ZOOLOGY 



57. Germinal Layers. The ectoderm and entoderm have 

 thus far been mentioned as the primary germinal layers of cells. 

 Some of the Invertebrates have only these two layers, but in 



PIG. 14. Modes of forming mesoderm (diagrams modified from Whitman andJSelenka). 

 A and B, special mesoblasts distinguishable early in segmentation (Annelid): A, surface view from 

 active pole; B, sectional view of same, ec, micromeres destined to form ectoderm; en. macromeres 

 destined to form entoderm; m, primitive mesoblast which produces the mesoderm. C, amoeboid 

 mesodermal cells (c) budding from entoderm into the segmentation cavity (j.c.), in an Echinoderm. 

 a, archenteron. 



FIG. 15. 



S.C. 



FIG. 15. Mesoderm formed by pouches from entoderm after gastrulation. A and B, early 

 and later stages in formation of mesoderm and ccelom. a, primitive gut; bp., blastopore; cat, body 

 cavity, formed from pockets of the archenteron; ec., ectoderm; en., entoderm; m., mesoderm; m.so, 

 body-wall mesoderm; m.sp,, visceral mesoderm; s.c., segmentation cavity. 



Questions on figures 14 and 15. Enumerate the three modes of mesoderm 

 formation figured here. In which type may the mesoderm be identified most early 

 in the embryonic development? By comparing with other texts determine in 

 what groups of animals the mesoderm is formed as in Fig. 15. 



most cases a third mass of cells comes to be situated between the 

 ectoderm and entoderm, from which important organs are de- 

 rived. The third or middle layer (mesoderm) differs somewhat 

 in its origin in the different groups of animals. It may originate 



