ORIGIN OF THE GERMINAL LAYERS. 



279 



The development of almost all the parasitic groups, i.e. the Trema- 

 toda,the Cestoda,the Acanthocephala, and the Linguatulida, and also 





A. 



FIG. 203. DIAGRAMMATIC FIGURES SHEWING THE DELAMINATION OF THE EMBBYO 



OF (TERYONIA. (After Fol.) 



A. Stage at the commencement of the delamination; the clotted lines x shew the 

 course of the next planes of division. B. Stage at the close of the delamination. 



cs. segmentation cavity; a. endoplasm; b. ectoplasm; ep. epiblast ; hy. hypoblast. 



of the Tardigrada, Pycnogonida, and other minor groups, is too imper- 

 fectly known to be classed with either the delaminate or invaginate 

 types. 



FIG. 204. SEGMENTATION AND FORMATION OF THE BLASTODERM IN CHELIFEK. 



(After Metschnikoff.) 



In A the ovum is divided into a number of separate segments. In B a number of 

 small cells have appeared (bl) which form a blastoderm enveloping the large yolk- 

 spheres. In C the blastoderm has become divided into two layers. 



