INTRODUCTION TO THE METAZOA 



83 



interior. The blastocoel is gradually obliter- 

 ated during invagination, while a new cavity, 

 the gastrocoel ( archenteron ) is established 

 and is bounded by the invaginated cells. The 

 gastrocoel lined by endoderm represents the 

 future gut cavity. The developmental stage 

 is now called a gastrula, and the process by 

 which it developed from the blastula is 

 termed gastrulation. In the embr)-ogeny of 

 many species of animals, invagination does 

 not occur, but certain cells of the blastula di- 

 vide and fill up the segmentation cavity as 

 in the hydra (Fig. 56, p. 116). 



Germ layers 



The gastrula of the simple metazoan 

 (sponges, coelenterates) consists of two 



Sperm 



germ layers, an outer ectoderm and an in- 

 ner endoderm. These phyla are said to be 

 diploblastic. However, in the most complex 

 animals, a third layer, the mesoderm, arises 

 as a result of gastrulation. Thus all the 

 higher metazoans arc triploblastic. The 

 origin of the mesoderm varies in different 

 groups, originating either as a result of multi- 

 plication of a few special blastomeres which 

 may be recognized in the early cleavage 

 stages, or from pouches arising from the 

 walls of the archenteron. All the tissues and 

 organs of the body are differentiated from 

 these germ (embryonic) layers. The struc- 

 tures that develop from the germ layers are 

 indicated in Fig. 42. 



Egg 



Zygote 

 (fertilized egg) 



Centra 



nervous 



system 



Epidermis 



Sense 

 receptors 



ENDODERM 



MESODERM 



Notochord 

 (eventuc 

 surround 

 by verteb 



Dermis 

 (in part) 



Muscles 



Most of 



reproductive 



system 



Circulatory 

 system 



Skeleton 



Peritoneum an 

 mesenteries 



Connective tiss 

 of digestive a 

 other organs of 

 endodermal origin 



Figure 42. 



Excretory 

 system 



Simplified diagram of the embr>onic differentiation in a vertebrate. 



Lining of 



respiratory 



system 



Lining of 



urinary 



bladder 



Most of 



digestive 



epithelium 



Lining of 

 ddle ear 



Thyroid 



and 



thymus 



