MARINE TELEOSTEAN DEVELOPMENT. 81 



ventriculum, and a median pair constituting the fourth ventricle. 

 Several days before escaping from the egg, great extension 

 of the mid-brain occurs. 



On hatching, the mesencephalon (optic lobes) embraces the 

 largest extent of the brain, and gives it the bulbous form so 

 characteristic of young Teleosteans. The cerebellum is almost 

 entirely covered by the posterior enlargements of the optic 

 lobes, a thickened ridge only projecting. The fore-brain forms 

 a narrow laterally compressed mass with a small dorsal cham- 

 ber. On the second or third day after hatching, a deep fold 

 divides this into anterior fore-brain (or cerebrum) and posterior 

 fore-brain (or thalamencephalon), in addition to the longitudinal 

 fold formerly present. 



Very early a posterior part of the floor of the thalamen- 

 cephalon is directed backwards as a prolongation beneath the 

 elevated medulla oblongata, and at the same time obliquely 

 downwards. The cells surround a cavity continuous above 

 with the third ventricle, or cavity of the thalamencephalon. 

 This is known as the infundibulum, which abuts on the roof 

 of the oral cavity. A loose mesh-work of cells lies behind the 

 infundibulum and into this the front end of the notochord 

 pushes. On the summit of the arch caused by the elevation of 

 the oral roof a mass of cells appears, proliferating from the oral 

 roof-cells. This is the pituitary body (hypophysis) and it lies in 

 front of the infundibulum. A small median swelling, not unlike 

 the hypophysis, lies in front of the other, i.e. behind the point 

 where the optic nerves cross. Such appears to form the 

 inferior lobes (hypoaria), so well developed in Percoids. 



The small extent of the roof of the thalamencephalon ex- 

 posed becomes very thin and much folded. In sections there is 

 seen a central aggregation of cells, which later push out as a papil- 

 liform process, containing a cavity which is continuous with the 

 third ventricle below. The cells become columnar and it forms 

 the pineal 'gland.' It becomes truncated, more or less plicated 

 and pressed against the arachnoid membrane. Later still its 

 lumen is obliterated and it forms a deeply folded mass of 

 cells. 



The spinal cord, when fairly advanced, proceeds quite to the 



M. F. G 



