I INTRODUCTORY 11 



C. Massive aggregates. 



i. Changes in volume. 



a. Unequal in different axes : e. g. when the spherical larva 

 becomes cylindrical in Dicyemidae. 



j8. Unequal at different points : e. g. the outgrowth of limb- 

 buds of Vertebrates and other forms, of the buds of plants, 

 ii. Rearrangement of material. 



a. Simple rearrangement of cells : e.g. in the formation of the 

 concentric corpuscles of the thymus, in the development of kidney 

 tubules in the metanephric blastema of Amniota, in the grouping 

 of the cells to form ectoderm, gut and atrium in the Salps. 



It, Development of an internal cavity : e. g. segmentation 

 cavities, lumina of ducts and blood-vessels, of the coelom and 

 many generative organs. 



c. Dispersion of the elements of an aggregate : e. g. in 

 gemmule formation in certain Sponges, in unipolar immigration 

 in some Sponges and some Coelenterates, in the liberation of the 

 germ-cells. 



iii. Division of masses. 



a. By constriction : e. g. the segmentation of the mesoderm 

 and neural crest. 



b. By splitting : e. g. the nervous system from the ectoderm 

 in Teleostei and many Invertebrates, the notochord from the 

 roof of the archenteron. 



iv. Fusion of masses : e. g. of originally separate nerve ganglia 

 (Vertebrates, Arthropods, Annelids), of myotomes, of somites in 

 Arthropods. 



v. Attachment of one mass to another : e. g. of sclerotome to 

 notochord. 



It will be seen that this resume of the principal kinds 

 of movement executed by the developing parts extends His's 

 principle of the local inequality of growth from flat layers to 

 linear and massive aggregates and at the same time includes 

 the movements of isolated cells. Davenport, however, is not 

 content merely to give a simple classification of the phenomena; 

 he goes further, and endeavours to express them in terms of 

 responses to stimuli, an idea due in the first instance to Herbst. 



