510 



COMPARATIVE ANATOMY. 



Fishes and Amphibia during the earlier stages of development. 

 The simple lamella is developed into a large body, in which, as in 

 the Crocodilini and Aves, the first part to be differentiated is the 

 middle. In the Marsupialia, however, this forms a delicate trans- 

 verse commissure for some time, while the lateral parts are developed 

 to a greater size. Transverse lamellae are developed in both regions, 

 and are arranged in various groups. The median portion is always 

 the larger in the Monotremata, and it is large in the Marsupialia, 

 Edentata, and Chiroptera. It is not till we come to the Carnivora 

 and Ungulata that we find the lateral parts, or ' ' hemispheres of 



the cerebellum," de- 

 ll veloped to a greater 

 size ; in most Primates 

 they are so much the 

 larger, that the median 

 portion diminishes, 

 and is known as the 

 " vermis." 



As the prosence- 

 phalon increases in 

 size, it gradually 

 covers the other di- 

 visions of the brain. 

 In many Marsupials, 

 and in Rodents (cf. 

 Fig. 287, ,4) and In- 

 sectivora, it does not 

 reach to the corpora 

 quadrigemina ; and, 

 in most of the other 

 Mammalia the meten- 

 cephalon is altogether 

 or very nearly free; 

 in the Primates, however, this portion is altogether below the 

 posterior lobes of the prosencephalic hemispheres ; in this point the 

 anthropoid Apes most closely resemble Man. When the hemi- 

 spheres of the metencephalon increase in size, a transverse commis- 

 sure is developed on the lower surface of the primitive myelencephalon 

 — the pons Varolii; this appears to unite the anterior portion of the 

 myelencephalon more closely to the cerebellum. This commis- 

 sure is feebly developed in the Monotremata and Marsupialia, and 

 most largely in the higher Primates. 



Fig. 2S7. Brain of the Rabbit. A From above, 

 B From below, lo Olfactory lobes. I Prosencephalon. 

 Ill Mesencephalon. IV Metencephalon. V Myelen- 

 cephalon. h Hypophysis. 2 Optic. 3 Ocnlo-motor. 

 5 Trigeminus. 6 Abducens. 7 8 Facial and auditory 

 nerves. In A the roof of the right hemisphere is 

 removed so that we can see into the lateral ventricle, 

 and make out the corpus striatum in front, and the 

 fornix with the commencement of the pes hippocampi 

 major behind. 



Miiialkovics, V. v., Entwickelungsgesch. des Gehirns. Leipzig, 1877- 



