ORIGIN OF THE GERMINAL LAYERS. 



337 



of many Crustacea is almost the same, but is subsequently 

 followed by an invagination (fig. 208), which apparently corre- 



A. B. 



FIG. 203. DIAGRAMMATIC FIGURES SHEWING THE DELAMINATION OF THE 



EMBRYO OF GERYONIA. (After Fol.) 



A. Stage at the commencement of the delamination ; the dotted 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; cp. epiblast ; hy. hypoblast. 



sponds to the normal invagination of other types. There are 

 strong grounds for thinking that the tracheate type of forma- 



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



(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. 



B. III. 22 



