GLOSSARY OF EMBRYOLOGICAL TERMS 273 



Diencephalon — portion of forcbrain posterior to telencephalon, including 

 second and third neuromeres. 



Differentiation — acquisition of specialized features which distinguish areas 

 from each other; progressive increase in complexity and organization, 

 visible and invisible; elaboration of diversity through determination 

 leading to histogenesis; production of morphogenetic heterogeneity; 

 process of change from a simple to a complex organism. (Syn., dif- 

 f erenzierung. ) 



Differentiation, Axial — variations in density of chemical and often indefin- 

 able inclusion in direction of one diameter of the egg, called egg axis. 



Differentiation, Dependent — all difi[erentiation that is not self-diflferentia- 

 tion; development of parts of organism under mutual influences, 

 such influences being activating, limiting, or inhibiting. Inability of 

 parts of organism to develop independently of other parts. 



Differentiation, Self perseverance in a definite course of development of 



a part of an embryo, regardless of its altered surroundings (Roux, 

 1912). 



Diocoel — cavity of diencephalon, ultimate third ventricle. 



Diploid — normal complement of chromosomes in somatic and primordial 

 germ cells, twice the haploid number characteristic of mature gam- 

 etes. 



Diplotene — stage in maturation following pachytene when chromosomes 

 again appear double and do not converge toward centrosome. Some- 

 times refers to split individual chromosomes. 



Discoblastula — disc-shaped blastula found in cases of discoidal (mero- 

 blastic) cleavage (e.g.. Cephalopoda and chick). 



Distal — farther from any point of reference, away from main body mass. 



Divergence, Ventral — divergence of material from mid-ventral line, com- 

 pensatory to process of dorsal convergence in gastrulation (Vogt). 



Diverticulum — blind outpocketing of a tubular structure (e.g., liver or 

 thyroid anlage). 



Dominance — parts of a system which have greater growth momentum and 

 also which gather strength from the rest, such as dorsal lip of blasto- 

 pore. 



Dorsal Mesentery — membrane formed by doubling of peritoneum from 

 mid-dorsal line of body cavity, which supports intestine. 



