454 Animal Biology 



(2) the upper ectoderm layer which gives rise to the brain, spinal cord, 

 outer skin, etc. Between the ectoderm and entoderm, on either side of 

 the median Une, and originating from both, there are formed two groups 

 of cells: (1) the membranous mesoderm (mesothelium) and (2) a 

 loosely arranged meshwork of cells called the mesenchyme. The ecto- 

 derm, mesoderm, and entoderm are known as the three primary germ 

 layers, because from them arise all the tissues and organs of the future 



oroanism. 



The tissues of the adult are derived from the primary germ layers as 

 follows : 



1. From the Ectoderm — The epidermis and its derivatives, as hair, 

 nails, glands, lens of the eye; nervous tissues, including the neuroglia; 

 the epithelium of the organs of special sense, of the mouth and its oral 

 glands, of the hypophysis, of the anus, the amnion; the chorion; the 

 smooth muscles of the iris (eye), and the sweat glands. 



/I. 



B. 



Fig. 225. — Human embryos. The membranes (amnion and chorion) have 

 been removed to show the embryo. The approximate sizes and ages are given for 

 each one. Drawn somewhat diagrammatically and somewhat enlarged. A, Thirty- 

 three days old and 5 mm. long, showing the umbilical cord, pharyngeal clefts, 

 tail, limb buds, eye, etc.; B, thirty-eight days old and 7.5 mm. long, showing the 

 enlarging heart (shown just below the pharyngeal clefts), the beginning of the 

 external ear, the myotomes (muscles) in the dorsal region; C, seven weeks old 

 and 17 mm. long; D, about eight weeks old and 24 mm. long. (Modified from 

 various sources.) 



2. From the Mesodenn — The epithelial lining of the pericardium, 

 pleura, peritoneum, urogenital system; striated muscles; smooth mus- 

 cles; notochord; connective tissues, including cartilage and bone; bone 

 marrow; blood; lining (endothelium) of the blood vessels and lymph 

 system; lymphoid organs; the cortex of the suprarenal gland. 



3. From the Entoderm — The epithelium of the pharynx and its deriva- 

 tives, the thyroid, parathyroids, thymus, tonsils and auditory tube; the 



