DAVENPORT : PROCESSES CONCERNED IN ONTOGENY. 



187 



not include the lateral organs, — eye vesicles, gills, and arms, — which 

 arise in a similar fashion as folds. 



Based npon the form of the growing area, two classes of folds may be 

 recognized : — 



(«). Pockets, from a circular area. 



(b). Linear folds, from an elongated area. 



Fig. 19. 



Good examples of the formation of pocket folds are seen in many cases 

 of gastrulation, and in the otocyst, shell gland, salivary gland, and sepia 

 sac of Figure 18. 



Fig. 19. Two stages in the development of the epiilermis and neural tube of tlie 

 chick. Both magnified to the same extent. (From W. His, '94, Arch. f. Anatomie, 

 Jahrg. 1894, pp. 74, 76 ) 



VOL. XXVII. — NO. 6. 2 



