278 COMPARATIVE ANATOMY 



recognised in the adult as a linear thickening of the peritoneum 

 lying at a little distance from the outer border of the kidney 

 and extending some way in front of it. In the female the 

 posterior part of the oviduct grows very much in length, is 

 thrown into many convolutions, and its walls in the middle 

 part of its course become thick and glandular. In the hinder 

 part of its course the walls remain thin, but the diameter is 

 greatly enlarged to form the so-called uterus. 



The epiblast of the embryo gives rise to the external 

 epithelium (continued at the mouth and anus into the 

 stomodaeum and proctodaeum), the central nervous system, 

 and the organs of special sense. The infolding of the neural 

 folds to form the neural tube, the enlargement of the anterior 

 end of the latter, and its division into fore, mid, and hind 

 brain has already been described. The fore-brain becomes 

 the thalamencephalon of the adult. Its sides are thickened, 

 but its roof and floor remain thin, and in consequence its 

 cavity, the third ventricle, is narrowed from side to side, but 

 has considerable vertical depth. The roof grows out to form 

 the pineal body, the floor is produced into the infundi- 

 bulum which meets and becomes attached to the pituitary 

 body. The cerebral hemispheres are formed as a pair of 

 hollow outgrowths of the anterior end of the primitive fore- 

 brain or thalamencephalon, and the olfactory lobes are in 

 turn formed as outgrowths from the cerebral hemispheres. 



The floor of the mid-brain becomes very thick from the 

 development of two diverging tracts of nerve fibres, leading 

 from the hind-brain to the fore-brain ; these are the crura 

 cerebri. The roof of the mid-brain grows out into a pair 

 of hollow ovoid projections, the corpora bigemina, and its 

 cavity, reduced by the thickening of the floor and sides, 

 remains as the Sylvian aqueduct or "iter." 



The floor and sides of the hind-brain are much thickened, 

 but the roof becomes very thin except for a strip in front, 

 which is thickened to form the cerebellum. The thin hinder 

 part of the roof, covering in the wide cavity of the fourth 

 ventricle, is thrown into numerous folds, into which a vascular 

 membrane penetrates, forming the choroid plexus of the fourth 

 ventricle. 



The walls of the spinal cord are much thickened, and 

 undergo hi'stological and other changes which cannot be 



