286 GLOSSARY OF EMBRYOLOGICAL TERMS 



Lecithin — fat from an. animal organism which is phosphorized in form of 

 phosphatides. 



Lens — thickening in head ectoderm opposite optic cup at about time of 

 hatching in frog embryo; it becomes a placode, invaginates to acquire 

 a vesicle, and then pinches off into space of optic cup as a lens. Inner 

 surface convex; substance fibrous. 



Lens Placode — early thickened ectodermal primordium of lens. 



Leptotene — stage in maturation which follows last -gonial division and 

 prior to synaptene stage, structurally similar to resting cell stage. The 

 chromatin material in form of a spireme. Term means thin, diffuse. 



Lipids — fats and fatty substances such as oil and yolk (lecithin) found in 

 eggs (e.g., cholesterol, ergosterol). 



Lips of the Blastopore — See Blastopore, Lips of. 



Localization — cytological separation of parts of the mosaic egg, each of 

 which has a known specific subsequent differentiation. There is often 

 a substratum associated with these areas, made up of pigmented 

 granules, but it is cytoplasm rather than pigmented elements in which 

 localization occurs. 



Macromere — ^larger of the blastomeres where there is a conspicuous size 

 difference. 



Malpighian Body — unit of functional kidney including Bowman's capsule 

 and glomerulus. (Syn., renal corpuscle, Malpighian corpuscle.) 



Mandibular Arch — rudiment of lower jaw, mesodermal, and anterior to 

 first or hyomandibular pouch. 



Mantle Fibers — those fibers of mitotic spindle which attach chromosomes 

 to centrosomes. 



Mantle Layer of the Cord — zone of developing spinal cord with densely 

 packed nuclei slightly peripheral to germinal cells from which they 

 are derived. Includes elongated cells of ependyma. 



Maturation — process of transformation of a primordial germ cell (sper- 

 matogonium or oogonium) into a functionally mature germ cell, the 

 process involving two special divisions, one of which is always meiotic. 

 Divisions known as equational and reductional. 



Mechanism — assumption that biological processes do not violate physical 

 and chemical laws but that they are more than the mere function- 

 ing of a machine because material taken into the organism becomes an 



