FROM SIMPLE CELL TO COMPLEX ANIMAL. 



35 



of the gastrula is the archenteron or embryonic digestive 

 tract; the opening into it, that is, the mouth of the gastrula, 

 is the blastopore (Fig. n, bp). In morulas in which the seg- 

 mentation cavity is small and the cells at the nutritive pole are 

 large (Fig. n, C, 4) this simple condition is much obscured, 

 and invagination as described above becomes impossible. Nev- 

 ertheless early in development the cells which produce the two 

 primitive layers are to be distinguished, and their relations are 

 always substantially as detailed. If the term gastrula is ap- 

 plied to these we have to say that they are formed in some 

 other way than by ordinary invagination. 



54. Library Reference. Let students report briefly on gastrulation by 

 overgrowth (epibole), and by delamination. Compare the results attained 

 by the various methods. Note what is constant in the methods and in the 

 results. 



55. 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 most cases a third mass of cells comes to be situated 

 between the ectoderm and entoderm, from which important 

 organs are derived. The third or middle layer (mesoderm 

 or mesoblasf) differs somewhat in its origin in the different 



FIG. 12. Modes of forming mesoderm (diagrams modified from Whitman and 

 Selenka). 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 ento- 

 derm into the segmentation cavity (s.c.), in an Echinoderm. a, archenteron. 



