By W. T. Hillier
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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. Eyespots are formed at the edge of the head region in which the tentacles are situated, and by this time the apical tuft of cilia has been shed.
If one sphere approached THEORY OF DEVELOPMENTAL HISTORY 85 the other directly, the exactly opposed movements would, it seems to me, only lead to mutual destruction of the two spheres. B y rolling together, the two spheres have p a r t at any rate of their motion in common and therefore each maintains its entity. I n the case of Polygordius, where the two pairs of spheres are involved, we m a y believe t h a t what applies to one pair holds also for the other. Like every other multicellular animal Polygordius begins as a blastula, which it actually realizes when it consists of sixteen cells.
8). The first sign of these changes is t h e sinking inwards of t h e two central cells of t h e under surface 5D 2 , 5D 12 , and the appearance of ridges bounding t h e depression on each side. These ridges are made u p of cells of the fifth quartette, with two fourth q u a r t e t t e cells a t their midpoints. These two cells are of large size, and begin t o close together, cutting t h e oval opening into two. Following this the hinder opening closes, by the fifth q u a r t e t t e cells there coming together, and as this is happening the whole of the oval area becomes still more t h r u s t into t h e 24 THE FORMATION OF ANIMALS now more conical animal and other lateral boundaries are formed, this time from cells of t h e second and third quartettes.
A Theory of the Formation of Animals by W. T. Hillier