332 



THE GAT. 



[CHAP. x. 



blast ; but although the nervous centres are thus formed, and though 

 the cranial nerves arise also as small imaginations of epiblast, yet 

 the mass of nerves which permeate the body in all directions cannot 

 be so derived, but must originate from mesoblastic tissue. The 

 ganglionic cells or nerve corpuscles are doubtless derived from 

 embryonic cells, which became transformed in nature and send forth 

 processes (very generally) in the way described.* 

 As to the nerve-fibres, they seem to be formed by the coalescence 



clu 



Fig. 151. LONGITUDINAL SECTION OF AN EMBRYO, SHOWING THE INCIPIENT VERTEBRAL 

 COLUMN WITH ITS VERTEBRAL CENTRA AND SPINOUS PROCESSES. 



1. Cerebral hemispheres. 



2. Vesicle of the third ventricle. 



3. Mid-brain. 



4. Cerebellum. 



5. Medulla oblongata. 



cli. Notochord passing forwards through the 

 bodies of the vertebrae into the basis cranii, 

 and terminating in the head between the 

 infundibulum and the pituitary body. 



s. The spinous processes of the vertebrae. 



n. The spinal cord. 



P. The pharynx. 



h. The heart. 



Z. The liver. 



i. The stomach and intestine. 



d. The cloaca. 



v. The urinary bladder and pedicle of the allan- 

 tois. 



11, u'. The umbilicus, or root of the umbilical 

 cord, containing the duct from the intestine 

 to the yelk-sac, the pedicle of the allantois 

 (or urachus), and the umbilical vessels. 



10. The Wolffian body, which appears as an 

 elongated viscus, with many transverse lines 

 situated above and between the two letters, 

 i and i ; whereof the left-hand i is placed 

 upon the stomach, and the right-hand i on 

 the large intestine close to the caecum and 

 entrance of the small intestine. 



in linear series of spindle-shaped cells, and to be at first of the 

 nature of pale or grey fibres, but afterwards, in great part, to acquire 

 a medullary sheath and to become white fibres. 



8. "We may next consider the origin of the organs, and first the 

 internal skeleton. 



The AXIAL SKELETON makes its appearance much earlier than 

 the appendicular skeleton, inasmuch as its foundations are laid in 

 the laminae dorsales bounding the medullary groove, and in the 

 notochord as already described. The distinction of the axial skeleton 

 into its vertebral and cranial portions is laid down from a very early 

 period, since the enlargement of the anterior end of the medullary 

 groove into the cerebral vesicles at once marks out the cranial part, 

 a distinction rendered yet plainer by the non- extension of the 

 chorda dorsalis forwards through it. 



* See ante, p. 255. 



