GLOSSARY. 367 



Capsule. A little case in which some organ or part of an organ is 



enclosed. 

 Cells. The nucleated elements of which the protozoa consist, and which 



in rnetazoa are built up into tissues. From the undijferentiated cells of 



the embryo arise such forms as the amoeboid (white blood-corpuscles), 



the columnar (epithelium), the flattened or squamous (epithelium, 



endothelium), the fibrous (muscle, nerve), the branched (connective 



tissue), and the ciliated (tracheal epithelium). 

 Centrolecithal. See Alecithal. 

 Ciliated. A term applied to those cells or organisms which have delicate, 



motile, hair-like processes or cilia. 

 Cleavage. A term applied to the splitting, division, or segmentation of 



the ovum into a number of cells or blastomeres ; also to the splitting 



of the mesoblast into two layers. 

 Ccelomata. Those organisms which have a separate body cavity, as 



opposed to the cozlenterata. 

 Columella. (1) A bone connected with the organ of hearing in the frog and 



fowl ; (2) the axis of the shell in the snail to which the columellar 



muscles are attached. 

 Commissure. A nerve band joining two ganglia or two parts of the brain. 



The term is sometimes reserved for transverse bands, longitudinal 



connections being termed connectives. 

 Conjugation. The union or fusion, temporary or permanent, of two protozoa. 



The phenomenon occurs in connection with the reproductive process. 



Development. Differential growth, involving a change in the organism as 

 it passes from the ovum to the mature state. 



Dioecious. Having the sexes distinct, the individuals being male and 

 female ; in contradistinction to monoecious or hermaphrodite, when 

 both sexes are combined in the same individual. 



Differentiation. The process by which cells or parts originally similar be- 

 come different in structure. It is accompanied by a specialisation of 

 function among the cells, sometimes spoken of as the physiological 

 division of labour. 



Distal. Applied to the end of an appendage, bone, or limb which is 

 furthest removed from the trunk : the other end being called proximal. 



Distribution. (1) The way in which organisms are distributed over the 

 earth in space and in time ; (2) the way in which nerves and blood- 

 vessels branch out to supply organs. The tenth nerve (e.g.) is distri- 

 buted to the larynx, heart, lungs, stomach, etc. 



Diverticulum. A blind tube opening out of another tube. 



Ecdysis. The throwing off of the outer skin or exoskeleton. 

 Embryo. The organism in the stages which intervene between the seg- 

 mentation of the ovum and birth or hatching. The term foetus is used 



