530 BASIC FEATURES OF VERTEBRATE MORPHOGENESIS 



entodermal lining give origin to smooth muscle tissue, connective tissue, etc. 

 (figs. 254C, D; 258; 260; 262; 278C). It is apparent, therefore, that the gut 

 throughout its length is formed from two embryonic contributions, namely, 

 one from the entoderm and the other from the mesenchyme given off by the 

 medial walls of the lateral plate or hypomeric mesoderm. 



(Note: The word splanchnic is an adjective and is derived from a Greek 

 word meaning entrails or bowels. That is, it pertains to the soft structures 

 within the body wall. The plural noun viscera (singular, viscus) is derived 

 from the Latin and signifies the same structures, namely, the heart, liver, 

 stomach, intestine, etc., which lie within the cavities of the body. It is fitting, 

 therefore, to apply the adjective splanchnic to the medial portion of the hypo- 

 mere because it has an intimate relationship with, and is contributory to, the 

 development of the viscera. The somatic mesoderm, on the other hand, is the 

 mesoderm of the lateral or body-wall portion of the hypomere. The word 

 splanchnopleure is a noun and it designates the composite tissue of primitive 

 entoderm and splanchnic mesoderm, while the word somatopleure is applied 

 to the compound tissue formed by the primitive lateral wall of the hypomere 

 (somatic mesoderm) plus the primitive ectoderm overlying it. The coelom 

 proper or splanchnocoel is the space or cavity which lies between the splanchnic 

 and somatic layers of the lateral plate or hypomeric mesoderm. During later 

 development, it is the cavity in which the entrails lie. 



3) Contributions of the Hypomere (Lateral Plate Mesoderm) to the Ex- 

 ternal (Ectodermal or Epidermal) Body Tube. The somatopleural mesoderm 

 gives origin to a mass of cellular material which migrates outward to lie along 

 the inner aspect of the epidermal tube in the lateral and ventral portions of 

 the developing body (fig. 252A, D). In the dorsal and dorso-lateral regions of 

 the body, contributions from the sclerotome and dermatome apparently aid 

 in forming this tissue layer. The layer immediately below the epidermis con- 

 stitutes the embryonic rudiment of the dermis. (See Chap. 12.) 



4) Contributions of the Hypomere or Lateral Plate Mesoderm to the Dorsal 

 Body Areas. Many cells are given off both from splanchnic and somatic layers 

 of the hypomeric mesoderm to the dorsal body areas above and along either 

 side of the dorsal aorta (fig. 254), contributing to the mesenchymal "pack- 

 ing tissue" in the area between the notochord and differentiating somite, ex- 

 tending outward to the dermis. 



5) Contributions of the Lateral Plate Mesoderm to the Walls of the Coe- 

 lomic Cavities. The pericardial, pleural, and peritoneal cavities are lined, as 

 stated above, by an epithelial type of tissue called mesothelium (fig. 254A-E). 

 These coelomic spaces (see Chap. 20) are derived from the fusion of the 

 two primitive splanchnocoels or cavities of the two hypomeres. External to 

 the mesothelial lining of the coelomic spaces, there ultimately is developed a 

 fibrous, connective tissue layer. Thus, mesothelium and connective tissue form. 



