Reptiles

(io.) The pre-sacral part of the vertebral column is usually less clearly divided into regions than in mammals and birds. There are two sacral vertebrae and a longer or shorter series of caudals. The atlas consists of a pair of neural arches, a single inter centrum and a centrum which forms an odontoid, though it may not be fused to the axis. There is sometimes a pro-atlas. The vertebra of the rest of the column always consists of a neural arch and a centrum with inter-centra forming chevron bones in the tail. Small inter-centra may be present throughout the col umn. Ribs are usually present on all vertebrae except the posterior caudals ; they may be single or double headed. A true sternum is usually present, connected to some of the dorsal ribs by sternal ribs.

The neural cranium is generally incompletely ossified, a good deal of the lateral walls anteriorly being membranous. It is often movably connected to the dermal bones of the skull roof and palate. There is a single occipital condyle, mainly basi-occipital but with contributions from the ex-occipital. A supra-occipital is present and articulates with the parietals. The inner ear lies within the opisthotic, usually fused with the ex-occipital, the pro otic and the supra-occipital. An ossification in front of the pro otic, in the side wall of the cranium, is absent in only two orders. There is an ossified basi-sphenoid, but the unossified pre-sphenoid is usually underlain by a para-sphenoid.

The dermal bones of the skull form a roof, which may be very incomplete or, indeed, absent, over the masticatory muscles, whilst the orbit is surrounded by a ring of bones which are continuous with the maxillae and nasals which enclose the anterior end of the head. In the palate the pterygoids are always large bones artic ulating with the basi-sphenoid and extending back to the quadrate. Pre-vomers and palatines are always present and ectopterygoids usually so. In many forms an epipterygoid is ossified. The lower jaw is complex, it articulates with the quadrate by an articular bone of endochondral origin, and at least five membrane bones contribute to its structure.

Fore and hind limbs are usually present, but either or both may be absent. The shoulder girdle consists of a pair of scapulae and "corticoids," both contributing to the glenoid cavity. There are generally clavicles and an inter-clavicle. The hand and foot are primitively pentadactyl, the fourth digit being the longest.

(I r.) Segmentation of the egg is incomplete (meroblastic).

No primitive streak is formed and a rudimentary archenteron with both roof and floor may be established. There is an amnion and an allantois, membranes developed for the protection, nutrition and respiration of the embryo.

Amphibian Ancestry.

The Amphibia, which were the an cestors of the reptiles, spent the greater part of their life in water, probably crawling on to land only to pass from one pool to another. They laid small eggs, which were fertilized after they had passed out from the body of the mother. These eggs de veloped into an aquatic larva which breathed by means of gills; subsequently, when this larva had reached a relatively large size, the gills were absorbed and the animal became dependent on the air for the main bulk of its oxygen. An aquatic animal may have, and in the case of the Amphibia did have, a soft skin which can only remain healthy if it be kept moist. Living Amphibia secure this condition by pouring out mucus and water from glands in their skin, which is therefore slimy. An animal which adopts this method has great difficulty in roaming fax from water, the possi bility of dying from desiccation being always present. Thus one of the first changes necessary to make an effectively terrestrial animal from an amphibian is to alter the character of its skin in such a way that it becomes water-tight, and has a dry outer sur face. Such a change in a vertebrate is most readily achieved by thickening the epidermis and laying down keratin in its outer layers; continuation of this process leads to the formation of the horny scales of reptiles, which are made by localized patches of skin exceptionally active in the production of keratin. As such a skin does not require to be kept moist, glands are very poorly developed in the skin of reptiles.

During the transition from water to air the sense organs neces sarily undergo great modifications. The olfactory organ, which had become adapted to the relatively large amounts of odorous substances which could come to it in solution in water, had to be made capable of recognizing the much smaller amounts brought to it as vapour through the air. In the intervening stage of the Amphibia the nose becomes double, one part of it, Jacobson's organ, functioning in water, the rest in air. When the reptiles be came completely terrestrial, Jacobson's organ took on the new function of smelling the material lying in the mouth, and the rest of the organ became the normal organ of smell.