908 EMBBYOLOG.Y. 



it stops about the middle of tte sacrum ia the equine foetus, but ascends 

 higher in the other species. During this apparently ascensional movement 

 is developed the^/ant terminale, and the nerves of the cauda equina. 



The walls of the medullary canal, at first very thin, increase in thickness 

 by the appearance of the nervous matter of the cord. Soon they divide 

 into two layers : an internal, the epithelium of the central canal ; the other 

 external, the greij substance of the cord. Gradually the canal contracts, and 

 the marrow is seen, with its longitudinal furrows. 



At the end of the first month, the inferior roots are in existence, as well 

 as the spinal ganglia ; the latter are developed at the expense of the proto- 

 vertebras, as will be shown presently. The superior roots are formed some 

 time afterwards. 



The envelopes of the cerebro-spinal centres are furnished by the proto- 

 vertebral lamina ; they are developed after the sixth week, progressing with 

 the growth of the parts they are destined to cover. 



3. Development of the Nerves. — 



Fig. 434. 



TRANSVERSE SECTION OF THE EMBRTO OE 

 A FOWL AT THE BEGINNING OF THE 

 THIRD DAT OP INCUBATION X 90-100. 



cA, Chorda dorsalis ; w-wh. Position of a 

 thinning, or cavity in the protoverte- 

 bral mass, dividing it into an anterior 

 and posterior portion ; hp, Parietal 

 lamina ; df. Intestinal fibrous lamina , 

 dd, Intestinal glandular lamina; dr, 

 Primitive intestinal groove ; h, Corneal 

 lamina ; mr. Medullary tube — spinal 

 cord ; m, Muscular lamina ; p, Pleuro- 

 peritoneal cavity; af, Fold of the 

 amnion ; ao, Primitive aorta ; vc, Vena 

 cardinalis ; un, Wolffian body ; ung, 

 Duct of the Wolffian body. 



or THE 



The development of the nerves is some- 

 what obscure. The motor roots seem to 

 arise from the cord ; but the spinal gan- 

 glia are formed separately in the pro- 

 tovertebrse, and perhaps originate the 

 sensitive roots. The nervous ramifica- 

 tions begin by elongated ramifying cells, 

 which become fused to each other by their 

 extremities ; the nuclei of the cells lying 

 at the periphery becoming the nuclei of 

 the white substance of Schwann, the 

 proper nervous tissue being afterwards 

 gradually deposited between the axis- 

 cylinder and the envelope. 



The great sympathetic is early seen 

 as a nodulated cord. It is probably de- 

 veloped in the same manner as the other 

 nerves. 



DEVELOPMENT OF THE OEGANS OF SENSE. 



The principal portion of the organs of 

 sense belong to the dependencies of the 

 nervous system, and are developed with it ; 

 the other parts belong to the external epithelial lamina, the skin, and the 

 germinative lamina. 



1. Visual Apparatus. — From the anterior cerebral vesicle are given off 

 two tubular prolongations, which are directed forward, and terminate by the 

 primitive eye-vesicles, traces of the ocular globes. The hollow protrusions 

 form the optic nerves ; the vesicles furnish the retina and choroid. The 

 crystalline lens, vitreous humour, cornea, and sclerotica arise from the 

 external blastodermic layer. These modifications take place in the follow- 

 ing manner : 



The external integument of the foetus passes over the front of the 

 primitive eye-vesicles. Here it shows a slight depression on its outer 

 surface, and on its inner face a cellular protrusion (Fig. 435, 1, o), which, 

 becoming developed, surrounds the external depression, completely closes it, 



