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

By W. T. Hillier

ISBN-10: 1483200345

ISBN-13: 9781483200347

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N m ,-'' 8 *-'' N Y Fig. —Graph, showing number of cells of quartettes. Before leaving the workshops, one last look at the embryo under construction shows us t h a t it is flattened out into an almost discoidal shape. We left t h e design as t h a t of a sphere which had been outlined by t h e concerted movements of an active meridian of each of t h e four phantom blastulse. The visit t o t h e works has shown t h a t the carrying out of this design is being done in p a r t by the original macromeres, and in p a r t b y t h e cells or micromeres of t h e asexual phase which follows upon the sexual union of the members of the two pairs of phantom blastulae.

There are some interesting points which the reader may take note of now. The stomach, inasmuch as it represents one blastular cavity stowed within another (which is the trochophore larva), is built up by all the four phantoms that enter into the composition of Polygordius. This is as it should be, for all metazoan animals exhibit a gastrular stage. The other points of interest concern the spiral curves. THEORY OF DEVELOPMENTAL HISTORY 55 I n order t o find t h e course which a spiral curve takes, provided it is a perfectly regular one, a very simple way is t o draw a large circle with a large number of evenlyspaced radii.

The beginning of the ciliated pits is seen close to the base of each tentacle. The internal structure of the organism has also been changing. A new lining to the oesophagus, coming from an area like an imaginal disc on each side, replaces t h e original lining. In the intestine there has been a great multiplication of the lining cells, certain special cells being detailed off for this purpose. In the North Sea species, the body of the worm, instead of protruding as it grows, is held puckered u p within an ' a m n i o t i c ' cavity, and while t h u s kept in confinement, the multiplying cells of the intestine are very closely packed and lie obliquely on one another.

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