LATERAL CONSTRICTIVE MOVEMENTS 493 



goes segmentation, the coelomic space within these areas becomes 

 segregated within the segments and, thus, is present in a discontinuous 

 condition. 

 (2) The early coelomic cavity in the shark and amphibian embryo, there- 

 fore, may be divided into three parts: (a) the myocoelic portion within 

 the epimeric mesoderm, (b) the nephrocoel within the nephrotomic 

 mesoderm, and (c) the splanchnocoel contained within the hypomeric 

 or lateral plate mesoderm. While the myocoelic and nephrocoelic re- 

 gions of the primitive coelom may become segmented and discontin- 

 uous, that within the splanchnocoel is continuous antero-posteriorly 

 in the trunk region. 



The coelomic cavities contained within the somites of the shark and am- 

 phibian embryo are soon lost. The coelomic cavity or nephrocoel within the 

 nephrotome is concerned with the development of the lumen within the tubules 

 and ducts of the excretory (urinary) system, while the splanchnocoels give 

 origin to the coelomic cavity proper of the adult. The lateral wall of the 

 splanchnocoel near the primitive epidermis is known as the somatopleural 

 mesoderm, and the inner or medial wall associated with the gut tube and 

 developing heart tissues constitutes the splanchnopleural layer. The epidermis 

 and somatopleural mesoderm together form the somatopleure, while the ento- 

 derm and splanchnopleural mesoderm form the splanchnopleure. 



In the embryos of higher vertebrates, the coelomic space of the somitic 

 portion of the primitive coelom (i.e., the myocoels) is less pronounced and 

 appears somewhat later in development than in the shark and amphibian 

 embryo, but it does tend to appear. This is true also of the nephrocoel or 

 coelomic cavity within the nephrotome. (See Chap. 18.) The coelomic con- 

 dition or splanchnocoel within the hypomere forms similarly in all vertebrates. 

 These matters will be described more in detail in Chapter 20. 



C. Notochordal Area 



The notochord is the elongated, median band of cells of the gastrula which 

 lies between the two mesodermal areas. The notochord thus may be regarded 

 as a specialized, median portion of the middle germ layer of mesodermal 

 tissue. During gastrulation and shortly after, there may be a tendency for the 

 notochordal material in certain forms to canalize or tubulate. Later, the noto- 

 chordal material becomes converted into a definite rod of notochordal cells 

 which represents the primitive skeletal axis of the embryo. The notochord 

 and its relation to the early skeletal system are discussed in Chapter 15. 



D. Lateral Constrictive Movements 



While the neural, epidermal, and entodermal tubulations are in progress, 

 a lateral constriction or invagination of the body wall occurs on either side 

 in all vertebrate embryos from the fishes to the mammals. These constrictions 



