BLASTULA, GASTRULA, AND GERM LAYERS 361 



the like; typical invaginations are seen in the formation of the optic 

 cup out of the optic lobe, the auditory sac, etc. When this form of 

 growth in extent is limited to certain directions instead of occurring 

 radially, the result is often the formation of a fold which bears some- 

 what the same relation to the dilation that the strand does to the bud. 

 The fold also may be above the surface of the membrane, forming a 

 sort of arch or hollow ridge, usually bounded by lateral depressions 

 (frog's pronephric duct), or below the surface forming a groove or furrow 

 bordered by lateral elevations (medullary groove) (Fig. 167). In some 

 instances a solid strand may be formed in this way instead of by the 

 simpler process of direct increase in thickness (Teleostean nerve cord, 



FIG. 166. Diagrams of four stages in the formation of an epithelial thickening, 

 several layers of cells deep. From Korschelt and Heider. 



in part). When the processes leading to the formation of a groove are 

 continued, the groove may be converted into a closed canal by the 

 approach, apposition, and fusion of the borders (neural tube) (Fig. 167). 



In those conditions where the proliferating cells become, to some 

 extent, separated either from one another or from the proliferating 

 region itself, we may note first, instances of actual cell migration. 

 This may be either emigration or immigration, according as to whether 

 we fix attention upon the source or the destination of the migratory 

 cells (mesenchyme cells) . Secondary processes of thickening or thinning 

 may accompany these processes. In other cases the movement of cells 

 may be described as rearrangement rather than migration; this may be 

 illustrated by the formation of mesodermal somites and blood islands 

 (chick). 



One of the common morphogenetic processes is a combination of 

 increase in thickness and cell rearrangement, such as the usual forma- 



