40 



ZOOLOGY 



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

 gastrula, is the blastopore (Fig. 13, bp). Such an organism as 

 Hydra (see Fig. 81) may be looked upon as a permanent gastrula, 

 somewhat modified in form. In morulas in "which the seg- 

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

 large (Fig. 13, 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 

 usually substantially as detailed. If the term gastrula is applied 

 to these we have to say that they are formed in some other way 

 than by ordinary invagination. 



56. Library Reference. — By reference to books on embryology the students 

 may 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. 



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 



Fig. 14. Modes of forming mesoderm (diagrams modiiied from Whitman and Selenka). 

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

 active pole; £. 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 (s.c), in an Echinoderm. 

 a, archenteron. 



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 



