Jan.s. 1888] 



NATURE 



225 



rudiments of the gill sense-organs and ganglia described 

 by Prof. Froriep in Mammalia. (Of these I hope to give 

 a fuller account in connection with other work.) I find 

 them in lizards, crocodiles, and birds ; and there can be 

 little doubt that they exist as rudiments in all animals 

 above fishes and amphibia. Their recurrence has its 

 explanation in that they probably form the Anlage for 

 certain portions of the cranial ganglia. 



It was Dohrn who first hinted, in his work on " Der 

 Urspruiig der Wirbeltiere," published in 1875, that the 

 hypophysis cerebri represented the last remains of the 

 old mouth, and that it must have opened on the dorsal 

 surface, after passing between the crura cerebri. 



This idea he soon gave up, and indeed, in the work 

 above mentioned, he inclined to the view that the opening 

 lay somevvhere in the region of the medulla oblongata. 

 Since then he has relinquished, for the time, the search 

 for the old mouth, and has advised others to do the 

 same. 



His first hypothesis has more recently been advanced 

 as new by Prof Owen and Mr. J. T. Cunningham. Both 

 of these writers hold very slightly different views from 

 those originally suggested by Dohrn. 



Some of the statements which I am about to make 

 appear on the surface to bear slight resemblance to Cun- 

 ningham's views, but, as I hope will be seen, nothing 

 could be further from the truth. Cunningham, starting 

 from Balfour's well-known, and now universally accepted, 

 belief that the spinal cord and brain were once an open 

 plate, advocated, as the latest discovery of Vertebrate 

 morphology, the view that the infundibulum, whose walls 

 consist of nervous matter and nothing else, is the vestige 

 of the old mouth which pierced the brain. 



One cannot but marvel at the rashness of an hypothesis 

 which annexes, without more ado, a portion of the nervous 

 system, and proclaims it to all the world as the remnant 

 of a former passage from the exterior to the stomach of 

 the animal ! 



Cunningham overlooks entirely the nature and ex- 

 ceedingly complicated development of the processus 

 infundibuli, or nervous portion of the hypophysis. 



Although, thanks to Rabl-Riickhard and others, we 

 have obtained a certain amount of light on the nature of 

 the pineal glanJ or epiphysis, the body (hypophysis), 

 at the opposite end of the third ventricle still remains 

 one of those organs on which all sorts of speculations 

 may be made, with impunity. Some of the explanations 

 offered are in accordance with certain facts of its develop- 

 ment. Others, on the contrary, accord with no known 

 fact of embryology. 



The nervous part — or, as I shall call it, the neural 

 hypophysis — has been considered by Rabl-Riickhard as 

 a gland secreting cerebro-spinal fluid. I must, however, 

 express a stro.ig opinion that such a glandular function 

 is extremely improbable, for the conversion of a piece of 

 nervous tissue into a gland is absolutely without parallel. 



Goette and Wiedersheim both regard the nervous 

 part as a remnant of a sense-organ ; against which 

 view a /r/tfr/ little or nothing can be said. The mouth 

 part or oral hypophysis was finally classed by Dohrn as 

 the rudiment of a pair of gill-clefis— a supposition not 

 wholly unsupported by its developmental history. It has 

 also, not unnaturally, been looked upon as a remnant of 

 a mouth-gland. 



Prof. Hubrecht made it the basis of his comparisons 

 of Nemerteaand Vertebrata, and saw in it the remains 

 of the Nemertean proboscis, the Vertebrate notochord 

 being the homologue of the proboscis sheath— compari- 

 sons which appear to me to be as little 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 hypophysis 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 : Profs. W. Miiller, Goette, Mihdikovics, 

 Kolliker, and Dohrn. In the following very brief sum- 

 mary I partly follow Kolliker's account (in his valuable 

 " Entwickelungsgeschichte 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, Cyclo- 

 stomata, Amphibia, Lizards, Snakes, Crocodiles, Birds, and 

 Mammals, mainly confirm Kolliker, who, in his turn, has 

 taken the greater portion of his account from the beautiful 

 classic of Mihdikovics. 



The hypophysis cerebri is composed of two parts : the 

 one, neural hypophysis, derived from the nervous system; 

 the other, oral hypophysis, 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 hypoblast, 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. It afterwards 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 hypo- 

 physis, on the other hand, develops 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 thalamen- 

 cephalon, it becomes solid below and converted into 

 indifferent tissue; the portion of the process which 

 remains hollow, and forms the base of the infundibulum, 

 alone retains a nervous structure. Kolliker records that 

 in pig embryos of 3 centimetres 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 Mammalia and also in 

 Dipnoi, the neural hypophysis becomes closely and almost 

 inseparably connected with the oral hypophysis. Usually 

 the Anlage of the oral hypophysis lies in the region of 

 the mouth epiblast ; in Petromyzon and Myxine it lies in 

 front of and outside the mouth. The process by which 

 it got into the mouth involution cannot be explained 

 without numerous figures. 



According to Dohrn, the oral hypophysis arises in 

 Petromyzon as an invagination of epiblast in front of 

 the mouth between the oral and nasal depressions. It 

 grows towards the base of the infundibulum, and comes 

 into close relationship with the end of the notochord, i.e. 

 with a structure derived from hypoblast, while it ap- 

 proaches a special process of hypoblast itself, with which, 

 however, in Petromyzon it does not fuse. In Myxine, 

 although the development is unfortunately not yet known, 

 we may assume that this fusion is effected, for in that 

 animal it opens throughout life into the gut (see figure, 

 O.M.). 



In Ammocoetes it gives off a certain number of gland- 

 follicles, which, according to Dohrn, become pinched off 

 in the Petromyzon. While I am not yet quite convinced 

 of the certainty of this latter point, I find, in Myxine, 

 numerous small glandular follicles opening into the oral 

 hypophysis. In Petromyzon and Myxine the neural hypo- 

 physis is present, and, as I believe, not rudimentary. It 

 appears to supply nerve-fibres to the oral hypophysis. 



Dohrn finds in Hippocampus traces of a paired origin 

 of the oral hypophysis. This is important. 



