CEREBRAL COMMISSURES IN THE VERTEBRATA. 479 



europma, the cells of the liippoeampus arc luucli more diffusely scattered than in Hydrosaurus and, to a 

 lesser degree, in Sphenodon also. This more diffuse scatteriug of cells points to the more primitive 

 condition of the hijjpocampus exhibited in the Amphibian brain. Osboru has given such an exeelleut 

 representation of it in Kmys, that it is quite unnecessary for me to do this here (Osl)orn, Morph. Jahrb., 

 Bd. xii. Taf. xiii. tig. I(i). In Chelone, hoHcver, the cells are much more compactly arranged, so as to 

 form a regular column which presents a close resemblance to that found in Sphenodon, which I have 

 already represented *. 



In the Chelonia, Crocodilia, and Rhyuchocephalia the characters of the distinctive column of cells of 

 the hippocampus arc iJi'actieally uniform throughout the whole length of the coluuni ; w hcrcas in the 

 Laccrtilia, and especially in the Ophidia, the cells become much more numerous and considerably smaller 

 in size in the neighbourhood of the paraterminal body. This fact is now well-recognized, and has been 

 successively noted by Herrick, Edinger, Meyer, and myself among many others ; and the condition has 

 been adinii:i!)ly rcjiresentcd in the excellent figures wiiicli accompany Edinger's memoir of 1888 

 (he. fit.) and Meyer's of 189;} [hc.cit.]. 



In the Chelonia, no such alteration in the characters of the ventral extremity of the hippocampal 

 column of cells can be detected, nor am I aijle to detect any trace of such a change in the brain of 

 Sphenodo/i. 



This peculiar feature of the hippocampus in the Saurians is significant. For it points to a more 

 rapid proliferation, resulting in the production of a large mass of cells of small size, in a position 

 analogous to that in which the fascia dcntata has been found to develop, by a similar process of rapid 

 proliferation and small-cell formation, in the Mammal. But it may l)e argued that the fascia dentata 

 in the Mammal is a group of cells set quite a[)art from the hippocampal column of pyramidal cells. 

 The developmental history of the fascia dentata, at which I have briefly hinted in the Monotreme, 

 and fliuch follows a similar course in the placental Mammal, shows that the fascia dentata, or rather its 

 "stratum grauulosum," is originally the ventral (or inner) extremity of the column of pyramids which 

 becomes separate at a later period of development. This process of separation can be actually seen in 

 the adult brain of any Monotreme or ■Marsupial — for in these Orders the fascia dentata becomes reduced 

 to very insignificant proportions at the cephalic extremity of the hippocampus; and as its cells are thus 

 brought into line with the stratum of pyramidal cells of the hippocampus proper, the picture of the 

 Saurian eonditi(m is complete {vide "The Fascia dentata," Anat. Anzeiger, 1896). It is not surprising 

 to find that this ditt'erentiation of the hippocampal column of cells is not found in all Reptiles, when we 

 recall the fact that in the Mammal the fascia dentata is late in making its appearance. And it is of 

 interest to note (although it is not quite pertinent to this discussion;, that, just as the fascia dentata is 

 the last part of the hippocampal formation to make its appearance, so it is the first part to dwindle 

 and disappear in the retrogressive changes which involve the anterior portitms (supra- and precommissural) 

 of the great hippocampal arc in the Eutheria, leaving a hippocampal vestige devoid of fascia dentata 

 {vide supra; also Trans. Linn. Soc. ser. 2, Zool., vol. iii. 1897, pp. 1-7-69; also Journal of Anatomy 

 and Physiology, vol. xxxii. 1897). 



The problem of the meaning of this peculiar change resulting in tiie development of the fascia dentata 

 presents itself for solution. It is an extremely difficult question, which does not peruiit of being definitely 

 settled in the present state of our knowledge. But the following suggestions may form a tentative 

 hypothesis until such a fuller knowledge is forthcoming. 



In the Reptile large numbers of nerve-fibres stream from the bulbus olfactorius into the neighbouring 

 area of hippocampus. In the Monotreme and Marsupial the corresponding fibres become collected 

 mainly at the ventral margin of the hippocampal formation (see Journal of Anatomy and Physiology, 



* Good figures of the brain of Chelone mydas will be found in Edinger's memoir in the Abhandl. d. Seiickenberg. 

 natnrf. (iescUscb., ItSltd, Taf. 2. figs. 5 & G. 



SECOKl) SERIES.^ZOOLOGV, VOL. VIII. 70 



