44 DEVELOPMENT OF THE DOG-FISH. 



transverse partition into two — an anterior and posterior (lower 

 and upper respectively, if cord be supposed to be horizontal.) 

 By the absorption of the roof of the posterior slit, the posterior 

 fissure is formed. The anterior slit remains as permanent 

 central canal of spinal cord. The anterior fissure, which appears 

 bef ore posterior, is formed by growing downwards of the parts of. 

 the cord on each side of the middle line, so as to enclose a some- 

 what linear space. 



Brain— Spinal canal dilates anteriorly into these vesicles — 

 anterior, posterior, and middle cerebral vesicles. 



From the anterior are produced — 



(1) Cerebral hemispheres. 



(2) Corpora striati. 



(3) Olfactory bulbs. 



(4) Optic thalami. 



(5) Pineal gland. 



(6) Pituitary body or gland (in part). 



(7) Third ventricle. 



(8) Optic nerve (primary). 

 From middle cerebral vesicle — 



(1) Optic lobes. 



(2) Aqueduct of Sylvius. 



(3) Optic nerve (secondarily). 

 From posterior — 



(1) Cerebellum. , 



(2) Medulla oblongata. 



(3) Fourth Ventricle. 



DEVELOPMENT OF ENDOSKELETON. 



(a) Vertebral Column — Protovertebrse form true vertebrae, by 

 providing the notochord with a cartilaginous sheath, which 

 becomes constricted to form bodies of the vertebrse, and sends 

 prolongations around spinal cord to form the neural canal. 



(b) Skull — Two bars of cartilage appear at anterior part of 

 notochord — the parachordal cartilages. In front of these other 

 two — the trabeculse cranii. These latter are at first separated 

 from one another and from the former, thus allowing a space to 

 intervene. Through this space the upper and back part of the 



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