126 INTRODUCTION. 



The mesoblast is only found in a fully developed condition 

 in the forms more highly organized than the Coelenterata. It 

 gives origin to the general connective tissue, internal skeleton, 

 the muscular system, the lining of the body-cavity, the vascular, 

 and excretory systems. It probably in the first instance origi- 

 nated from differentiations of the two primary layers, and in all 

 groups with a well-developed body-cavity it is divided into two 

 strata. One of them forms part of the body-wall and is known 

 as the somatic mesoblast, the other forms 

 part of the wall of the viscera and is known 

 as the splanchnic mesoblast. 



A very large number not to say the 

 majority of organs are derived from parts of 

 two of the germinal layers. Many glands 

 for instance have a lining of hypoblast which 

 is coated by a mesoblastic layer. 



The processes by which the germinal 

 layers take their origin are largely influenced 

 by the character of the segmentation, which, FIG DIAGRAM 

 as was shewn in the last chapter, is mainly OF A GASTRULA. 

 dependent on the distribution of the food- m bl?stopore; b. 



yolk. When the segmentation is regular, archenteron; c. hypo- 

 ' blast ; d. epiblast. 



and results in the formation of a blastosphere, 

 the epiblast and hypoblast are usually differentiated from the 

 uniform cells forming the wall of the blastosphere in one of the 

 two following ways. 



(1) One-half of the blastosphere may be pushed in towards 

 the other half. A two-layered hemisphere is thus established 

 which soon elongates, while its opening narrows to a small pore 

 (fig- 55)- The embryonic form produced by this process is 

 known as a gastrula. The process by which it originates is 

 known as embolic invagination, or shortly invagination. Of 

 the two layers of which it is formed the inner one (c) is known as 

 the hypoblast and the outer (d} as the epiblast, while the pore 

 leading into its cavity lined by the hypoblast is the blastopore 

 (a). The cavity itself is the archenteron (b}. 



(2) The cells of the blastosphere may divide themselves by 

 a process of concentric splitting into two layers (fig. 56, 3). The 

 two layers are as before the epiblast and hypoblast, and the 



