PLAN OF OilGANIZATION, 



3 



Fig. 1. DIAGRAM OP AN EARLY HUMAN EMBRYO. (Allen Thomson.) 



.?, s, indications of the vertebral divisions along the line of the back ; r, u, upper limb ; t, f, lower 

 limb ; u, umbilical cord. In the cranial part the divisions of the brain are indicated, together with 

 the eye, and an, the auditory vesicle ; near 6, the visceral arches and clefts of the head, forming inter 

 "fin the rudiments of the upper and lower jaws. 



Fig. 2. SEMIDIAGRAMMATIC VIEW OF A LONGITUDINAL SECTION OF THE EMBRYO REPRESENTED IN 



FIGURE 1 ; SHOWING THE RELATIONS OF THE PRINCIPAL SYSTEMS AND ORGANS. (Allen Thomson. ) 



1, 2, 3, 4, 5, primary divisions of the brain in the cranial part of the neural canal ; n, n, spinal 

 cord in the vertebral part of the canal ; s, spinous process of one of the vertebrae ; ch, chorda dorsalis run- 

 ning through the axis of the vertebral centra : ch', the same extending into the base of the cranium : a, 

 dorsal aorta ; p, pharyngeal cavity ; i, i, alimentary canal : h, ventricular part of the heai't, with 

 which the arterial bulb is seen joining the aorta by arches ; 6, visceral arches of head ; I, liver ; w, 

 Wolffian body ; r, urinary vesicle or allantois, joining the intestine in the cloaca, cl ; u, u l } umbilicus. 



Fig. 3. TRANSVERSE SECTION (DIAGRAMMATIC) OF THE TRUNK OF THE EMBRYO THROUGH THE UPPER 



LIMBS. (Allen Thomson.) 



MI, spinal cord ; n, neural or dorsal arch, including bone, muscle, skin, roots of the nerves, &c. ; 

 ch, chorda dorsalis, surrounded by the vertebral body or centrum ; v, ventral or visceral arch, or wall 

 of the body ; p, p, body cavity ; i, alimentary canal ; 7t, heart ; I, I, the rudimentary limbs. 



Fig. 4. FIRST DORSAL VERTEBRA WITH THE FIRST RIB AND UPPER PART OF THE STERNUM, SEEN FROM 



ABOVE. J. 



C', centrum ; N t neural cavity ; V, cavity of the chest, visceral cavity. 



B 2 



