4 INTRODUCTORY I 



suggested that some of the principal organs might be moulded 

 by mere local inequalities of growth the ventricles of the brain, 

 for instance, the alimentary canal, the heart and he further 

 succeeded in imitating the formation of these organs by folding, 

 pinching, and cutting india-rubber tubes and plates in various 

 ways. This analysis, however, deals only with the foldings of 

 flat layers, and must be supplemented by a more exhaustive 

 catalogue of the processes concerned in ontogeny, such as that 

 more recently suggested by Davenport. Davenport resolves the 

 changes in question into the movements of cells or cell aggre- 

 gates, the latter being linear, superficial, or massive, and within 

 the limits of these categories the phenomena are susceptible of 

 further classification. The catalogue proceeds as follows : 



I. THE MOVEMENTS OF SINGLE CELLS. 



1. Migration of nodal thickenings in a network of protoplasm : 

 e. g. the migration of the ' cells ' to the surface of the Arthropod 

 ovum to form a blastoderm, the movements of vitellophags, and 

 yolk-nuclei (Fig. 1). 



a. &. 



FIG. 1. Sections of the egg of Geophilus ferrug'meus showing two 

 stages in the formation of the blastoderm : II, blastoderm ; dp, yolk 

 pyramids ; gr, groups of blastoderm cells on what will be the dorsal side ; 

 k, nuclei surrounded by masses of protoplasm. (After Sograff, from 

 Korschelt and Heider.) 



