32G 



EMBRYOLOGY. 



a 



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, distinct from 

 one another at first, now 

 become connected in the 



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

 (corvus corone) nearly four lines in lum ig gtm d andw ide ; 

 length, drawn under the simple lens. The ., ., ., r , i 



amnion, a, a, surrounds it closely on the pituitary body ap- 

 every side ; the allantois, 6, protrudes pears ; the cerebellum is 

 from the abdominal sulcus ; the extremi- formed ; but the fourth 

 ties are visible as simple lamellae ; nume- ventricle is still widely 

 rous segments of the vertebrae and the ftnd g over ^ 



several cerebral cells are conspicuous; 1 , 



behind the corpora quadrigemina appears a dee P posterior furrow of 

 the cerebellum, and then the depression the spinal cord. The eye 

 for the fourth ventricle; the ear is seen is developed in every part, 

 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. 



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 



and is very large ; the 

 external opening of the 

 ear is conspicuous, and 

 connexion with the 



in 



