478 GENERAL CONSIDERATIONS 



In the ctenophores for the first time a distinct mesoderm lying between 

 the ectoderm and entoderm is encountered, although it is composed of 

 few cells. From the flatworms onward, however, the mesoderm makes 

 up a large part of the mass of the body and as a result of differentiation 

 forms a variety of tissues. 



The mesoderm includes both mesothelium and mesenchyme (Sec. 146). 

 The mesenchyme is derived from ectodermal or entodermal cells which 

 are freed in the blastocoel and form a loose meshwork; this appeared for 

 the first time in the ctenophores and became well developed in the plana- 

 rian. Cavities may appear in the mesenchyme; when these form spaces 

 between the viscera and appear like portions of a body cavity but 

 contain blood, they form what is known as a hemocoel. In some forms 

 the mesothehum is derived by a process of delamination from the ento- 

 derm; in others, however, it is produced by outpocketings of the wall 

 of the archenteron which become cut off from that cavity and form 

 what are known as mesodermal pouches. These pouches are metameri- 

 cally arranged in pairs. Each pouch extends upward and downward 

 and becomes divided into three portions known respectively as the 

 epimere dorsally, mesomere laterally, and hypomere ventrally. The 

 hypomere is divided into the somatic and splanchic layers, the space 

 between these two layers being the true coelom. 



493. Tissue Formation and Organogeny.— From the three germ layers 

 develop all the tissues of the mature animal. Organogeny has been 

 defined as the development of organs by the association of tissues and 

 leads to the development of systems. Organogeny takes place in a 

 variety of ways in the different phyla, and references in various places 

 earher in this text, especially in the preceding chapter, have indicated 

 certain details in the development of the organs and organ systems. 



494. Postembryonic Development. — In all the lower animals as long 

 as the embryo remains within the egg it is spoken of simply as an embryo. 

 In these forms when the organism escapes from the egg it is extremely 

 simple, and a considerable degree of growth and development is neces- 

 sary before it becomes mature. During this period it is known as a 

 larva. In higher forms, however, the organism is much more complex 

 when it is freed from the egg and may be quite similar to the adult, in 

 which case it is simply recognized as young. In birds the young animal 

 within the egg is given the same name as after it has hatched ; an example 

 is the chick. The young \^^thin the body of the mammal is called an 

 embryo until about one-third of the time during which it is retained in 

 the uterus has elapsed, after which it is called a fetus. 



If pronounced changes take place during larval Hfe, the phenomenon 

 is known as metamorphosis. This has been noted in the discussion of the 

 biogenetic law and in connection with the development of several types 

 including the sheep liver fluke (Sec. 202), echinoderms (Sec. 241), 



