Book - Vertebrate Zoology (1928) 8

From Embryology

Vertebrate Zoology G. R. De Beer (1928)

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Chapter VIII Lacerta : A Chordate Living Entirely on Land

Externals

In general shape, the lizard is not very dissimilar from the newt, but it differs from it in one very important respect, which is characteristic of all the animals (Reptiles) of the group to which the lizard belongs. The body is covered with scales formed from the epidermis, and therefore totally different from the true scales of fish, which are always formed from the (mesodermal) dermis. To mark this distinction, the scales of reptiles are called corneoscutes. They cover the whole body including the limbs and head, and on the latter their arrangement does not correspond with that of the underlying bones. On the last phalanges of the fingers and toes, the scales form horny claws. Underlying the corneoscutes of the head there are ossifications of the dermis forming osteoscutes, which fuse with the underlying bones of the skull.


The skin is dry and devoid of glands. The eyes have upper and lower eyelids, and also a so-called " third eyelid " or nictitating membrane. Behind the eye is a circular area sunk slightly below the level of the skin, and covered over like a drum by the tympanic membrane or ear-drum. The external nostrils are on the side of the snout ibove the mouth. At the base of the tail is the cloaca, which in the male is provided with a pair of protrusible copulatory organs.

Skull

The skull of the lizard, although well ossified, has several holes in it, separating the membrane-bones. The nasal aperture is bounded by the premaxilla, nasal and maxilla. The orbit is limited by the prefrontal, frontal, and postfrontal above, and the lachrymal, jugal, and postorbital beneath. Behind the orbit is an aperture called a temporal fossa, in between the parietal, supratemporal, squamosal, postorbital, and postfrontal. In Lacerta this fossa is covered over by the osteoscutes mentioned above. This is not the case in Varanus, a form related to Lacerta, and in which the relations of the temporal fossa may be conveniently studied. There is a postorbital bar formed from the postfrontal, postorbital, and jugal, separating the orbit from the temporal fossa ; and a horizontal temporal bar formed by the postorbital and squamosal, forming the lower border of the temporal fossa. (It should be mentioned that there remains an element of doubt concerning the homologies of the bones here called supratemporal and squamosal. See p. 440.) On the floor of the nasal capsules are the septomaxillaries, which overlie Jacobson's organs.



Fig. 48. — Varanus : view of the skull from behind. bo, basioccipital ; bp, basipterygoid process of basisphenoid ; bs, basi-sphenoid ; fm, foramen magnum ; p, parietal ; pf, post-temporal fossa ; pp, paroccipital process (opisthotic and exoccipital) ; pt , pterygoid ; q, quadrate ; s, supratemporal ; so, supraoccipital.


The quadrate abuts against the fused opisthotic and exoccipital which form the paroccipital process. The quadrate also articulates by a loose joint with the pterygoid, and is movable relatively to the squamosal and brain-case (a condition known as streptostylic, see p. 292) ; in connexion with this arrangement the upper jaw can be raised relatively to the brain-case.


The foramen magnum is bounded by the basioccipital, supraoccipital, and exoccipital bones. In front of the basi- occipital, on the floor of the skull, is the basisphenoid, which has a pair of basipterygoid processes for articulation with the pterygoids. In front of the basisphenoid is the parasphenoid.


On the palatal surface the pterygoids are long bones lying to each side of the middle line. Behind, they connect with the quadrate, their inner surfaces articulate with the basipterygoid processes of the basisphenoid, and in front each pterygoid is connected with two bones : the transpalatine laterally and the palatine medially. The transpalatine is the representative of the ectopterygoid of Gadus. In front of the palatines are the prevomers. The margin of the upper jaw is made by the premaxilla? and maxillas. The ascending process is present and ossified as the epipterygoid, which rises as a slender pillar from the pterygoids. As in other animals, it separates the ophthalmic from the maxillary branches of the trigeminal nerve (see Figs. 138, 148, and 150).


In the lower jaw, the posterior region of Meckel's cartilage is ossified as the articular ; in addition there are the following membrane-bones : dentary, angular, supra-angular, splenial, and coronoid. Teeth are carried on premaxilla, maxilla, palatine, and dentary.


The ventral portions of the hyoid and branchial arches form a " hyoid " skeleton beneath the tongue. The hyomandibula is represented by the columella auris, a slender rod which connects the ear-drum or tympanic membrane with the fenestra ovalis in the side of the auditory capsule. This change of function of the hyomandibula from the condition in Scyllium and Gadus, where it supports the quadrate, is made possible by the autostylic method of suspension of the quadrate.


It is to be noted that the skull articulates with the vertebral column by one median condyle, and that in the formation of the skull, two more segments have been incorporated than in the Amphibia. This accounts for the fact that the hypoglossal nerves emerge from the skull, instead of behind it.


Vertebral Column

In Reptiles and all higher vertebrates, the first two vertebrae are peculiarly modified. The first is called the atlas, and its anterior surface is hollow to receive the condyle of the skull. Its centrum has, however, been separated from it and attached to that of the second vertebra, forming the odontoid peg. Round this peg the atlas and skull are free to rotate. The second vertebra is called the axis. The subsequent vertebrae of the neck and thorax are normal, and consist of centra with neural arches and spines, and zygapophyses. The centra are concave anteriorly, and convex posteriorly, a condition described as proccelous.


The vertebrae of the tail are peculiar in that they are split transversely, and when the lizard sheds (autotomises) its tail the break occurs at one of these splits. Under some of the tail-vertebrae are Y-shaped haemal arches, ossified as " chevronbones." Ribs are carried by all vertebrae in front of the sacrum except the first three ; they articulate with the centra of their respective vertebrae. The ribs belonging to the vertebrae of the neck (cervical) are short, those of the anterior region of the thorax are attached ventrally to the sternum (five pairs). The more posterior ribs (" floating ") do not touch the sternum. The two sacral vertebrae bear stout transverse processes which are attached to the ilia of the pelvic girdle.


Pectoral Girdle and Limb

The cartilage-bones of the pectoral girdle are the scapula and coracoid, both contributing to the glenoid cavity into which the head of the humerus fits. The anterior borders of these bones are characteristically indented. The membrane-bones consist of a pair of clavicles, and a median Y-shaped interclavicle. The forelimb is typical, and similar to that of Triton except that it has five fingers, each ending in claws.


Pelvic Girdle and Limb

The acetabulum is bordered by ilium, ischium, and pubis. The ilium points backwards towards its articulation with the sacral vertebrae. Both the ischium and the pubis meet their fellow-bones of the opposite side in the middle line, forming symphyses. On each side, the pubis and ischium are separated by the ischio-pubic foramen.


The hind limb is similar to that of Triton, but the tarsal bones undergo a modification. The proximal bones are fused into one which is attached to the tibia and fibula ; the distal bones are reduced to two, which become attached to the metatarsals. The result is that the ankle can only bend in one place, at the so-called mesotarsal joint. The 5th metatarsal is worthy of notice on account of its peculiar hook-shaped appearance.


Fig. 49. — Lacerta : pectoral- girdle and forelimb, seen from the left side.



Fig. 50. — Lacerta : sacrum, pelvic- girdle and hind limb, seen from the left side. c, coracoid ; ca, carpals ; cl, clavicle ; /, femur ; fi, fibula ; h, humerus ; ic, interclavicle ; il, ilium ; ip, ischio-pubic foramen ; is, ischium ; mc, metacarpal ; mts, metatarsal (note hook-like shape of that of fifth digit) ; p, phalanges ; pu, pubis ; r, radius ; s> sternum ; sc, scapula ; sv, sacral vertebrae ; t, tibia ; ta, tarsals ; tp, transverse processes of sacral vertebrae ; u, ulna.


Alimentary System

The tongue is long, bifid at the tip, and protrusible. It is supported by the " hyoid " skeleton. On the roof of the mouth the palatal folds appear, extending inwards from the sides. At the back of the mouth-cavity, the Eustachian tubes open. These represent the cavity of the spiracle of the dogfish, and each is closed laterally by the ear-drum or tympanic membrane. These cavities are also called tympanic cavity, and " middle-ear," and will be referred to again in connexion with that sense-organ. The mouth is provided with salivary glands, which assist digestion.


The glottis leads to the larynx and lungs. The remaining viscera do not differ sufficiently in detail from those of Triton to necessitate a specific redescription.



++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++


Fig. 51. — Lacerta : ventral view of the pectoral girdle and sternum.


c, coracoid ; cl, clavicle ; h, humerus ; ic, interclavicle ; r, ribs ; s } sternum ; sc, scapula.


Respiratory System

The lungs are sacs with very vascular walls, and they are the only respiratory organs, for the skin no longer functions as such. Another change from the amphibian condition is shown by the method of breathing. Instead of raising and lowering the floor of the mouth, the ribs are pulled forwards by muscles which run obliquely from rib to rib. At rest, the ribs slope backwards, and when pulled forwards the effect is to increase the volume of the thoracic cavity and of the lungs. Air then rushes in.


Vascular System

The heart consists of sinus venosus, two auricles and a single ventricle. The truncus arteriosus has been split into three right down to its base, so that the ventricle opens directly into three arteries. The more ventral of the three opens into the right side of the ventricle, and leads to the lungs, dividing as it goes into two pulmonary arteries. These no longer connect with the lateral dorsal aorta or systemic arches. The pulmonary circulation is therefore distinct. The other two vessels are the right and left systemic arches (corresponding to the systemic arches of the newt and the 4th arterial arches of fish). The right systemic arch springs from the left side of the ventricle, and the left arch from the right side of the ventricle. These vessels run up the 4th visceral arch and join dorsal to the gut to form the dorsal aorta. Now, the left side of the ventricle is occupied mostly by arterial (oxygenated) blood from the left auricle and pulmonary veins ; consequently the right systemic arch receives pure blood, more or less. But the left arch and the pulmonary vessel are on the right side of the ventricle, which contains venous blood from the right auricle, sinus venosus, and the veins of the body. In addition, there is an incomplete septum dividing the ventricle, so that while the pulmonary artery receives venous blood as would be expected, the left systemic arch receives mixed blood. The carotid arches spring from the base of the right systemic arch and therefore receive pure blood, as indeed they need, for they supply the brain. The carotid arches run dorsally in the 3rd visceral arch and when dorsal to the gut run forwards into the head as the internal carotid arteries. In addition, however, the carotid arteries connect back with the systemic arches by what are really remnants of the lateral dorsal aorta. These connexions are known as the ductus caroticus.


The arteries given off to the viscera are on the whole similar to those of Triton.


The venous system is likewise similar to that of Triton, but it is necessary to mention three new points. In correlation with the lack of respiratory function on the part of the skin, the cutaneous vein is not found. The posterior cardinal veins are likewise much reduced ; that on the left disappears altogether, that on the right is now known as the azygos vein. Lastly, the renal portal system is less well developed, and this is associated with the fact that the functional kidney in the adult is a new structure, the metanephros.


Urino-Genital System

The kidneys are paired structures lying in the roof of the posterior part of the coelomic cavity, which connect with the base of the allantoic bladder (where the latter opens into the cloaca) by means of ducts called ureters. These kidneys are not the same as the mesonephric kidneys of the animals previously described ; they are metanephric kidneys, and serve excretory functions only, never connecting with the genital organs. Otherwise, they are similar in structure to the mesonephros, and consist of Malpighian corpuscles with glomeruli and tubules. The metanephros develops later than the mesonephros, and out of the way of the posterior cardinal veins. The mesonephros is present in early stages of development, but does not function as an excretory organ in the adult. As the renal portal system is associated with the mesonephros, the disappearance of the one is correlated with the reduction of the other.


In the female, the mesonephros disappears in the adult, together with the Wolffian duct. The Miillerian duct persists as the oviduct and serves to convey the eggs (which drop from the ovary into the ccelom) to the exterior via the cloaca. Glands in the oviduct secrete a shell round the egg, for it is laid on dry land, and not in water. The embryo develops within a membrane, the amnion, for which reason reptiles, birds, and mammals are called Amniota. In the male, the Miillerian duct is absent, but the mesonephros and Wolffian duct persist, serving only to evacuate the sperms. The testis is connected with the mesonephros by vasa efferentia in the ordinary manner, and the tubules of the mesonephros, through which the sperms pass, form the epididymis. The epididymis is really very long, and when unravelled it forms a tube which in man is over twenty feet long. During their passage through it, the sperms are acted on by a secretion, as a result of which they complete their development and acquire the power of individual movement. The Wolffian duct is the vas deferens leading to the cloaca. As already mentioned, the wall of the cloaca bears two eversible copulatory organs, for since the egg is surrounded by a shell when it leaves the oviduct, it is obvious that fertilisation must take place in the oviduct itself.


Fig. 53. — Method of formation of the kidneys in amniotes.


A, larval condition, with Mullerian duct (Md), Wolffian duct (Wd), mesonephric funnels (mi) opening into the splanchnocoel (sc) ; i, intestine. The ureter (u) arises from the base of the Wolffian duct and divides into a number of tubes which eventually connect with the metanephric capsules (mc). B, condition in the adult male. The Mullerian duct has disappeared and the Wolffian duct persists as the vas deferens (vd), receiving the sperms from the testis (t) by means of the vasa efferentia (ve). The ureter drains the urine from the metanephros (m) to the bladder (b). C, condition in the adult female. The Mullerian duct persists as the oviduct (od), the opening of which into the splanchnocoel is the Fallopian tube (Ft) ; o, ovary. The Wolffian duct has disappeared.


Nervous System

The brain is built on the same plan as that oL Triton, but it shows an advance in the increased size of the cerebral hemispheres. In these there is a small amount of superficial grey matter or cortex ; in lower forms the grey matter is almost entirely within the white. This is a very important advance from the point of view of the evolution of the human brain. The sides of the telecephalon (corpus striatum) and of the diencephalon (thalamus) are enlarged, and the cavities of the 1st, 2nd, and 3rd ventricles are consequently reduced. There is a well-developed pineal eye, arising from the diencephalon, and connected with the right habenular ganglion. The cranial nerves are similar to those of Triton except for the fact that the hypoglossus (12th nerve) is included among them, and that there is a spinal accessory nerve (nth nerve). The spinal accessory supplies the dorsal muscles of the shoulder girdle, and represents a specialised portion of the vagus of lower forms.

Sense-organs

The lateral line sense-organs are no longer present, and the sole representative of the system to which they belong is the ear. In addition to being an organ of balance, the ear is also stimulated by vibrations in air, or sound. (This is also the case in Amphibia such as the frog, but not so typically in Triton in which the ear is degenerate (see p. 100).) The vibrations impinge on the ear-drum (tympanic membrane) and are communicated to the columella auris (hyomandibula) ; the latter conveys the vibrations across the cavity of the middle-ear or tympanic cavity (spiracular visceral cleft) to the fenestra ovalis in the wall of the auditory capsule. The auditory capsule contains the auditory vesicle ; between the latter and the wall of the capsule is a fluid called perilymph, while the auditory vesicle itself contains endolymph. The vibrations brought by the columella auris are imparted through the fenestra ovalis to the perilymph, which in turn passes them on through the wall of the auditory vesicle to the endolymph. Here the vibrations stimulate the special sensory cells. The wall of the auditory capsule has a second opening (the fenestra rotunda), situated ventrally to the fenestra ovalis. The fenestra rotunda is covered by a membrane separating the perilymph from the tympanic cavity, and its function is to damp down and deaden the vibrations in the perilymph when they reach it. With regard to the auditory vesicle itself, the utricle has the usual three semicircular canals, and the saccule, which is better developed than in lower forms in connexion with the perfecting of the sense of hearing, has a ductus cochlearis.


The retina of the eye contains mostly cones with very few rods. The lens changes its degree of convexity, and thereby its focal length, as a result of the contraction of the circular iris-muscle. The sclerotic is strengthened by bony plates. Mention has already been made of the three eyelids. The lining of the lids is in places modified into glands. At the inner side of the eye is the Harderian gland which lubricates the " third eyelid " (nictitating membrane) ; at the outer angle is the lachrymal gland. The transparent nictitating membrane, which is really a fold of the conjunctiva, is activated by a muscle derived from the retractor bulbi, and like it innervated by a branch of the abducens. The lower lid is depressed by a special muscle.


The pineal eye, already seen in Petromyzon, is remarkably developed. Its stalk rises up from the roof of the diencephalon and swells out into a vesicle of which the lower portion forms the sensory layer, and the upper forms the lens. This eye lies below the foramen between the parietals ; it is, however, covered over by connective tissue and a corneoscute.


The cavity of the nose is enlarged, and a shelf projects inwards from the side wall, increasing the surface of the nasal epithelium and forming a so-called concha. Ventral to the nasal cavities are a pair of pockets, originally formed from the nasal cavities, and lying just above the prevomers. Each opens into the mouth cavity a little way in front of the choanae, or internal nostrils. These structures are known as Jacobson's organ, and they probably serve to smell food in the mouth.


Coelom

The splanchnocoel is represented by the pericardium and perivisceral coelomic cavities. The lungs project backwards into the latter on each side of the stomach, supported by the pulmonary folds of the coelomic epithelium (accessory mesenteries). These folds also connect ventrally with the liver, by the pulmo-hepatic ligaments. The gut is of course suspended from the roof of the coelomic cavity by the dorsal mesentery, and connected with the liver ventrally by the so-called lesser omentum,* also a fold of epithelium. The mesentery ventral to the liver mostly disappears, but persists anteriorly as the falciform ligament. In this manner the coelomic cavity becomes divided up into a number of intercommunicating spaces. On each side of the gut and mesentery, and median to the pulmonary folds and pulmo-hepatic ligaments, is a pulmo-hepatic recess which ends blindly in front, and opens posteriorly into the main cavity. Owing to the kinking of the stomach to the left, the first portion of the intestine (duodenum) recurves to the right, and the right pulmo-hepatic recess forms part of a pocket, the omental cavity. This cavity communicates with the main coelom on the right side by an opening, the upper and front borders of which are formed by the right pulmonary fold and pulmo-hepatic ligament ; the lower and hind borders are formed by the lesser omentum running from the duodenum to the liver, and by the mesentery supporting the duodenum. The opening is the foramen of Winslow, and its relations are important with regard to the inferior vena cava which runs down its upper and anterior border, and the hepatic portal vein, the hepatic artery, and the bile-duct which run along its lower border (see Fig. 125).


  • The lesser omentum is equivalent to the gastro-hepatic ligament and the duodeno-hepatic ligament : portions of mesentery connecting the liver with the stomach and the duodenum respectively.


Characters of Lacerta > lacking in lower forms , and common to Amniota :

Embryos develop on land within an Amnion and a shell ;

Metanephros and ureter ;

Spinal accessory and Hypoglossal nerves emerge from the skull ;

Superficial grey matter (cortex) in the cerebral hemispheres ;

Breathing effected with the help of the ribs ;

Jacobson's organ ;

Development of saccule, tympanic cavity (spiracular pouch), and conversion of the hyomandibula into the columella auria. (Already present in the frog, though feebly developed in Triton) ;

Atlas and Axis vertebrae differentiated ;

Copulatory organs developed ;

Ischio-pubic foramen present.


Characters of Lacerta which are typical of Reptiles :

Skin covered with horny scales ;

Ventricle of the heart incompletely divided (except in Crocodiles).

Literature

Parker, T. J. A Course of Instruction in Zootomy (Vertebrates). Macmillan, London, 1884.


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Pages where the terms "Historic" (textbooks, papers, people, recommendations) appear on this site, and sections within pages where this disclaimer appears, indicate that the content and scientific understanding are specific to the time of publication. This means that while some scientific descriptions are still accurate, the terminology and interpretation of the developmental mechanisms reflect the understanding at the time of original publication and those of the preceding periods, these terms, interpretations and recommendations may not reflect our current scientific understanding.     (More? Embryology History | Historic Embryology Papers)
Vertebrate Zoology 1928: PART I 1. The Vertebrate Type as contrasted with the Invertebrate | 2. Amphioxus, a primitive Chordate | 3. Petromyzon, a Chordate with a skull, heart, and kidney | 4. Scyllium, a Chordate with jaws, stomach, and fins | 5. Gadus, a Chordate with bone | 6. Ceratodus, a Chordate with a lung | 7. Triton, a Chordate with 5-toed limbs | 8. Lacerta, a Chordate living entirely on land | 9. Columba, a Chordate with wings | 10. Lepus, a warm-blooded, viviparous Chordate PART II 11. The development of Amphioxus | 12. The development of Rana (the Frog) | 13. The development of Gallus (the Chick) | 14. The development of Lepus (the Rabbit) PART III 15. The Blastopore | 16. The Embryonic Membranes | 17. The Skin and its derivatives | 18. The Teeth | 19. The Coelom and Mesoderm | 20. The Skull | 21. The Vertebral Column, Ribs, and Sternum | 22. Fins and Limbs | 23. The Tail | 24. The Vascular System | 25. The Respiratory system | 26. The Alimentary system | 27. The Excretory and Reproductive systems | 28. The Head and Neck | 29. The functional divisions of the Nervous system | 30. The Brain and comparative Behaviour | 31. The Autonomic Nervous system | 32. The Sense-organs | 33. The Ductless glands | 34. Regulatory mechanisms | 35. Blood-relationships among the Chordates PART IV 36. The bearing of Physical and Climatic factors on Chordates | 37. The origin of Chordates, and their radiation as aquatic animals | 38. The evolution of the Amphibia : the first land-Chordates | 39. The evolution of the Reptiles | 40. The evolution of the Birds | 41. The evolution of the Mammalia | 42. The evolution of the Primates and Man | 43. Conclusions | Figures | Historic Embryology



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