18 



capable of support as those of the same investigator between the 

 Vertebrate and Nemertean nervous systems. 



And so after all, on turning to Wiedersheim's latest book "Der 

 Bau des Menschen", we read: "The hour of the release of the hypo- 

 physis cerebri from its obscure position has not yet struck, and the 

 problems it presents are rendered more difficult in that it develops 

 from two different points, from the brain (Infundibulum) and from 

 the epiblast of the primitive pharyngeal involution." For what we 

 know of the facts of its anatomy and development we are mainly 

 indebted to five distinguished morphologists : to Profs. W. Müller, 



GOETTE, MlHALKOVICS, KÖLLIKER and DOHRN. 



In the following very brief summary I partly follow Kölliker's 

 account in his valuable "Entwicklungsgeschichte des Menschen" (1879), 

 which for the time it was written , is by far the most complete 

 we possess. 



My own researches on Sharks, Ganoids, Dipnoi, Cyclostomata, 

 Amphibia, Lizards, Snakes, Crocodiles, Birds and Mammals mainly 

 confirm Kölliker, who in his turn has taken the greater portion of 

 his account from the beautiful classic researches of Mihalkovics. 



The hypophysis cerebri is composed of two parts ; the one, neural 

 hypophysis, derived from the nervous system, the other, oral hypo- 

 physis, from the epiblast in the region of the mouth. 



The oral hypophysis is formed early in development as an epiblastic 

 involution towards the end of the notochord, i. e. towards the hypo- 

 blast, and in the direction of the base of the brain. In some cases 

 it may even grow in the direction of a process of hypoblast immediately 

 below the anterior end of the notochord. But, except in Myxine, it 

 never fuses with the hypoblast. Afterwards it becomes pinched off" 

 from the pharynx, and gets thus to lie on the floor of the skull, 

 becoming finally converted into a compound gland-like organ. 



The neural hypophysis, or hinder lappet of the hypophysis, on 

 the other hand, developes ventrally as a process of the basal portion 

 of the thalamencephalon , or hinder part of the fore-brain. At first 

 composed of tissue of exactly the same character as the rest of the 

 thalamencephalon it becomes solid below, and converted into indifiereut 

 tissue; the portion of the process which remains hollow, forming the 

 base of the infundibulum, alone retains a nervous structure. 



Kölliker records that in pig embryos of 3 centim. in length 

 longitudinal bundles of nerve fibres pass into the developing neural 

 hypophysis, or processus infundibuli as it is called, from the base of 

 the thalamencephalon. In most cases, especially in Mammals, and 



