256 



AN INTRODUCTION TO ZOOLOGY 



optic vesicle, responsible for the production of the retina, at its outer 

 end, and a tubular portion, the optic stalk, connecting it with the 

 brain and marking the position of the future optic nerve. The level 

 where the stalk joins the brain is marked by an optic groove running 

 across the floor. This persists in the adult as the pre-optic recess, 

 and it forms an important landmark in the brain, since it marks the 

 hinder end of the lamina terminalis, and also the ventral posterior 

 limit of the telencephalon. The dorsal limit is soon laid down by 

 the appearance of a fold destined to form the velum transversum. 

 The antero-dorsal walls of the telencephalon now grow forward on 

 each side in the neighbourhood of the neuroporic recess, to form the 

 paired cerebral hemispheres, but the brain does not grow actually 



FIG. 84. Diagram of development of brain of Scy Ilium. 



I.-IV., four successive stages in development; II., stage of cranial flexure; IV., late stage, 

 for full naming compare with section of adult brain ; C., cerebrum ; CB., cerebellum ; F., fore- 

 brain, prosencephalon ; H., hind-brain, rhombencephalon ; I., infundibulum ; M., mid-brain, 

 mesencephalon ; Mt., metencephalon ; My., myelencephalon ; OO., optic groove; O.X., optic 

 chiasma ; P., pineal outgrowth ; Pi., pituitary body ; T., telencephalon ; Th., thalamencephalon, 



in the middle line. Hence even in the higher animals where the 

 cerebral development is great, the lamina terminalis always remains 

 to mark the end of the embryonic fore-brain. 



The next part of the brain, the thalamencephalon, is early marked 

 by two median outgrowths, while its roof remains for the most part 

 non-nervous. The dorsal projection is the beginning of the epiphysis 

 cerebri, while the ventral one is the infundibulum. The hypophysis 

 cerebri, however, is not an outgrowth from the brain at all. It arises 

 as a pocket-like diverticulum from the roof of the stomodoeum, which 

 then forms a closed vesicle with a solid stalk. The stalk disappears 

 and the vesicle takes up its position beneath the infundibulum. 



The roof of the mid-brain vesicle gives rise to two enlargements, 

 the future optic lobes, while its walls and floor become markedly 



