EXPLANATION OF PLATE V. 325 



the parts are perfectly developed, but diagrammatically reduced and sim- 

 plified, in order to exhibit clearly their relative positions and their relations 

 to the four secondary germ-layers. In the brain, the five original brain- 

 bladders (Fig. 15, m 1 -m s ) have been differentiated and transformed in the 

 manner peculiar to the higher mammals: m l , fore brain (cerebrum), out- 

 weighing and covering all the other four brain bladders; m t , twixt brain 

 (" the centre of sight ") ; m 3 , mid brain (" the four bulbs ") ; m 4 , hind 

 brain (cerebellum) ; m 5 , after brain, or prolonged marrow (medulla 

 oblongata), passing into the spinal marrow (m 6 ). The brain is enclosed in 

 the skull (s), the spinal marrow by the vertebral canal: above the spinal 

 marrow the vertebral arches and spinal processes, under it the vertebral 

 bodies (wk). The intestinal tube has differentiated into the following partB 

 lying one behind another : the mouth-cavity, the throat-cavity (in which at 

 an earlier period the gill-openings, ks, were situated), the trachea (Ir) with 

 the lungs (lu), the oesophagus (sr), the stomach (mg), the liver (lb), with 

 the gall-bladder (i), the ventral salivary gland, or pancreae (p), the small 

 intestine (dd), the large intestine (dc), and the rectum with the anus (a). 

 The body-cavity, or coelom (c), is divided by the diaphragm (z) into two 

 distinct cavities ; the breast-cavity (c), in which the heart (hz) lies in. front of 

 the lungs, and the ventral cavity in which most of the intestines lie. In front 

 of the rectum lies the sheath (vagina, vg), which leads into the uterus (/) ; 

 in this the embi'yo, indicated here by a small germ-membrane vesicle (e), is 

 developed. Between the uterus and the os pubis lies the vesica urines (hb), 

 the remains of the stalk of the allantois. The horn-plate (h) as the outer 

 skin, covers the whole body, and also forms the coating of the cavities of 

 the mouth, the anus, the vagina, and the uterus. The milk glands, or mamma 

 ( md), are also originally formixl from the horn. plate. 



