360 GENERAL EMBRYOLOGY 



tion of the more common and important processes, a few of which have 

 already been mentioned earlier in this chapter. 



While the morphogenetic processes within the embryo show the 

 greatest diversity and vary almost infinitely in specific details, yet it is 

 possible to include them all under a few heads, when these matters of 

 detail are omitted. Again it should be recalled that we are limiting 

 our description to the Chordata. 



The primary condition of morphogenesis is cell multiplication. After 

 each division the daughter cells increase to practically the size of the 

 parent cell; and numerical increase in cells, together with their growth, 

 i.e., cell proliferation, play either a primary or a secondary part in every 

 morphogenetic process. When the process of cell division is quite 

 general throughout the extent of the germ disc or layer, the result is an 

 increase in the thickness or in the extent of the sheet, respectively, when 

 the plane of the cell divisions is in general parallel with, or perpendicular 

 to, the plane of the whole layer. If there should be little or no regularity 

 in the positions of the division planes, the membrane would increase in 

 al] directions (Fig. 166). 



Ordinarily, in embryogeny, cell multiplication and growth are more 

 intense in restricted areas of the blastoderm or germ layer. It is 

 convenient then to distinguish between (a) those processes in which the 

 multiplying cells tend to remain associated in the same general region, 

 and (fr) other processes where they become more or less separated, either 

 from one another or from their seat of origin. Under the former head 

 we must again distinguish between the results of increase in thickness 

 and in extent. A localized increase in thickness is frequently termed a 

 bud; buds may project either above (limb bud) or below (Teleostean lens) 

 the free surface where they are formed. If the thickening region is 

 elongated the result may be the formation of a strand or plate of cells, 

 again either a ridge-like structure above the surface of the layer (genital 

 ridge), or a keel-like thickening below the surface (Teleostean nerve 

 cord, in part). 



Increase in extent of a localized area frequently involves the obstruc- 

 tive action of the region bounding the area. When this form of growth 

 occurs generally, so that the tendency to extension occurs in every 

 direction from the middle of the area concerned, the result is frequently 

 an arching, either outward or inward. This may take the form of a 

 simple arching as in the Teleostean blastula (Fig. 150, C), or the same 

 process may be carried farther and followed by a constriction near the 

 base. Such processes are very common indeed and are termed in- 

 vagination and evagination, according as the growth is below or above 

 the free surface. Simple illustrations of evagination are afforded by 

 the formation of intestinal villi, the rudiments of lung or thymus, and 



