6 INTRODUCTORY I 



2. Splitting. 



a. At the end, that is, branching : e. g. of nerves, blood- 

 vessels, kidney tubules, glands, tentacles. 



/?. Throughout the length : e. g. the segmental duct of 

 Elasmobranchs, the truncus arteriosus of Mammalia. 



3. Anastomoses : e. g. of the dorsal and ventral roots of the 

 spinal nerves, of nerve plexuses, of capillaries, of bile capillaries, 

 of the excretory tubules of Platyhelmia. 



4. Fusion with other organs : e. g. of a nerve with its end- 

 organ, of the vasa efferentia with mesonephric tubules, of 

 nephridia with the coelom in Annelida. 



B. Superficial aggregates, 

 i. Increase of area. 



a. Growth of a sphere. 



1. Equal in all directions : e. g. the blastula of Echino- 

 derms. 



2. Unequal. 



a. Unequal in different axes : e. g. the conversion of 

 a spherical blastula into an ellipsoid Planula in Coelenterata, 

 or into an ellipsoid Sponge larva, or of the spherical into the 

 ellipsoid blastocyst in Mammalia. 



/3. Unequal at different poles : e. g. the formation of 

 ovoid forms, such as Planulae, the club-shaped gland of 

 Ampkioxiis, the auditory vesicle of Vertebrata. 



b. Growth of a plane surface. 



1. Equal in all directions : e. g. the growth of the 

 blastoderm over the yolk in Sauropsida, or Cephalopoda. 



2. Unequal. 



a. When parts lying in one plane move out of that 

 plane : e. g. invaginations and evaginations of all descriptions 

 (Fig. 2). 



ft. When parts e. g. a row of cells lying in one plane 

 are moved in that plane : e. g. the germ-bands of Clepsine, by 

 the growth of the epiblast (Fig. 3). 

 ii. Alterations of thickness. 



a. Increase : thickenings : e. g. the formation of the central 

 nervous system in Teleostei, the formation of gonads from the 



