Histogenetic and Organogenetic Processes in the Development of Specific Characters in some South African Tadpoles1

Ever since it was discovered that embryonic inductions can occur even where the inductor and the reacting tissues belong to different species of animals, it has been accepted that the specific nature of the reaction depends mainly on the properties of the reacting cells and not on the properties of the inductor. The embryonic cells appear to have only a limited set of possible reactions, determined by the genotype, whereas the inducing factors in different animals seem to be very similar and readily interchangeable. So the tissues of a species of salamander not normally possessing a balancer cannot develop one even if transplanted to another species which normally has that organ (Mangold, 1931; Rotmann, 1935a). Ectoderm of a frog embryo transplanted to the oral region of a salamander embryo will develop mouth parts, but these will be the mouth parts of a frog larva, with horny jaws and horny teeth, and not those of a salamander larva (Spemann & Schotté, 1932; Spemann, 1938).

On the Head of the Liopelmid Frog, Ascaphus Truei

This paper gives the first account of the larval cranial anatomy of either of the genera of Liopelmid frogs. A single, partly grown larva of Ascaphus truei, Stejneger, has been studied in transverse sections and in two-dimensional reconstructions. Its chondrocranium, jaws, gill arches, and head muscles are described and figured. Comparisons are made throughout with similar structures of Urodeles and certain other frogs, particularly Discoglossus pictus and Kana temporaria. A summary of the characters which Ascaphus shares with the Urodeles is given on pp. 175-7 and with Discoglossus on pp. 177-8. The reader is referred to these lists as an important part of this summary. Noble (1931, &c.) considers Ascaphus (with Liopelma) to be one of the two most primitive living frogs. The findings of this paper are in full agreement with this view. Thus larval Ascaphus is shown to be a persistently primitive ‘link-animal’ whose cranial structures, in almost every case, differ from those of other frogs--often radically--and throw much light on the evolution of the modern-type frog tadpole from the unknown (larval) ancestor. Ascaphus is shown to have more characters in common with the Urodeles than any other frog larva yet described. Most of these are probably a simple retention of an ancestral Amphibian plan which led on to the frogs and Urodeles (contrast the writings of Holmgren and Säve-Söderbergh). Others seem to link these two orders even more closely together. Such are: (1) The presence of ‘urobranchial’ prongs on the basibranchial copula and the attachment to them of Subarcuales obliqui and Eecti cervicis muscles; (2) the presence of a pair of Branchio-hyoideus externus muscles and other similarities of the musculature. The relationship of Ascaphus to the Gymnophiona is far less marked. Ascaphus, however, has remained more primitive than the present-day Urodeles by retaining: (1) a? Vth gill bar, with its Subarcualis rectus and S. obliquus muscles, and (2) four pairs of S. obliqui muscles instead of two. In these points it is, in fact, the most primitive living tetrapod. Ascaphus is, however, somewhat specialized in relation to a sucker mechanism and to a peculiar method of larval progression which it employs. These have led to an exaggerated autostyly of the palatoquadrate which has developed an additional fusion to the anterior tip of the auditory capsule; to a rigid fusion of the central part of the supra-rostral system to the skull; to the general heavy build of the head cartilages and to the great size of several of the mandibular and hyoid muscles; to a general consolidation and widening of parts of the hyobranchial apparatus; to a widening of the posterior jaw cartilages and the development from them of unique posterior spurs. A pre-oral mouth cavity and a long ‘posterior narial tube’ to the inner nostril are also parts of this specialization. Among the frogs Ascaphus is shown to be most nearly related to the Discoglossidae, which appear to have been derived from an ancestor with many Ascaphus-like characters. This is in further agreement with Noble's classification and is particularly true of Discoglossus pictus. Ascaphus is unique among frogs in the posterior position of its splanchnic head structures; see the list on p. 147. A forecast is made of the probable evolution of the moderntype tadpole's jaw system from that of the unknown ancestor and this is diagrammatically summed up in Text-fig. 7, pp. 158-9. Evidence is collected to show that the ‘anterior basal process’ (= commissura quadrato-cranialis anterior) is not an ethmoidal structure by origin, as has been held up to now, and consequently Säve-Söderbergh's use of it to explain an ethmoidal structure in a Stegocephalian Amphibian is criticized. An account of the muscles has been given in summary form on pp. 126 to 146 and cannot be further condensed here. But it may be noted that Edgeworth's theories (1935) of the primitive muscular content of a single branchial segment break down when applied to Ascaphus. It is now probable that a single segment could simultaneously contain a Subarcualis rectus, a S. obliquus, and a Transversus ventralis muscle. Further, Edgeworth's term ‘Transversus ventralis II’ must be changed to ‘S. obliquus II’ in the frogs and his ‘S. rectus IV’ must probably be changed to ‘S. recti IV, III, and II’ in the Urodeles.

THE BEHAVIOR OF THE LEUCOCYTES DURING COINCIDENT REGENERATION AND THYROID-INDUCED METAMORPHOSIS IN THE FROG LARVA, WITH A CONSIDERATION OF GROWTH FACTORS

1. Coincident progressive and regressive changes are induced in the tail of the frog larva by simultaneous thyroid administration and removal of a portion of the tail. The rate of growth of the regenerating tail is greatest in animals not treated with thyroid, next greatest in animals treated with thyroid 2 days after tail removal, somewhat less in animals treated with thyroid simultaneously with tail removal, and least in those treated with thyroid 2 days before removal of the tail. 2. The leucocytes chiefly concerned are neutrophilic granulocytes, lymphocytes, hemoblasts, and lymphoid phagocytes (monocytes, macrophages). The neutrophils further tissue lysis, the lymphoid phagocytes remove tissue débris. The lymphocytes are apparently not numerous enough to exert any important growth-promoting influence. The hemoblasts represent merely a part of the general reaction to thyroid treatment. Eosinophils are absent except for a few that appear in the late stages. Basophils of mesenchymal origin are present in the later stages of thyroid-treated animals. These appear to differentiate further into eosinophils. 3. The rate and amount of growth are correlated directly with the degree of crowding of cells, and inversely with the degree of vascularity. The greatest crowding and least vascularity occur in animals not treated with thyroid, somewhat less crowding and greater vascularity in those treated with thyroid 2 days after tail removal, still less crowding and somewhat greater vascularity in those treated with thyroid at the time of tail removal, and finally, least crowding and greatest vascularity occur in animals treated with thyroid 2 days before tail removal. These results support the views of Burrows (4) upon the mechanism of growth stimulation of cells.