362 



GENERAL EMBRYOLOGY 



tion of the notochord, or the process of delamination, which consists 

 in the splitting of a single thickened sheet into two separate layers, 

 either as a whole or in localized areas (formation of mesoderm in the 

 frog, or division of the mesoderm into somatic and splanchnic layers) ; 

 in some instances the initial thickening may not be very apparent. 



Occasionally cells of different layers, or of different rudiments, meet 

 and fuse, forming a continuous rudimentary mass (pituitary body) . 



c 



FIG. 167. FIG. 168. 



FIG. 167. Diagrams of the formation of the medullary canal or neural tube, in 

 the Vertebrates. From Korschelt and Heider. 



FIG. 168. Diagrams of the formation of the mouth and stomodseum in a 

 typical form. From Korschelt and Heider. ec, ectoderm; md, pharynx; 

 vd, stomodaeum. 



Finally we may mention certain morphogenetic processes of a wholly 

 different kind, namely, resorption, and changes in the form and size of 

 cells. From this point of view merely, the process of resorption may be 

 regarded as the reverse of proliferation. Definite rudiments may appear 

 first as spaces thus formed by the gradual dissolution and absorption of 

 cells in certain areas. Thus the oral and anal openings, gill-clefts, etc., 

 are usually formed as "perforations" by the resorption of areas where 

 previously separate and continuous layers became united by a process 

 of fusion (Fig. 168). In other cases rudiments once established may 



