By W. T. Hillier
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Extra info for A Theory of the Formation of Animals
Showing annelidan cross. ,-, , -, molluscan cross, the arms are vertical and horizontal. The cells of the other quartettes and the four macromeres are to be seen composing the under surface. The egg a t this time is beginning t o flatten out, and b y t h e time t h a t t h e two next cleavages have been completed the embryo is nearly flat and has a squarish outline with very much rounded corners. At t h e seventh cleavage, a fifth q u a r t e t t e of micromeres is budded off (Fig. 7). W i t h t h e exception of the cells l q 1 1 1 and l q 2 " all the cells of the embryo divide.
For showing the manner in which t h e design of the viscera is sketched out, t h e reader will, I think, learn more from considering the diagrams given in Fig. 21 t h a n by trying t o follow a long wordy description. Let it suffice to say t h a t while a meridian in turning round the axis of a sphere describes the entire surface contour of the sphere, the same meridian if transported along a straight line (perpendicular t o t h e plane in which t h e meridian lies) would describe a trough or gutter.
Six other nerves pass out radially from t h e supra-cesophageal ganglion, and there are rings of nerve strands associated with both t h e prototroch and metatroch. Besides these there is a diffuse network of nerves immediately under the skin. This curious feature of the systems of nerves is noted by Woltereck as reminiscent of t h e condition of affairs in ctenophores. The six lesser nerve strands, the two rings, and t h e diffuse network, have counterparts in the form of muscular fibres, which, like the nerves, belong to the larval 28 THE FORMATION OF ANIMALS life only and are cast off a t t h e metamorphosis.