262 GLOSSARY OF EMBRYOLOGICAL TERMS 



Amphiblastula — double-structured blastula as in Porifera (sponges). 



Amphimixis — mixing of germinal substances accomplished during fertiliza- 

 tion. 



Amphitene — one end of chromosome is thick, one end is thin, moving 

 toward full pachytene during maturation. 



Amplexus — sexual embrace of female amphibian by male, a process which 

 may (frogs and toads) or may not (urodeles) occur at time of 

 oviposition. 



Anal Plate — thickening and invagination of mid-ventral ectoderm which 

 meets evaginating endoderm of hindgut, later to be perforated as 

 proctodeum (anus). (Syn., cloacal membrane.) 



Analogy — similarity of parts in respect to function rather than to structure. 



Anamniota — forms which never develop an amnion, e.g., cyclostomes, 

 fishes, amphibia. 



Anaphase — phase of mitosis when paired chromosomes are separating at 

 equatorial plate and begin to move toward ends of spindle. 



Anastomosis — joining together, as of blood vessels and nerves, generally 

 forming a network. 



Androgen — hormonal secretion of interstitial tissue of testis. 



Androgenesis — development of an egg with paternal (sperm) chromosomes 

 only, accomplished by removing or destroying egg nucleus before 

 syngamy. 



Angioblast — migratory mesenchyme cell associated with formation of 

 vascular endothelium. 



Animal Pole — region of egg where polar bodies are formed; region of 

 telolecithal egg containing nucleus and bulk of cytoplasm; gives rise 

 largely to ectodermal derivatives. (Syn., apical pole or hemispheres.) 



Anlage — rudiment; group of cells which indicate a prospective develop- 

 ment into a part or an organ. (Syn., ebauche or primordium.) 



Anterior — toward head; head end. (Syn., cephalic, cranial, rostral.) 



Anura — tailless amphibia (e.g., frogs and toads). (Syn., Salientia.) 



Aortic Arch — blood vessel which connects dorsal and ventral aortae by 

 way of visceral arch. 



Aqueduct of Sylvius — ventricle of mesencephalon (mesocoel) becomes 

 aqueduct of Sylvius, connecting with cavities of optic lobes. (Syn., 

 iter.) 



