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

By W. T. Hillier

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The ' w o r m ' , then, m a y be regarded as an asexual phase of which the members remain connected for a longer time t h a n is generally t h e case. Having observed these things, it is now easy for us t o see t h a t the successive quartettes of micromeres which are budded off from the four macromeres of Polygordius represent an asexual phase following upon the sexual union of the members of the two pairs of macromeres. I t is the fact t h a t the four cells of each quartette are budded off simultaneously, which makes it seem probable t h a t the anterior and t h e posterior pair of original blastulse were in a larval stage when they united to produce t h e annelid.

Now I wish to take him to t h e drawing office t o see t h e designs by which these various operations are guided. These designs, be it remembered, are abstract things—-not concrete—but t h e y govern t h e other phenomena. On the other hand, the animal is a very concrete thing as far as the material composing it is concerned, so t h a t the living animal m a y be looked upon as a compromise between the characters of t h e theoretical design and of the substances in which t h e design is carried out.

The paths of 3 ^ Ρ , 3dxP do not seem t o be under the same influence; they move quite as much away from the middle line as they do towards it, and form t h e greater p a r t of the metatroch on the surface of t h e animal and do not enter into the composition of the viscera. Woltereck's figures of the early stages of invagination show first of all an oval impression, the edges of which then become pinched in to form a figure-of-eight. Next they meet, and divide the opening into two—an anterior one the mouth, which never closes u p , b u t widens out transversely, and a posterior one, the anus, which before long closes u p .

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