202 Symmetrij of Egg and Sijmmetry of Einbrijo in the Frog 
In the Sea-urchin Toxopneustes Wilson and Matthews were able to show that 
after the extrusion of the polar bodies at the animal pole of what may be called 
the primary egg-axis, the female pronucleus wandered in the cytoplasm to any 
position till met by the male pronucleus. The segmentation nucleus formed of 
their union then took up an excentric position near the centre, so determining 
a new egg-axis and a new animal pole. This axis became the axis of cleavage, 
the meridian of the First Furrow being determined by the point of entrance of 
the spermatozoon. 
In the Ascidian Cynthia according to Conklin, the immature oocyte has a 
radial symmetry about an axis determined by the excentric position of the 
germinal vesicle ; there is a peripheral layer of yellow cytoplasm surrounding 
a central grey yolk. Upon the entrance of the spermatozoon near the vegetative 
pole the nucleus breaks down and from it a clear substance is formed. Some of 
this remains at the animal pole as a small patch containing the female pronucleus ; 
the rest flows, with the peripheral yellow substance, to the vegetative pole, where 
it lies in a shallow layer over the latter. 
The symmetry is still radial. It soon, however, becomes bilateral, for the 
yellow and clear substances stream up on one side, the future posterior side, 
to the equator of the egg, the sperm being can-ied along in the latter to meet the 
female nucleus, which has meanwhile descended from the animal pole. The two 
together pass into the centre, along with the clear substance, and the fertilization 
spindle is formed at right angles to their path. The First Furrow, and later the 
Sagittal Plane, lie therefore in the plane of egg symmetry which is established 
during fertilization. 
Again, in the Mollusc Dentalium we know from Wilson's account that when 
the sperm enters, the centrally placed germinal vesicle breaks down and its 
substance becomes confluent with a small clear area at the animal pole, a larger 
clear area at the vegetative pole. Later the brick-red yolk is interposed between 
animal and vegetative hyaline areas, the latter being soon extruded as the polar 
lobe. 
A few other instances may be cited. In the Nematode Diplogaster it is the 
point of union of the two pronuclei which determines which end of the elliptical 
egg — the anterior ovarian end at which the polar bodies are formed, or the posterior 
oviducal end at which the sperm enters — shall become the animal and ectodermal 
end (Ziegler). In another Nematode Strongylus WandoUeck has shown that the 
yolk, originally uniformly distributed, becomes placed at one end, and the egg 
therefore telolecithal when fertilization occurs. Similarly in Cirrhipedes (Groom), 
in the Snail Physa (Kostanecki and Wierzejski), and in Turbellarians (Lang). In 
Ctenophora in the same way the micromere-forming substance becomes aggregated 
at one pole (Agassiz), and in Teleostei the blastodisc is formed by the local accumu- 
lation of the periblast. Lastly may be mentioned the polar rings of Leeches and 
Oligochaets (Whitman, Vejdovsky, Foot). 
