GLOSSARY OF EMBRYOLOGICAL TERMS 299 



Recapitulation Theory — theory that embryonic development reviews major 

 steps in evolutionary history. (See qualifications under Biogenetic Law.) 



Rectum — narrowed posterior portion of hindgut, lined with thickened en- 

 dodermal epithelium, which opens directly into cloaca. 



Reductional Maturation Division — one of the two important divisions in 

 the maturation of gametes which results in separation of allelomorphic 

 (homologous) pairs of chromosomes so that resulting cells are in- 

 variably haploid. Opposed to equational division. (Syn., meiotic divi- 

 sion, disjunctional division.) 



Regeneration — repair or replacement of lost part or parts, a power gradu- 

 ally lost in the ontogeny of most animals. 



Regions, Presumptive — regions of blastula which, by previous experimen- 

 tation, have been demonstrated to develop in certain specific direc- 

 tions under normal ontogenetic influences. 



Regulation — reorganization toward the whole; power of pre-gastrula em- 

 bryos to utilize materials remaining, after partial excision, to bring 

 about normal conditions; more flexible power than regeneration. 



Renal Portal System- — venous system which carries blood to kidneys, in- 

 volving lateral portions of caval veins (really parts of posterior cardi- 

 nals), iliacs, and dorso-lumbars. Found in adult amphibia as the most 

 striking evidences of recapitulation. 



Rete Cords — strands of epithelial cells containing many primordial germ 

 cells which connect with seminiferous tubules and later become vasa 

 efferentia, in the bird. (Syn., rete testis.) 



Retinal Zone — ectodermal derivatives of optic cup consisting of internal 

 limiting membrane, retinal and lenticular zones, and outer pigmented 

 layer. Retina proper includes portions from internal limiting membrane 

 to rods and cones, inclusive. 



Rhombencephalon — See Hindbrain. 



Saccule — -outer and ventral portion of inner ear from which are derived 

 cochlea associated with eighth or auditory nerve. (Syn., sacculus.) 



Saccus Endolymphaticus — original endolymphatic duct, closed off from 

 exterior, which (in 20 mm. stage of tadpole) grows up over rhomben- 

 cephalon to join other sac and form a vascular covering of the brain. 



Sachs' Law — all cells tend to divide into equal parts and each new plane 

 of division tends to intersect the preceding one at right angles. 



