorhina. This 

 is " a terminally situate sensory organ." A series of longitudinal sec- 

 tions at different stages is discussed and brought, as far as possible, 

 into harmony with the sturgeon series. An unpaired thickening of 

 the epidermis at the dorsal portion of the truncated cephalic extermity 

 of the ( primarily solid ) nerve tube is the olfactory plate. The 

 hypophysis at this stage is a slight depression at the ventro-cephalic 

 angle of the tube. At the eighth day the olfactory plate has become 

 depressed producing a beak-like angle of the nerve tube at its dorsal 

 end which is identified as the olfactorius impar. The brain at this 

 period is divided into three segments. 



Larvas 4 mm. long have the free rounded end of the olf. impar re- 

 tracted from the olfactory organ but a short fibrous cord connects the 

 two points. This cord Kupffer regards as a transient olfactory nerve. 



These studies prove ( what was a priori to be expected ) that the 

 so-called anterior epiphysis is homologous with the paraphysis of other 

 vertebrates. It arises at the caudal limit of the poorly-differentiated 

 prosencephalon. 



When Ammocoetes becomes 7 mm. long a retrograde metamor- 

 phosis of the brain begins, exhibiting itself in the decrease of the cere- 

 bellum, and of the commissures. The olfactorius impar and vesicle 

 of the prosencephalon have already disappeared. 



Kupffer discovers that there is a special decussation of fibres 

 cephalad of the supracommissure without recalling that such a double 

 commissural system has been described by C. L. Herrick in a large- 

 number of different vertebrates. The statement that the fibres after 

 decussation apply themselves externally on the paraphysis should be 

 verified in various stages. The author corrects Studnicka's error in 

 deriving the " parapineal organ" or paraphysis from the dienceph- 



1 Kupffer, C. V. Studien zur vergleichenden Entwickelungsgeschicte der 

 Kopfes der Kranioten, 2. J. F. Lehrnann Munich-Leipzig 1894. 10 marks. 



