GLOSSARY OF EMBRYOLOGICAL TERMS 283 



Hypochordal Rod — transitory string of cells constricted off between dor- 

 sal wall of midgut and notochord of amphibian embryo, between level 

 of pancreas and tail, and disappearing before hatching time. (Syn., 

 sLib-notochordal rod.) 



Hypomere — most ventral segment of mesoderm out of which develop 

 somatopleure, splanchnopleure, and coelom. (Syn., lateral plate 

 mesoderm.) 



Hypophysis — ectodermally derived solid structure arising anterior to 

 stomodeum and growing inwardly toward infundibulum to give rise 

 to anterior and intermediate parts of pituitary gland. 



Hypoplasia — undergrowth or deficiency in elements composing a part. 



Hypothesis — complemental supposition; presumption based on fragmentary 

 but suggestive data offered to bridge a gap in incomplete knowledge 

 of the facts. May be offered as an explanation of facts unproved, un- 

 til subjected to verification or disproof. 



Idiosome — material out of which acrosome is formed during metamorphosis 

 of spermatid to spermatozoon. (Syn., spermatosphere, centrosphere. ) 



Induction — successive and purposeful influences which bring about 

 morphogenetic changes within embryo. 



Inductor — a loose word which includes both organizer and evocator (Need- 

 ham). Generally means a piece of living tissue which brings about 

 differentiation within otherwise indifferent tissue. 



Infundibulum of the Brain — funnel-like evagination of floor of diencephalon 

 which, along with hypophysis, will give rise to pituitary gland of adult. 



Infundibulum of the Oviduct — See Ostium Abdominale Tubae. 



Ingression — inward movement of yolk endoderm of amphibian blastula 

 (Nicholas, 1945). 



Insemination — process of impregnation; fertilization. 



Interauricular Septum — longitudinal sheet of mesodermal tissue which 

 grows ventrally from roof of atrial chamber to divide it into right and 

 left halves. 



Interkinesis — resting stage between mitotic divisions. 



Intermediate Cell Mass — narrow strip of mesoderm which, for a time, joins 

 dorsal epimere with ventral hypomere, being made up of a dorsal 

 portion continuous with dorsal wall of somite and somatic meso- 

 derm and a ventral portion continuous with ventral wall of somite and 



