326 



EMBEYOLOGY. 



the rudiments of the ribs become more conspicuous ; the imme- 

 diate tegument of the brain and spinal cord is perceived to be 



composed of two layers ; 

 the largely developed 

 corpora quadrigemina 

 seem to advance with 

 less rapidity of growth 

 towards the end of the 

 seventh day, and the he- 

 mispheres soon equal 

 them in size (fig. 353, 

 c, c, d, d) ; the fornix is 

 evolved over the still open 

 third ventricle ; the cor- 

 pora striata and thalami 

 become conspicuous ; the 

 optic nerves, distinctfrom 

 one another at first, now 

 become connected in the 



Fig. 350. Embryo of the jackdaw chiasma ; the infundibu- 

 (corvus cor&ne) nearly four lines in 

 length, drawn under the simple lens. The 

 amnion, a, a, surrounds it closely on 

 every side ; the allantois, b, protrudes 

 from the abdominal sulcus ; the extremi- 

 ties are visible as simple lamellae ; nume- 

 rous segments of the vertebra and the 

 several cerebral cells are conspicuous ; 

 behind the corpora quadrigemina appears 

 the cerebellum, and then the depression 

 for the fourth ventricle ; the ear is seen 

 as a pediculated vesicle, c, springing from 

 the medulla oblongata : under it lie the 

 branchial arches and fissures ; d is the 

 eye ; e, the nasal fossa, behind which the 

 heart is perceived. 



lum is still deep and wide; 

 the pituitary body ap- 

 pears ; the cerebellum is 

 formed ; but the fourth 

 ventricle is still widely 

 open, and passes over into 

 a deep posterior furrow of 

 the spinal cord. The eye 

 is developed in every part, 

 and is very large ; the 

 external opening of the 

 ear is conspicuous, and 

 in connexion with the 



auditory vesicle the semi- 

 circular canals and cochlea are formed ; the nasal depression 

 has lengthened downwards into a nasal passage, which runs 

 between the superior maxillary bone and the frontal process, 

 the opposite halves of which have now become united. In 

 the extremities, the arm and thigh, both extremely short, can 

 be distinguished ; in the hand the rudiments of the three 

 digits, and in the foot those of the four toes, can be made 



