26 



INTRODUCTION 



forms. Mesoderm forms on either side of the primitive streak and 

 grows into the space between the ectoderm and the endoderm, as 

 in preceding groups. The mesoderm mass spKts into two sheets, 

 a somatic layer associated with ectoderm to form somatopleure and 

 a splanchnic layer associated with endoderm to form splanchnopleure. 

 Organogeny.— The development of similar organs differs in details 

 in different classes of vertebrates and a complete account belongs to 

 the field of comparative embryology. Later, as each tissue and 

 system of organs is considered, reference to early embryology will 

 be made wherever it is considered essential to an understanding 

 of the microanatomy of the organ in question. The following table 

 summarizes the structures usually considered as developing from 

 ectoderm, mesoderm and endoderm: 



From Ectoderm. I From Mesoderm. From Endoderm. 



Epidermis of skin, hairs, Cartilage, bone, peri- Epithelium of digestive 

 nails, sweat and sebaceous c ho ndrium, periosteum, ten- tract from pharynx to rec- 

 glands, claws, superficial ' dons, ligaments; other con- ' turn; glands of this tract. 



portion of feathers, arrec- 



tor pili muscles in skin; | cells; almost all muscle tis 



lens of eye; part of cornea 



retina; conjunctiva; lacri 



mal gland; most of pituit- 



ary body; epithelium lining neys; gonaducts and uri- 

 mouth and nasal cavities; nary ducts: cortex of adren- 

 sensory cells of many sense als; pleura?; pericardium; 



nective tissues; microglia | inckiding jjancreas and 

 liver; epithelium of larynx, 

 sue; blood cells; marrow; trachea, and lungs; gland- 

 blood vessels and lymph I ular epithelium of thyroid 

 vessels; gonads and kid- , and parathyroid glands; 

 epithelium lining the Eus- 



tachian tube and middle 

 ear. 



organs; pineal gland; me- 

 dulla of adrenal gland; 



peritoneum; lymph organs; 

 scales in fishes and bony 



enamel of teeth; all nerve plates in reptiles; major 

 cells and most neuroglia ' part of feathers. ! 



cells. 



In the past it has been a common ])ractice to emphasize the origin 

 of the tissues and organs with regard to the germ layers, but embryo- 

 logical studies show considerable \-ariations with regard to such 

 origins. Until the embryologists have settled the debatable points 

 we are not justified in placing nuich emphasis on origins. 



REFERENCES. 



Arey, Ij. B. 1934. Developmental Anatomy, Philadelpliia, W. B. Saunders 

 Company. 



Adelmann, H. B. 1932. The development of the prechordal plate and meso- 

 derm of Amblystoma punctatum, Jour. Morph., 54, 1-53. 



DE Beer, G. R. 1934. An Introduction to Exp3rimental Embryology, Oxford, 

 England, The Clarenden Press. 



Keith, A. 1933. Human Embryology and Morpliology, 5th ed., Baltimore, 

 William Wood & Co. 



Richards, A. 1931. Outline of Comparative Embryology, New York, John 

 Wiley & Sons, Inc. 



Shumway, W. 1935. Vertebrate Embryology, 3rd Ed., New York, J. Wiley 

 & Sons. 



