CHANGES liY THE OVUM. ^X 



are in existence, as well as the spinal ganglia, which are developed at the 

 expense of the protovertebrae ; the superior roots are not distinguishable 

 for some time after. The envelopes of the nervous centres are furnished 

 by the protovertebral laminae, and are developed after the sixth week, fol- 

 lowing the formation of the parts they are destined to cover. 



The fierves are not so definite in their development, and some obscurity 

 prevails with regard to them. It would appear that the motor roots origi- 

 nate in the cord, but that the ganglia are formed separately in the proto- 

 vertebrae, and perhaps become the point of departure of the sensitive 

 roots. The nervous ramifications grow from elongated ramified cells, 

 which are joined by their extremities. The nuclei of the cells, joined to 

 the periphery, become the nuclei of the sheath of Schwann, and the nerve 

 tissue is afterwards deposited gradually between the axis-cylinder and 

 the envelope. The great sympathetic nerve is perceived at an early date 

 as a nodulated cord ; it is probably developed in the same manner as the 

 other nerves. 



The Organs of Sense. 



The principal portion of the organs of sense belong to the nervous sys- 

 tem, and are, of course, developed with it ; the other portions belong to 

 the external epithelial layer, and to the derm or germinative layer. With 

 regard to the organs of vision, two tubulous prolongations arise from the 

 anterior cerebral vesicle and passing forward terminate in the primary 

 ocular vesicles, traces of the ocular globes ; the hollow prolongments 

 forming the optic nerves, and the vesicles furnishing the choroid and 

 retina. The crystalline lens, vitreous body, cornea, and sclerotica are 

 derived from the external blastodermic layer. The part of the integu- 

 ment not required to form the lens constitutes the envelope of the globe. 

 The latter forms the sclerotica and cornea ; while the epiderm furnishes 

 the epithelium to the latter, which becomes distinct from the sclerotica 

 in about the fourth month. A slit occurs at the lower part of the fibrous 

 envelope of the globe ; this is related to the development of the vitreous 

 humor, a prolongation of the derm passing through this slit and enter- 

 ing between the lens and the anterior wall of the secondary ocular vesi- 

 cle which appeared shortly before. There this prolongation becomes 

 developed and transformed into the vitreous humor, which at one period 

 is surrounded by vessels, but shows none immediately before gestation. 

 It exhibits in its centre a transverse canal, which lodges a branch of the 

 arteria centralis of the retina. 



The optic nerve is developed in the pedicle connecting the ocular vesi- 

 cle with the anterior cerebral vesicle, and the reti?ia is formed by the in- 

 ner la3^er of the secondary ocular vesicle ; it extends to the lens in chang- 

 ing its character in front. The choroid coat is constituted by the posterior 

 layer of the ocular vesicle ; it extends as far as the lens, and is then in- 

 flected in front of that body to form the iris. The borders of the pupil 

 embrace the vascular envelope of the lens, and the anterior face as well 

 as this pupil are covered by a very vascular membrane, the membrana 

 pupillaris ; behind it is the equally vascular covering, the membrana cap- 

 sulo-pupillaris, that passes through the pupil to the lens to envelop it in a 

 kind of sac that disappears towards the end of gestation. Previous to 

 this time this aperture is very wide ; but as the iris is developed the pupil 

 contracts, and the vessels of the vascular or pupillary membrane dimin- 

 ish in size and number, until at last only a few are seen crossing the 

 transparent membrane. 



