New PDF release: A Theory of the Formation of Animals

By W. T. Hillier

ISBN-10: 1483200345

ISBN-13: 9781483200347

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Extra info for A Theory of the Formation of Animals

Sample text

Masses of plasticine were weighed out proportional to the number of cells formed from each quartette of micromeres, a different colour being used for each quartette. There were thus six masses of plasticine, one for each quartette and one for the residual macromeres. These masses were then moulded in as even a thickness as possible in the form of an upper and lower circular cap and four intermediate zones over the surface of an india-rubber R ball, so as to cover it in entirely. This of course was done on the assumption A c t h a t the ideal form of the embryo at this stage is a sphere, and t h a t the ideal D arrangement of the quartettes of micromeres is in successive belts.

The two anterior pairs of cells m a y do so, but the two hindmost cells meet by rotating through half a circle, and some other cells at t h e same time show remarkable POLYGORDIUS 25 movements. Two cells, namely, 3c2PPa, 3d2PPa, wander into the blastoccelic cavity to form p a r t of the primitive excretory organs or archinephridia. Three other cells on each side swing round from their original position to form the lower lip of t h e large slit-like mouth. One other cell on each side becomes drawn out into a long band and forms much of the lesser ciliated band or metatroch.

When the original upper ends of the main axes (which for convenience m a y be called their north poles) are below, so t h a t the main axes slope downwards while still pointing outwards, the middle of the under surface begins to be tucked in because the active meridians are now rolling together, those of phantoms Blue and Green towards one another, and those of phantoms Red and Yellow towards one another. B u t t h e active meridians of phantoms Blue and Green are more advanced along their course t h a n are those of phantoms Red and Yellow, so t h a t while the main axes (of phantoms Blue and Green), as they turn round their secondary axes, gradually approach one another as they become more vertical in position again, the active meridians are turning away from one another and so the mouth opening is never closed.

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A Theory of the Formation of Animals by W. T. Hillier


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