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GLOSSARY 



CLEAVAGE, HOLOBLASTIC - complete division 

 of the egg into blastomeres, generally equal 

 in size (Asterias, Arbacia) although not 

 necessarily so (Amphioxus, Frog). Syn. , 

 total cleavage. 



CLEAVAGE, HORIZONTAL - (See cleavage, 

 equatorial.) 



CLEAVAGE, INDETERMINATE - cleavage re- 

 sulting in qualitatively equi-potential blas- 

 tomeres in the early stages of development. 

 When such blastomeres are isolated from 

 each other they tend to give rise to complete 

 embryos. Opposed to mosaic development. 

 Syn. , regulatory cleavage, or development. 



CLEAVAGE, LATITUDINAL - (See cleavage, 

 equatorial. ) 



CLEAVAGE LAWS - (See specific laws under 

 names of Balfour, Hertwig, and Sachs. ) 



CLEAVAGE, LEVOTROPIC - cleavage result- 

 ing in left-handed or counter-clockwise 

 production of daughter blastomere(s) as in 

 some cases of spiral cleavage. 



CLEAVAGE, MERIDIONAL - cleavage along the 

 egg axis, opposed to equatorial. Generally 

 the first two cleavages of any egg. Syn. , 

 vertical cleavage. 



CLEAVAGE, MEROBLASTIC - (See discoidal 

 cleavage. ) 



CLEAVAGE NUCLEUS - the nucleus which con- 

 trols cleavage. This may be the syngamic 

 nucleus of normal fertilization; the egg 

 nucleus of parthenogenetic or gynogenetic 

 eggs; or the sperm nucleus of androgenetic 

 development. 



CLEAVAGE PATH - path taken by the syngamic 

 nucleus to the position awaiting the first 

 division. 



CLEAVAGE, RADIAL - holoblastic cleavage 



which results in (two) super-imposed tiers 

 of cells as early as the 8-cell stage. Op- 

 posed to spiral cleavage. 



CLEAVAGE, SPIRAL - cleavage at an oblique 

 angle with respect to the egg axis so that 

 the resulting blastomeres (generally upper 

 micromeres at the 8-ceIl stage) lie in an 

 interlocking fashion within the furrows of 

 the original blastomeres. The shift in ex- 

 pected position is due to intrinsic genetic 

 factors rather than external pressure, 

 (e. g. , MoIIsuca. ) Opposed to radial clea- 

 vage. 



CLEAVAGE, SUPERFICIAL - cleavage around 

 the periphery of centrolecithal eggs. Syn. , 

 peripheral cleavage. 



CLEIODOIC - refers to eggs that are more-or- 

 less closed off from their environment 

 (e. g. , Chick). 



CLINOSTAT - apparatus (or keeping objects in 

 constant rotation. 



COADAPTATION - correlated variation in two 

 mutually dependent organs. 



COELOBLASTULA - spherical ball of cells 



(blastomeres) developing in early cleavage 



as a result of segmentation, provided with 

 a large central cavity (blastocoel). (e. g. , 

 Echinodermata. ) 



COELOM - mesodermal body cavity of chor- 

 dates, from the walls of which develop the 

 gonads. It is subdivided in higher forms 

 into pericardial, pleural, and peritoneal 

 cavities. Extended as the exocoel or extra 

 embryonic body cavity of chick embryo. 



COENOBLAST - the layer which will give rise 

 to the endoderm and mesoderm (obsolete). 



COLLOID - dispersed substance whose particles 

 are not smaller than l\i nor larger than 

 100(Jl, approximately. Physical state of 

 protoplasm. 



COLORLESS PIGMENT CELL - same as de- 

 pendent or potential pigment cell of 

 DuShane and Hamilton. Syn. , farblose 

 pigmentzellen. 



COMPETENCE - state of reactivity, of dis- 

 equilibrium in a complex system of re- 

 actants. Possessing labile determination 

 (Reakionsfahig) or having reaction possi- 

 bility (Raven). Competencies may appear 

 simultaneously or in sequence within a 

 given area, some to disappear later even 

 without function. Embryonic competence 

 seems to be lost in all adult tissues but 

 may be reclaimed in a blastema. It is a 

 name for the state of the cell area at or 

 before the time when irritability is resolved 

 and a developmental path is chosen. The 

 word supercedes the older words of 

 "potence", "potency", or "potentiality". 



COMPRESSION - either the acceleration of de- 

 velopment or the extension of a certain 

 (e.g., pre-hatching) period, resulting in the 

 completion (or omission) of certain (larval) 

 stages, in an unbalanced time schedule. 



CONCRESCENCE - the coming together of pre- 

 viously separate parts (cell areas) of the 

 embryo, generally resulting in a piling up 

 of parts. One of the corollaries of gastru- 

 lation where a bottle-neck of cell move- 

 ments occurs at the lips of the blastopore. 

 Original meaning (His, 1874) referred to 

 presumed pre-formed parts of the fish germ 

 ring. (See confluence. ) 



CONE, EXUDATION - (See cone fertilization.) 

 Term used by Fol. 1879. 



CONE, FERTILIZATION - a conical projection 

 of the cytoplasm from the surface of the egg 

 to meet the spermatozoon which is to invade 

 the egg cortex. The cone makes contact and 

 then draws the sperm into the egg. Not 

 universally demonstrated but seen in the 

 starfish (Chambers). Syn. , exudation cone. 



CONES OF GROWTH - the enlarged outgrowth 

 of the neuroblast forms the axis cylinder or 

 axone of the nerve fiber and is termed the 

 cone of growth because the growth processes 

 by which the axone increases in length are 

 supposed to be located there. 



