Turn now to the full-grown larval form, Ammocoetes, an animal in the case of _Petromyzon Planeri_ as large as the full-grown Petromyzon, and seek for this musculature. There is, apparently, no sign of it, no suctorial apparatus whatever, only, as already mentioned, an oral chamber bounded by the lower and upper lips and the remains of the septum between it and the respiratory chamber--the velar folds. Attached to its walls a number of tentacles are situated, which form a fringe around and within the mouth. Most extraordinary is the contrast here between the larval and the adult stages; in the former, no sign of the suctorial apparatus, but simply tentacles and velar folds; in the latter, no sign of tentacles or of velar folds, but a massive suctorial apparatus. In order, then, to understand the origin of the muscles of mastication, it is necessary to study the changes which occur at transformation, and thus to find out how the suctorial apparatus of the adult arises. This most important investigation has been undertaken by Miss Alcock, and owing to the kindness of Mr. Millington, of Thetford, we have been able to obtain a better series in the transformation process than has ever been obtained before. Miss Alcock has not yet published her researches, but has allowed me to make use of some of her facts. An enormous proliferation of muscular tissue takes place with great rapidity during this transformation, which causes the disappearance of the tentacles, and gives origin to the suctorial apparatus. The starting point of this proliferation can be traced back in all cases to little groups of embryonic tissue found below the epithelial lining of the oral chamber in Ammocoetes. Of these groups the most conspicuous one is situated at the base of the large median ventral tentacles. Others are situated at the base of the tentacular ridge. Further, although this extraordinary change takes place in the {288}peripheral organ, no marked difference occurs in the arrangement of the nerves issuing from the trigeminal motor centre, no new nerves are formed to supply the new muscles, but every motor nerve-fibre and the motor cell from which it arises increases enormously in size, and these giant nerve-fibres thus formed split into innumerable filaments corresponding with the proliferation of the muscular elements. The clue, then, to the origin of the suctorial apparatus and of the nature of the original organs supplied by the trigeminal is afforded in this case, as in all other similar inquiries, by the central nervous system and its outgoing nerves. Here is always the citadel, the fixed seat of government, here is 'headquarters,' from which the answers to all our inquiries must originate. [Illustration: FIG. 114.--DISTRIBUTION OF TRIGEMINAL NERVE IN AMMOCOETES.