THE F(ETUS. 



1035 



bral laminfe ; they are developed after the sixth week, progressing with the 

 growth of the parts they are destined to cover. 



4. Dn'elopmmt of the nerves. — 'The development of the nerves is somewhat 

 obscure. The motor roots seem to arise from the cord ; but the spinal ganglia 

 are formed separately in the protovertebrte, and perhaps originate the sensitive 

 roots. The nerve-branches 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 

 nerve-tissue being afterwards gradually deposited between the axis-cylinder and 

 the envelope. 



Fig. 570. 



Fig. 569. 



TRANSVERSE SECTION OF THE EMBRVO 

 OF A FOWL AT THE BEGINNING OF 

 THE THIRD DAY OF INCUBATION, X 



90-100. 

 ch, Chorda dorsalis ; wwh, 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; A, corneal 

 lamina j mr, medullary tube (spinal 

 cord); m, muscular lamina; /), pleuro- 

 peritoneal cavity ; af, fold of the 

 amnion ; ao, primitive aorta ; t)C, vena 

 cardinalis ; un, Wolffian body ; ung, 

 duct of the Wolffian body. 



LONGITUDINAL SECTION OF THE EYE OF EMBRYO 

 FOWL. 



1, From an embryo at about the sixty-fifth hour of 

 incubation ; 2, from an embryo a few hours 

 older; 3, from an embryo at the fourth day of 

 incubation, h, Corneal lamina; /, lens in fig. 1, 

 still connected with the corneal lamina, and 

 possessing a small cavity, o, in its interior (in 

 figs. 2 and 3 it is seen detached, but still 

 hollow) ; r, introverted portion of the primitive 

 optic vesicle, subsequently becoming the retina ; 

 a, posterior j>art of the optic vesicle, which, 

 according to Remak, probably becomes the 

 choroid coat, ciliary processes, and iris, and in 

 figs. 1 and 2 is still connected with the brain 

 by the hollow optic nerve ; x, thickening of the 

 corneal lamina around the spot from which the 

 lens has detached itself; gl, vitreous body, or 

 humour. 



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 Organs 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 following 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. 570, 1 o), which, becoming developed, 



