264 GLOSSARY OF EMBRYOLOGICAL TERMS 



Axis of the Embryo — imaginary line representing antero-posterior axis of 

 the future embryo. 



Balancers — cylindrical and paired projections of ectoderm with mesen- 

 chymatous cores, used as adhesive organs in place of (anuran) suckers 

 by many urodele amphibia. 



Balfour's Law — "The velocity of segmentation in any part of the ovum 

 is, roughly speaking, proportional to the concentration of the proto- 

 plasm there; and the size of the segments is inversely proportional to 

 the concentration of the protoplasm." The intervals between cleavages 

 increase in proportion to the amount of yolk which a cell contains in 

 its protoplasm. 



Basal Plate — ventro-lateral wall of myelencephalon, separated from dorso- 

 lateral alar plate by sulcus limitans. 



Basophil — cell constituents having an affinity for basic dyes, often used as 

 an adjective for an entire cell. (See Acidophil.) 



Bidder's Organ — anterior portion of anuran pro-gonad, somewhat ovarian 

 in character, developing from part of gonad rudiment consisting 

 wholly of cortex; its development indicates failure of medullary sub- 

 stance to diffuse to anterior extremity of gonad rudiment. 



Biogenetic Law — embryos of higher species tend to resemble embryos of 

 lower species in certain respects but are never like adults of lower 

 species. Embryonic development is a gradual deviation from the more 

 general (phylogenetic) to the more specific characters of the in- 

 dividual species. Not to be confused with recapitulation theory. 



Blastema — indifferent group of cells about to be organized into definite 

 tissue; nev/ly formed cells covering a cut surface, functional in regenera- 

 tion of tissues. 



Blastocoel — cavity of blastula. (Syn., segmentation or subgerminal cavity.) 



Blastoderm — living portion of egg from which both embryo and all of its 

 membranes are derived. The cellular blastodiscs. "Because the embryo 

 chooses this as its seat and its domicile, contributing much to its con- 

 figuration out of its own substance, therefore, in the future we shall 

 call it blastoderm" (Pander, 1817). 



Blastomere — cellular unit of developing egg or early embryo, prior to time 

 of gastrulation. Smaller blastomeres are micromeres; intermediate 

 ones are mesomeres; larger ones are macromeres, where there is great 

 disparity in size. 



