10 INTRODUCTION 



Heteretxonia or BiJateria. In the latter the blastopore 

 undergoes a secondary shifting, so that the later chief axis 

 can no longer he identified with the primitive axis. 



The layered structure of the Metazoa becomes more com- 

 plicated by the appearance of a cell-layer introduced between 

 the ectoderm and entodei-m, which takes a position in the 

 primitive body cavity, the remnant of the cleavage cavity, 

 and i« designated as mesoderm or middle germ-layer. This 

 name is applied to any cell-layer introduced between ecto- 

 dei'm and entoderm and separated from both by a sharp 

 boundary, but it is not intended thereby to maintain the 

 homology of this layer for all the Metazoa. On the contrary, 

 it appears that in the Protaxonia mesodermic layers were 

 independently acquired in •various ways. Even in the 

 Bilateria the homology of the mesoderm in all groups is 

 not absolutely established, although it may be assumed as 

 probable. 



The mesoderm of the Bilateria arises as a rale out of the 

 primary entoderm, which in such cases is divided into two 

 parts : mesoderm and secondary entoderm. In regard to 

 the mode of origin, we can distinguish two sharply separated 

 types: the formation out of two pplmitive mesoderm cells 

 and the formation by the production of diverticula of tlie 

 arcJienteron} The first type is widely distributed among the 

 Bilateria. At an early period there become conspicuous at 

 the prostoma of the gastrula-stage two peculiar cells, by 

 whose position the median plane, which passes between the 

 two, is determined. These cells are known as the primitive 

 mesoderm cells. They move into the space between the 

 ectoderm and entoderm (therefore into the primitive body 

 cavity), and by proliferation give rise to two paired cell- 

 bands, which are called the mesoderm bauds, and out of 

 which the oi^gans of the mesoderm are constructed. The 

 formation of the mesoderm by the production of diverticula 



' As a third type of mesoderm formatiou one might perhaps cite the 

 formation of a tnesenchyma (compare p. 11) in those cases in which, as 

 in the Nemerteans and Echinoderms, numerous wandering cells migrate 

 into the blastocoele at an early period. Yet this type could perhaps be 

 reduced to one of those mentioned above. 



