The Head of Xenopus laevls. By Nellie F. Paterson, D.Se., Ph.D., Department of Zoology, University of the Witwatersrand, Johannesburg. With Plates 9 to 16. CONTENTS. PAGE INTRODUCTION 161 LATERAL LINE SENSORY ORGANS 163 MUSCULATURE 165 BLOOD-VESSELS ......... 172 THE CHONDROCRANIUM ........ 175 1. Metamorphosis ........ 183 2. Olfactory Eegion 188 3. Nasal Cavities 191 4. Auditory Eegion ........ 193 THE HYOBRANCHIAL SKELETON ....... 196 THE CRANIAL NERVES 198 Ganglion Pro-oticum ........ 199 Nervus Trigeminus ........ 200 1. Ramus Mandibularis 200 2. Ramus Ophthalmicus Profundus 203 NERVUS FACIALIS 209 1. Truncus Supra-orbitalis 210 2. Ramus Hyomandibularis . • - .211 3. Ramus Palatinus ........ 214 NERVI GLOSSOFHARYNGEtTS AND VAGUS . - .216 1. Nervus Glossopharyngeus . • • • .217 2. Nervus Vagus 220 SUMMARY OF COMPOSITION AND DISTRIBUTION OF NERVES . 226 SUMMARY 227 REFERENCES 228 INTRODUCTION. THE Aglossa, comprising only the genera Xenopus, Pipa, Propipa, Hymenochirus, and Pseudohymeno- chirus, are characterized among other things by the absence of a tongue and by a pectoral girdle that exhibits considerable deviation from that of typical Anura Phaneroglossa. NO. 322 M 162 NELLIE F. PATBESON The Aglossa are usually classified as the lowest of the A n u r a, but as Gadow in his account of the Amphibia in the ' Cambridge Natural History' (1909) indicates, their characteristic features are not necessarily primitive ones. A tongue is lacking in the majority of truly aquatic forms, and in the Aglossa the shoulder girdle and other parts of the body are doubtless specialized in response to their particular habits. It is therefore not surprising to find that the Aglossa present some striking morphological similarities with the aquatic Urodela on the one hand, and with certain genera of the Phaneroglossa on the other, but it is very doubtful if these resemblances are of any conse- quence. Probably they are merely accidental or superficial likenesses, as for instance the similarities of skeletal structures indicated in the course of the present study as existing between Xenopus laevis and the Phaneroglossangenera Hemisus, Breviceps, and Probreviceps. The resemblances be- tween Xenopus and such Urodela as Siren and Pro- teus, especially in regard to the arrangement of the nervous system, seem to be more significant, but even this may be due to adaptation to somewhat similar habitats. The anatomical peculiarities of X. laevis Daud., the ' clawed toad' or ' platanna' of Southern Africa, have for many years been favourite subjects of investigation by students of morphology, but despite the valuable contributions that have appeared, there still remain many problematic points. X. laevis abounds in ponds and dams throughout southern and tropical Africa. Eose (1929) in his book 'Veld and Vlei' introduces the reader to a discussion on Xenopus by stating that 'the plathander has an even stronger claim than the springbok to be considered a typically South African animal, for whereas horned antelopes are found in many other countries, clawed toads are found in southern and tropical Africa and in no other part of the world'. The larvae have been described and figured by Bles (1904) and Peter (1930), and are chiefly remarkable for the extreme trans- parency of the tissues. They may also be recognized by the presence of a pair of long trailing' tentacles' (tent, fig. 30 a, PL 16), which disappear at metamorphosis, and by the characteristic HEAD OF XENOPUS 163 attitude they take up in the water; they usually remain more or less stationary with the head directed downwards and with the thin tapering extremity of the tail constantly vibrating and thus enabling them to maintain this position. The present study of X. laevis was begun with the inten- tion of investigating the distribution of the cranial nerves, but as it was necessary to make a somewhat intensive study of other associated cephalic structures and, as certain of these observations have proved of interest, they have been included in the dissertation. For the purpose of this study a series of larvae in progressive stages of development and measuring from 5 mm. to 60 mm. long were sectioned after having been fixed in aqueous Bouin. In order that comparisons between larval and adult conditions might be made possible, and conclusions might be arrived at regarding the changes occurring during the process of meta- morphosis, several transforming specimens as well as some very young frogs were also examined. The latter were sectioned after having been decalcified for several days in 3 per cent, nitric acid as prescribed in the eighth edition of Bolles Lee's 'Vade Meeum', 1921, pp. 252-3. The thickness of all the sections was 10/*, and the stains used were Hansen's haematoxylin and eosin. The preparations were found to be very satisfactory for a general study of the various organs and systems in the head, including the cranial nerves. LATERAL LINE SENSORY ORGANS. As Xenopus is one of those interesting Anura in which the lateral line sensory organs persist throughout life, due no doubt to their aquatic existence, a comparison of their arrange- ment in larval and adult stages is of some interest. Escher (1925), who gives a diagram of the organs in the adult, failed to observe them in the larva. It is to be admitted, however, that, owing to the light sprinkling of somewhat star-shaped patches of pigmentation, and more particularly to the remark- able transparency of the head region, the larval sensory organs are rather obscured and are only determined with difficulty. The ventral organs are easily distinguished as white markings 164 NELLIE F. PATERSON on the darkly coloured abdominal region in all larval stages. Even in young tadpoles measuring about 15 mm. in length and in which the hind-limbs have not yet been protruded, the organs of the dorsal and median rows (osl., msl, fig. 30 a, PI. 16) are sharply contrasted with the darker body coloration between the fore- and hind-limb buds. On the dorsal and ventral sur- faces of the head in front of this region the organs are indis- tinguishable macroscopically from the general surface of the body. That they are developed, however, is evident from sections of the anterior part of the body. Certain organs (csl., fig. 30 a, PI. 16) are also obvious in these early stages on the ventral fin anterior to the cloaca, where they at first appear as a lateral row of small rounded dots which show a gradual elongation during subsequent stages of development. In older larvae and in those undergoing metamorphosis these cloacal sensory organs decrease to a few vertically elongate organs on each side of the fin fold, and with the gradual absorption of the tail they come to lie even nearer to the cloacal aperture. Thus in the mature frog they are only observed with difficulty as indistinct ridges on the ventral fold of skin immediately anterior to the cloacal aperture. All the sensory organs become more apparent just before metamorphosis, when the skin is more deeply and more uni- formly pigmented and the whitish organs show up by contrast. It is then evident that the arrangement in the larva is essentially similar to that of the adult, and that both conform in general to the plan drawn up by Escher (1925) for U r o d e 1 e Amphibians. The dorsal, median, and ventral rows of trunk sensory organs are strongly developed, the two former systems extending on to the tail (fig. 30 b, PI. 16). The dorsal row may be continued only for a short distance caudally in some specimens, but the median organs are apparent throughout the length of the tail at the dorsal ends of the myomeres. The cephalic organs are arranged into supra-, infra-, and post- orbital series above, and oral and gular rows below. In addition, as Escher (1925) has indicated in the adult Xenopus, there are hyomandibular organs (hso., fig. 30 b, PI. 16) which are laterally continuous with the median line on the body, an HEAD OF XENOPUS 165 accessory row (ace, fig. 30 b, PL 16) which arches dorsally, and several small round parietal organs (par., fig. 30 b, PI. 16) situated dorsally midway between the supra-orbitals. The chief difference between larval and adult arrangements is found in the orbital grouping. In the larva the supra-orbitals (sorb., fig. 30 b, PI. 16) and post-orbitals (porb., fig. 30 b, PL 16) are quite distinct, but during metamorphosis they gradually approach the eye, so that in the adult they are arranged into a single circumorbital series. Closer inspection reveals that there is some considerable variation in the arrangement and number of organs in the several rows, not only in different specimens but also in the two sides of an individual. In general, however, each series of sensory organs is as a whole sufficiently constant to allow of their relative positions being easily determined. MtTSCULATUBE. Edgeworth (1929, 1935) has reported upon the cephalic muscles of the larva of X. fraseri in the course of his well- known dissertations on the muscles of Vertebrates, and the entire musculature of the adult X. laevis has been studied in detail by Grobbelaar (1924). The present series of prepara- tions of both larval and adult X. laevis have made it possible to draw conclusions regarding the changes undergone by the musculature of the head during metamorphosis. The muscles associated with the chondrocranium up to the time of metamorphosis are essentially similar to those of X. fraseri. The levatores mandibulae anterior and posterior are two rather broad muscles arising dorsally on the palatoquadrate and having their insertions on the dorsal surface of the Meckel's cartilage. The levator mandibulae posterior (lev.post, figs. 7, 8, PL 11) lies ventrally to the levator mandibulae anterior (lev.ant., figs.