120 



METAZOA IN GENERAL 



entoderm. This third layer is termed mesoderm. If the mesoderm is 

 composed of a meshwork of scattered cells which have passed from either 

 of the other layers into the blastocoel, it is known as mesenchyme (Fig. 

 47 7). In the embryogenies of certain animals the mesoderm cells are 

 formed by an outpocketing of entoderm cells, which pushes into the 

 space between the entoderm and ectoderm (Fig. 48). In those of other 

 animals they are separated from the wall of the archenteron as solid 

 masses of cells, which later become hollow. In both of the latter cases 

 the cells surrounding these cavities form mesotheliu7n (Fig. 48 H and 7). 



Ecfoderrn 



Blastocoel Blasfoderm 



G Blastopore H Archenteron I 



Fig. 47. — Diagrams illustrating the steps in an ideal embryogeny. A, the egg. B, 

 the two-cell embryo. C, the four-cell stage. D, the eight-cell stage. E, the sixteen-cell 

 stage. F , the morula, a solid mass. G, section of the blastula, with the blastocoel. H, 

 section of the gastrula. /, gastrula in which the mesoderm cells are appearing in the 

 blastocoel. These mesoderm cells will form mesenchyme. 



From the three germ layers tissues are developed. The tissues then 

 become arranged to form organs, the process being termed organogeny. 



This series of stages and processes may be outlined in the following 

 manner: 

 First stage: The egg cell (normally previously fertilized). 



First process: Cleavage, or segmentation. 

 Second stage: The morula. 



Second process: Formation of the cleavage, or segmentation, cavity. 

 Third stage: The blastula {monohlastic, or one-layered, embryo). 



Third process: Development of the archenteron, or gastrulation. 



