24 HUMAN ANATOMY. 



from the first, from those of the mesoblast, their component elements being more 

 compact in arrangement and early manifesting a tendency to acquire the character- 

 istics of covering cells or epithelium. 



The mesoblastic elements, on the contrary, soon assume irregular forms and 

 are loosely held together by intercellular substance, thus early foreshadowing the 

 special features which distinguish the subsequently differentiated connective tissues. 

 This early distinction becomes more marked as differentiation proceeds, the epithelial 

 tissues possessing elements of comparatively regular form, separated by minute 

 amounts of intercellular substance ; the latter in the connective tissues, on the con- 

 trary, becomes conspicuous on account of its excessive quantity and the resulting 

 profound modifications in the physical character of the tissue; the cells of the con- 

 nective tissues rapidly assume the irregularly stellate or triangular form so charac- 

 teristic in young tissues of this class. Since the three primary layers give rise to all 

 the tissues of the organism, a brief synopsis presenting these genetic relations here 

 finds an appropriate place. 



DERIVATIVES OF THE BLASTODERMIC LAYERS. 



From the ectoderm are derived — 



The epithelium of the outer surface of the body, including that of the conjunc- 

 tiva and anterior surface of the cornea, the external auditory canal, to- 

 gether with the epithelial appendages of the skin, as hair, nails, sebaceous 

 and sweat-glands (including the involuntary muscle of the latter). 



The epithelium of the nasal tract, with its glands, as well as of the cavities 

 communicating therewith. 



The epithelium of the mouth and of the salivary and other glands opening into 

 the oral cavity. 



The enamel of the teeth. 



The tissues of the nervous system. 



The retina ; the crystalline lens, and perhaps part of the vitreous humor, . 



The epithelium of the membranous labyrinth. 



The epithelium of the pituitary and pineal bodies. 



From the mesoderm are derived — 



The connective tissues, including areolar tissue, tendon, cartilage, bone, den- 

 tine of the teeth. 



The muscular tissues, except that of the sweat-glands and dilator pupillae. 



The tissues of the vascular and lymphatic systems, including their endothelium 

 and circulating cells. 



The sexual glands and their excretory passages, as far as the termination of the 

 ejaculatory ducts and vagina. 



The kidney and ureter. 



From the entoderm are derived — 



The epithelium of the digestive tract, with that of all glandular appendages 

 except those portions derived from ectodermic origin at the beginning 

 (oral cavity) and termination of the tube. 



The epithelium of the respiratory tract. 



The epithelium of the urinary bladder and urethra. 



The epithelium of the thyroid and thymus bodies, the modified primary epithe- 

 lium of the latter giving rise to Hassall's corpuscles. 



The Primitive Streak and the Gastrula. — Examined from the surface 

 during the formation of the primary layers, the mammalian blastodermic vesicle, as 

 represented by that of the rabbit, presents a circular light-colored field, the embryonic 

 area, which corresponds to the expansion of the original embryonic spot, the latter 

 becoming larger with the extension of the ectoblast and the entoblast differentiated 

 from the inner cell mass. At first circular, the embryonic area later becomes oval 

 or pyriform in outline (Figs. 22, 23), the larger end corresponding with the cephalic 



