CLEAVAGE AND DIFFERENTIATION IIQ 



it does not occur until after the end of the cleavage period (as in 

 Amphibia) , the substances are precluded from this type of movement 

 through their being confined within cell-membranes, and redistri- 

 bution can only be effected by the movements of cell-regions. 



The examples so far given concern the migration of organ-form- 

 ing substances during cleavage. Other forms show striking localisa- 

 tion phenomena in the uncleaved egg, usually initiated by polar 

 body formation, as the result of fertilisation. The classical ex- 

 ample of this is the Ascidian Styela {Cynthia). Before fertilisation, 

 the egg contains a cap of clear cytoplasm at its animal pole, a central 

 mass of yolk, and a superficial layer of yellow cytoplasm laden with 

 mitochondria. The clear cytoplasm is chiefly derived from the 

 breaking down of the large germinal vesicle. Almost immediately 

 after the entry of the sperm, the polar bodies are given off, and the 

 clear cytoplasm and the yellow cytoplasm flow down to the vege- 

 tative pole, leaving the animal pole occupied by the yolk, except for 

 a very small cap of clear cytoplasm. Next, the sperm moves to- 

 wards the centre of the &gg, along an apparently predetermined 

 path (indicating that a plane of bilateral symmetry already exists 

 in the tgg), and another rearrangement of the cytoplasmic regions 

 ensues. The sperm appears to drag much of the yellow cytoplasm 

 with it into the interior of the egg, and this yellow cytoplasm now 

 forms a crescent on the surface, beneath the equator, with its horns 

 extending a quarter of the way round the egg on each side. The 

 clear cytoplasm forms a crescent immediately above the yellow 

 cytoplasm, and the centre of these crescents marks the future 

 ventro-posterior side of the embryo. After the first cleavage, 

 another crescent, light grey in colour, is formed opposite the yellow 

 crescent (fig. 56). A pattern almost precisely similar is found in 

 Amphioxus.^ Thus, in these forms, both radial and bilaterally 

 symmetrical localisation are effected prior to cleavage. 



Other examples of such rearrangements are afforded by other 

 Ascidians (e.g. Ciona) and by Myzostoma, in which a green vegeta- 

 tive area is formed in the oocyte, while fertilisation results in the 

 withdrawal of a red substance to the animal pole, leaving a clear 

 equatorial zone.- In the leech Clepsine, the pole-plasms, or areas 

 of clear cytoplasm at the two poles, only form after the polar bodies 



^ Conklin, 1933. - Driesch, 1897. 



