450 GENERAL CONSIDERATIONS 



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. 144). 

 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 the case of 

 some forms the mesothelium is derived by a process of delamination from 

 the entoderm; in other cases, 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 

 metamerically arranged in pairs. Each pouch extends upward and down- 

 ward and becomes divided into three portions known respectively as 

 the e-pimere dorsally, mesomere laterally, and hijpomere ventrally. The 

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

 between these two layers being the true coelom. 



491. 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 

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

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



492. 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 within 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 life, 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 hver fluke (Sec. 200), echinoderms (Sec. 239), 

 insects (Sec. 313), tunicates (Sec. 338), and amphibians (Sec. 400). 

 Metamorphosis may be varied in degree, but the terms complete and 

 incomplete are applied only to metamorphosis in the insects, depending 



