SECTION II. 

 DEVELOPMENT OE THE OEGANS OF THE BODY. 



Classification. The organs of the body are generally classified according to 

 the germinal layer from which their primary or specific elements are derived, but 

 the middle layer enters into the construction of all organs inasmuch as their 

 connective-tissue framework, sheathing or covering membranes, and blood-vessels 

 are derived therefrom. From the histological point of view such a classification 

 is of importance, because the tissues springing from the several layers show a 

 certain specific character, both in their history and structural features. This ia 

 especially true of the epithelia derived from the two primary layers, and it holds 

 also, though less rigidly, for the derivatives of the dorsal, gastral, or epithelial 

 mesoderm. The second order of mesoderm the so-called mesenchyme is r 

 however, a blastema of more heterogeneous characters. 



With these reservations, the following classification will form the general basis 

 of the descriptive account of organogeny. From the ectoderm are derived the 

 epidermis and dermal appendages (hair and nails) ; the epithelium of the sebaceous 

 and sweat glands as well as of the mammary gland ; of the mouth (in part) ; of the 

 anal canal (in part) ; of the nasal passages as well as of the glands and cavities 

 opening thereinto ; the glandular part of the pituitary body ; the enamel of the 

 teeth ; the whole central and peripheral nervous system ; the epithelium of the 

 sense-organs ; the sympathetic-chromophil system, and medulla of the suprarenal 

 body. From the entoderm are derived the epithelium of the alimentary canal 

 and all the glands connected therewith ; of the Eustachian tube and tympanum ; of 

 the larynx, trachea, bronchi, and pulmonary alveoli ; of the thyroid and thymus ; of 

 the urinary bladder and of part of the urethra. From the epithelial mesoderm 

 are derived the voluntary muscles ; the epithelium of the Wolffian and Miillerian 

 ducts, and of the excretory tubules both of Wolfnan body and kidney ; the 

 epithelial lining of the body-cavity ; the cortex of the suprarenal body ; the genital 

 cords of ovary and testis (and perhaps the germ -cells). From the mesenchyme 

 are derived the connective tissues, the involuntary muscular tissue, the spleen, 

 haemolymph and lymphatic glands, the endothelial lining of the heart, blood and 

 lymph vessels, the red blood-corpuscles, and perhaps the lymph-corpuscles. 



DEVELOPMENT OF THE SKIN, CUTANEOUS GLANDS, ETC. 



In the section dealing with the formation of the embryo on the blastoderm, it 

 has already been noted that the ectoderm is early differentiated into a thickened 

 axial plate, the neural plate, and thinner lateral portions. When the neural canal 

 has become closed and the embryo is separated from the yolk-sac, the surface 

 ectoderm forms a continuous layer which gives rise to the epidermis. The 

 ectoderm at first consists of an epithelial layer of more or less columnar cells ; 

 but in the stage represented in fig. 81, p. 56, it has taken the form of a thin 

 lamella of apparently continuous granular protoplasm with a single layer of 

 nuclei regularly and closely disposed. By the end of the first or beginning of 



