THE FCETUS. 907 



and\r?inflS''^"™T ^W^ remain adherent to the vertebral laminro, 

 ^r^l,ai • . !• ^ inwards with them to form the anterior portion of the 

 cephalo-mtestmal cavity. The latter is divided into two parts : the iri- 

 geal and the oesophageal cavities. ^ pnaryn- 



clos?iateSThwK^'l°P'''' externally by the month; it is partially 

 nffcr« « H^l .^' 1^ t^Plf ryngeal arches. The oesophageal cavity soon 

 f ritorfJ I r^"^'- ^^/'^ '.^^^'^^y ^**^' commmiicates with the Jleuro- 

 L"£ir*^^ *'^^ diverticulum lodges the heart, and is nam'ed the 



The cephalic laminae also form the derm of the cranium, and the fibrous 

 layer m whicH some of the cranial bones are developed. 



Article II.— Development op the various Organs of the Animal 



Economy. 



development op the nervous system. 



We wiU at first glance at the development of the nervous centres— the 

 brain and spinal cord ; then their peripheral parts, the nerves. 



1. Development op the Brain.— We have seen in the preceding 

 article how the primitive groove appears, and afterwards the medullary 

 cavity. The latter, the first trace of the nervous centres, offers an en- 

 largement at each of its extremities ; the posterior, or rhomboidal sinus, gives 

 off the nerves of the sacro-lumbar plexus ; the anterior gives rise to the 

 brain. 



The latter enlargement is resolved into three successive dilatations — the 

 cells or cerebral vesicles — distinguished as anterior, middle, and posterior. 

 They are filled with fluid, and the middle slightly surmounts the other two, 

 which gives the three together a triangular shape. 



The vesicles increase irregularly in volume ; their walls are gradually 

 developed, and form the nervous tissue; while their cavity persists, and 

 becomes the ventricles of each portion of the encephalon. Thus, the 

 anterior vesicle represents the cerebral hemispheres, the thalami optici, and 

 the lateral ventricles. The middle vesicle forms the cerebral crura, the 

 corpora quadrigemina, and the aqueduct of Sylvius or middle ventricle. 

 The posterior vesicle gives rise to the medulla oblongata, pons Varolii, 

 and the fourth ventricle. 



The middle vesicle is that which, at first, increases most rapidly in 

 volume ; it soon, however, ceases, to allow the anterior cell to become 

 developed, when the encephalon assumes its ovoid form, with a predominance 

 of the anterior part. 



Towards the end of the first third of intra-uterine life, nearly all the 

 parts of the brain are distinct : the two hemispheres have become isolable 

 by the development of the septum lucidum ; the convolutions appear on their 

 surface ; the corpora quadrigemina and cerebral crura are distinctly defined. 

 It is not until a little later that the cerebellum is distinguishable, as well 

 as the pons Varolii, medulla oblongata, corpora restiformia, and corpora 

 pyramidalia. 



2. Development op the Spinal Coed. — The medullary (spinal) canal 

 is the first trace of the spinal cord ; it occupies the whole length of the 

 vertebral column, and its cavity communicates, in front, with the fourth 

 ventricle. When the spine is developed, the marrow does not increase 

 proportionately in quantity, and appears to ascend in the vertebral canal ; 



