Book - The Elements of Embryology - Chicken 8

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Foster M. Balfour FM. Sedgwick A. and Heape W. The Elements of Embryology (1883) Vol. 1. (2nd ed.). London: Macmillan and Co.

The Elements of Embryology 1883 1 Chicken : Hen's egg and the beginning of incubation | Whole history of incubation | day 1 of incubation | first half of day 2 | second half of day 2 | day 3 | day 4 | day 5 | day 6-21 | Appendix | Figures as Gallery 2 Mammalian: General Development of the Embryo | Embryonic Membranes and Yolk-Sac | Organs from Epiblast | Organs from Mesoblast | Alimentary Canal | Appendix | Figures as Gallery

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The changes which take place on the fifth day

ON opening an egg about the middle of the fifth day, the observer's attention is not arrested by any new features ; but he notices that the progress of development, which was so rapid during the later half of the fourth day, is being continued with undiminished vigour.

The allantois

The allantois, which on the fourth day began to project from the pleuroperitoneal cavity, has grown very rapidly, and now stretches away from the somatic stalk far over the right side of the embryo (which it will be remembered is lying on its left side) in the cavity between the two amniotic folds (Fig. 9, K.\ It is very vascular, and already serves as the chief organ of respiration.

The blastoderm has spread over the whole of the yolk-sac, and the yolk is thus completely enclosed in a bag whose walls however are excessively delicate and easily torn. The vascular area extends over about twothirds of the yolk.

The splanchnic stalk

The splanchnic stalk or vitelline duct has now reached its greatest narrowness ; it has become a solid cord, and will undergo no further change till near the time of hatching. The space between it and the somatic stalk is still considerable, though the latter is narrower than it was on the fourth day.

The embryo remains excessively curved, so much so indeed that the head and the tail are nearly in contact.

The limbs

The limbs have increased, especially in length ; in each a distinction is now apparent between the more cylindrical stalk and the flattened terminal expansion ; and the cartilaginous precursors of the several bones have already become visible.

The fore and hind limbs are still exceedingly alike, and in both the stalk is already beginning to be bent about its middle to form the elbow and knee respectively.

The angles of both knee and elbow are in the first instance alike directed outwards and somewhat backwards. By the eighth day, however, the elbow has come to look directly backwards and the knee forwards. In consequence of this change, the digits of the fore limb point directly forwards, those of the hind limb directly backwards. This state of things is altered by a subsequent rotation of the hand and foot on the arm and leg, so that by the tenth day the toes are directed straight forwards, and the digits of the wing backwards and somewhat ventralwards, the elbow and knee almost touching each other.

While these changes are taking place the differences between wing and foot become more and more distinct. The cartilages of the digits appear on the fifth day as streaks in the broad flat terminal expansions, from the even curved edge of which they do not project. On the sixth or seventh day the three digits of the wing (the median being the longest) and the four (or in some fowls five) digits of the foot may be distinguished, and on the eighth or ninth day these begin to project from the edge of the expanded foot and wing, the substance of which, thin and more or less transparent, remains for some time as a kind of web between them. By the tenth day the fore and hind extremities, save for the absence of feathers and nails, are already veritable wings and feet.

Within the mesoblast of the limbs a continuous blastema becomes formed, which constitutes the first trace of the skeleton of the limb. The corresponding elements of the two limbs, viz. the humerus and femur, radius and tibia, ulna and fibula, carpal and tarsal bones, metacarpals and metatarsals, and phalanges, become differentiated within this, by the conversion of definite regions into cartilage, which probably are at first united. These cartilaginous elements subsequently ossify.

Early Skeleton

The pectoral girdle

The scapulo-coracoid elements of the shoulder girdle are formed as a pair of cartilaginous plates, one on each side of the body. The dorsal half of each plate ossifies as the scapula, the ventral as the coracoid. The clavicles are probably membrane bones.

The pelvic girdle

The pelvic girdle is derived from a pair of cartilaginous plates, one on each side. Each of them is developed in continuity with the femur of its side. The dorsal half of each plate ossifies as the ilium ; the ventral half becomes prolonged into two processes, the anterior of which ossifies as the pubis, the posterior at the ischium.

Ribs and sternum

The ribs appear to arise as cartilaginous bars in the connective tissue of the body walls. They are placed opposite the intervals between the muscle-plates, and are developed independently of the vertebrae, with the transverse processes of which they subsequently become closely united by fibrous tissue.

The sternum appears to be formed from the fusion of the ventral extremities of a certain number of the ribs. The extremities of the ribs unite with each other from before backwards, and thus give rise to two cartilaginous bands. These bands become segmented off from the ribs with which they are at first continuous, and subsequently fuse in the median ventral line to form the unpaired sternum.

The skull

Two distinct sets of elements enter into the composition of the avian skull. These are (1) the cranium proper, (2) the skeleton of the visceral arches.

The cranium=

As we mentioned in the last chapter, the formation of the primitive cranium commenced upon the fourth day. This primitive cranium, in its earliest stage, inasmuch as it is composed of condensed but otherwise only slightly differentiated mesoblast, may be spoken of as the membranous cranium. On the sixth day true hyaline cartilage makes its appearance as a differentiation within the membranous cranium. The cartilaginous cranium is composed of the following parts.

(1) A pair of cartilaginous plates placed on each side of the cephalic section of the notochord, and known as the parachordals (Fig. 76, iv.). These plates, together with the notochord (nc.) enclosed between them, form a floor for the hind- and mid-brain. The continuous plate, formed by them and the notochord, is known as the basilar plate.

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FIG. 76. YlEW FROM ABOVE OF THE PARACHORDALS AND OF THE TRABECULE ON THE FIFTH DAY OF INCUBATION. (From Parker.)

In order to shew this the whole of the upper portion of the head has been sliced away. The cartilaginous portions of the skull are marked with the dark horizontal shading.

c.v. 1. cerebral vesicles (sliced off), e. eye. nc. notochord. iv. parachordal. 9. foramen for the exit of the ninth nerve. d. cochlea, h.s.c. horizontal semi-circular canal, q. quadrate. 5. notch for the passage of the fifth nerve. Ig. expanded anterior end of the paracbordals. pt.s. pituitary space, tr. trabeculse. The reference line tr has accidentally been made to end a little short of the cartilage.

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(2) A pair of bars forming the floor for the forebrain, and known as the trabeculce (tr.). These bars are continued forward from the parachordals, with which, in the chick, they are from the first continuous. United behind where they embrace the front end of the notochord, they diverge anteriorly for some little distance and then bend in again in such a way as to enclose a space the pituitary space. In front of this space they again unite and extend forwards into the nasal region.

(3) The cartilaginous capsules of the sense organs. Of these the auditory and olfactory capsules unite more or less intimately with the cranial walls, while the optic capsules, forming the sclerotics, remain distinct.

The parachordals and notochord

The first of these sets of elements, viz. the parachordals *and notochord, forming together the basilar plate, is an unsegmented continuation of the axial tissue of the vertebral column. It forms the floor for that section of the brain which belongs to the primitive postoral part of the head, and its extension is roughly that of the basioccipital of the adult skull.

Laterally it encloses the auditory sacs (Fig. 76), the tissue surrounding these (forming the so-called ' periotic capsules') is in. the chick never separate from the basilar plate. In front it becomes narrowed, and at the same time excavated so as to form a notch on each side (Fig. 76, 5) through which the fifth nerve passes ; and in front of this it again becomes expanded.

In order to render our subsequent account more intelligible, we may briefly anticipate the fate of the basilar plate. Behind it grows upwards on both sides, and the two outgrowths meet above so as completely to enclose the medulla oblongata, and to circumscribe a hole known as the ' occipital foramen? And it is at this point only that the roof of the skull is at any period formed of cartilage.

It will be convenient to say a few words here with reference to the notochord in the head. It always extends along the floor of the mid- and hind-brains, but ends immediately behind the infundibulum. The front end of the notochord becomes more or less ventrally flexed in correspondence with the cranial flexure ; its anterior end being in some animals (Elasmobranchii) almost bent backwards (Fig. 77).

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FIG. 77. LONGITUDINAL SECTION THROUGH THE HEAD OF A YOUNG PRISTIURUS EMBRYO.

cer. commencement of the cerebral hemisphere, pn. pineal gland. In. infundibulum. pt. ingrowth from mouth to form the pituitary body. mb. mid-brain, cb. cerebellum, ch. notochord. al. alimentary tract. laa. artery of mandibular arch.

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Kolliker has shewn that in the Rabbit, and a more or less similar phenomenon may also be observed in Birds, the anterior end of the notochord is united to the hypoblast of the throat in immediate contiguity with the opening of the pituitary body ; but it is not clear whether this is to be looked upon as the remnant of a primitive attachment of the notochord to the hypoblast, or as a secondary attachment.

Within the basilar plate the notochord often exhibits two or more dilatations, which have been regarded by Parker and Kolliker as indicative of a segmentation of this plate ; but they hardly appear to be capable of this interpretation.

The trabeculae

The trabeculse, so far as their mere anatomical relations are concerned, play the same part in forming the floor for the front cerebral vesicle as the parachordals for the mid- and hind-brains. They differ however from the parachordals in one important feature, viz. that, except at their hinder end, they do not embrace between them the notochord.

The notochord constitutes, as we have seen, the primitive axial skeleton of the body, and its absence in the greater part of the region of the trabeculse would probably seem to indicate, as pointed out by Gegenbaur, that these parts, in spite of their similarity to the parachordals, have not the same morphological significance.

While this distinction between the parachordals and the trabeculse must be admitted, there seems to be no reason against supposing that the trabeculse may be plates developed to support the floor of the fore-brain, for the same physiological reasons that the parachordals have become formed at the sides of the notochord to support the floor of the hind-brain. By some anatomists the trabeculse have been held to be a pair of branchial bars ; but this view has now been generally given up. They have also been regarded as equivalent to a complete pair of neural arches enveloping the front end of the brain. The primitive extension of the base of the fore-brain through the pituitary space is an argument, not without force, which has been appealed to in support of this view.

In the majority of the lower forms the trabeculse arise quite independently of the parachordals, though the two sets of elements soon unite ; while in Birds (Fig. 76) and Mammals the parachordals and trabeculse are formed as a continuous whole. The junction between the trabeculse and parachordals becomes marked by a cartilaginous ridge known as the posterior clinoid. The trabeculse are somewhat lyre-shaped, meeting in front and behind, and leaving a large pituitary space between their middle parts (Fig. 76). Into this space there primitively projects the whole base of the fore-brain, but the space itself gradually becomes narrowed, till it usually contains only the pituitary body. The carotid arteries pass through it in the embryo ; but it ceases to be perforated in the adult. The trabeculso soon unite together, both in front and behind, and form a complete plate underneath the fore-brain, ending in two horns in the interior of the fronto-nasal process. A special vertical growth of this plate in the region of the orbit forms the interorbital plate (Fig. 78, ps.), on the upper surface of which the front part of the brain rests. The trabecular floor of the brain does not long remain simple. Its sides grow vertically upwards, forming a lateral wall for the brain, in which two regions may be distinguished, viz. an alisphenoidal region (Fig. 78, as.) behind, growing out from what is known as the basisphenoidal region of the primitive trabeculse, and an orbitosphenoidal region in front growing out from the presphenoidal region of the trabeculse. These plates form at first on each side a continuous lateral wall of the cranium. At the front end of the brain they are continued inwards, and more or less completely separate the true cranial cavity from the nasal region in front. The region of the trabeculse in front of the brain is the ethmoidal region ; it forms the anterior boundary of the cranial cavity. The basal part of this region forms an internasal plate, from which an internasal septum, continuous behind with the interorbital septum, grows up (Fig. 78); while the lateral part is known as the lateral ethmoid region, which is always perforated for the passage of the olfactory nerve.

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FIG. 78. SlDE VIEW OF THE CARTILAGINOUS CRANIUM OF A FOWL ON THE SEVENTH DAY OF INCUBATION. (After Parker.)

pn. preuasal cartilage, aln. alinasal cartilage, ale. aliethmoid ; immediately below this is the aliseptal cartilage, eth. ethmoid, pp. pars plana. ps. presphenoid or inter-orbital. pa. palatine, pg. pterygoid. z. optic nerve, as. alisphenoid. q. quadrate, st. stapes, fr. fenestra rotunda, hso. horizontal semicircular canal, psc. posterior vertical semicircular canal : both the anterior and the posterior semicircular canals are seen shining through the cartilage, so. supraoccipital, eo. exoccipital. oc. occipital condyle. nc. notochord. mJc. Meckel s cartilage, ch. cerato-hyal. bh. basihyal. cbr. and ebr. cerato-branchial. bbr. basibranchial.

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The sense capsules

The most important of these is the auditory capsule, which, as we have seen, fuses intimately with the lateral walls of the skull. In front there is usually a cleft separating it from the alisphenoid region of the skull, through which the third division of the fifth nerve passes out. This cleft becomes narrowed to a small foramen. The sclerotic is free, but profoundly modifies the region of the cranium near which it is placed. The nasal investment is developed in continuity, and is closely united, with the ethmoid region.

The cartilaginous cranium, the development of which has been thus briefly traced, persists in the adult without even the addition of membrane bones in certain fishes, e.g. the Elasmobranchii. In the Selachioid Ganoids it is also found in the adult, but is covered over by membrane bones. In all other types it is invariably present in the embryo, but becomes in the adult more or less replaced by osseous tissue.

The bones

in the adult skull may be divided roughly into two categories according to their origin.

(1) Cartilage bones, i.e. ossifications in the primitive cartilaginous cranium.

(2) Membrane bones, i.e. ossifications in membrane without any cartilaginous precursors.

The names which have been given to the various parts of the cartilaginous cranium in the above account are derived from the names given to the bones appearing in the respective regions in the more developed skull.

The skeleton of the visceral arches

The visceral arches were all originally branchial in function. They supported the walls between successive branchial clefts.

The first arch (mandibular) has in all living forms lost its branchial function, and its bar has become converted into a supporting skeleton for the jaws.

The second arch (hyoid), with its contained bar, though retaining in some forms (Elasmobranchii) its branchial function, has in most acquired additional functions, and has undergone in consequence various peculiar modifications.

The succeeding arches and their contained bars retain their branchial function in Pisces and some Amphibia, but are secondarily modified and largely aborted in the abranchiate forms.

The ordinary visceral arches in the chick are, as we have seen, sufficiently obvious, while as yet their mesoblast is quite undifferentiated ; but in the three anterior of them rods of cartilage are subsequently developed and begin to make their appearance about the fifth day.

The first arch (mandibular), it will be remembered, budded off a process called the superior maxillary process. The whole arch, therefore, comes to consist of two parts, viz. a superior and an inferior maxillary process ; it is in the latter of these that the cartilaginous rod on each side is developed. The membranous tissue in the superior maxillary process is called, from its subsequent fate, the ptery go-palatine bar, and is in the chick ossified directly without the intervention of cartilage. In the inferior maxillary process two developments of cartilage take place, a proximal and a distal. The proximal cartilage is situated (Figs. 76 and 79, q.) at the side of the periotic capsule, but is not united with it. It is known as the quadrate, and in the early stage is merely a small knob of cartilage. The quadrate cartilage ossifies as the quadrate bone, and supplies the permanent articulation for the lower jaw. The distal rod is called Meckel's cartilage (Fig. 79, mk.) ; it soon becomes covered by investing (membrane) bones which form the mandible ; and its proximal end ossifies as the articulare.

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FIG. 79. VIEW FROM BELOW OF THE PAIRED APPENDAGES OF THE SKULL OF A FOWL ON THE FlFTH DAT OF INCUBATION. (From Parker.)

cv. 1. cerebral vesicles, e. eye. fn. fronto-nasal process, n. nasal pit. tr. trabeculse. pts. pituitary space, mr. superior maxillary process, pg. pterygoid. pa. palatine, q. quadrate, mk. Meckel's cartilage, ch. cerato-hyal. bh. basihyal. cbr. ceratobranchial. ebr. proximal portion of the cartilage in the third visceral arch. bbr. basibranchial. 1. first visceral cleft. 2. second visceral cleft. 3. third visceral arch.

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In the next arch, usually called the second visceral or hyoid arch, there is a very small development of cartilage. This consists of a central azygos piece, the 'basi-hyal' (Fig. 79, bh.), and two rods, one on each side, the ' cerato-hyals ' (Fig. 79, ch.).

In the third arch, which corresponds with the first branchial arch of the Ichthyopsida, there is on each side a large distal cartilaginous rod (Fig. 79, cbr.), the ' cerato-branchial/ and a smaller proximal piece (Fig. 79, ebr.), between the two arches lies an undefined mass (Fig. 79, bbr), the ' basibranchial.' In the arches behind this one there is in the bird no development of cartilage.

The lower part of the hyoid arch, including the basi-hyal, unites with the remnants of the arch behind to form the hyoid bone of the adult.

The fenestra ovalis and fenestra rotunda appear on the seventh day as spaces in the side walls of the periotic cartilage. The former is filled up by a small piece of cartilage, the stapes (Fig. 78, st.), which in the adult forms part of the columella (see pp. 166, 167).

The columella is believed by Huxley and Parker to represent the independently developed dorsal element of the hyoid, together with the stapes with which it has become united.

For further details of the development of the skull we must refer the student to Professor Parker's Memoir upon the Development of the Skull of the Common Fowl (Gallus domesticus), Phil. Trans., 1866, Vol. CLVL, pt. 1, and to the chapter on the Bird's skull in the Morphology of the Skull, by Professor Parker and Mr Bettany.

We shall conclude this account by giving a table of those bones which are preformed in cartilage, and of the purely splint or membrane bones.

Parts of the bird's skull which are either preformed in cartilage or remain cartilaginous.

Formed from the parachordal cartilages and their upgrowths around the foramen magnum. Supraoccipital. Exoccipital. Basioccipital.

Formed in the periotic cartilage. Epiotic. Prootia Opisthotic.

Formed from the trabeculse and their upgrowths, Alisphenoid. Basisphenoid. Orbitosphenoid. Presphenoid. Ethmoid. Septum nasi, turbinals, prenasal and nasal cartilages.

Articulare and quadrate belonging to the first visceral arch. Skeleton of the second and third visceral arches and stapes.

Splint-bones not preformed in cartilage.

Parietals. Squamosals. Frontals. Lacrymals. Nasals. Premaxillse. Maxillae. Maxillo-palatines, Vomer. Jugals. Quadrato-jugals. Dentary and bones of mandible. Basi-temporal and rostrum. Pterygoid and palatine (superior maxillary process).

The face

Closely connected with the development of the skull is the formation of the parts of the face.

After the appearance of the nasal grooves on the fourth day the mouth (Fig. 80, M.) appears as a deep depression inclosed by five processes. Its lower border is entirely formed by the two inferior maxillary processes (Fig. 80, F. 1), at its sides lie the two superior maxillary processes S. M., while above it is bounded by the fronto-nasal process nf.

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FIG. 80. A. HEAD OF EMBRYO CHICK OF THE FOURTH DAY VIEWED FROM BELOW AS AN OPAQUE OBJECT. (Chromic acid preparation.)

C.ff. cerebral hemispheres. FB. vesicle of the third ventricle. Op. eyeball, nf. naso-frontal process. M. cavity of mouth. S.M. superior maxillary process of F. 1, the first visceral fold (mandibular arch). F. 2, F. 3, second and third visceral folds. N. nasal pit.

In order to gain the view here given the neck was cut across between the third and fourth visceral folds. In the section e thus made, are seen the alimentary canal al with its collapsed walls, the neural canal ra.c., the notochord ch., the dorsal aorta AO.j and the jugular veins V.

B. The same seen sideways, to shew the visceral folds. Letters as before.

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After a while the outer angles of the fronto-nasal process, enclosing the expanded termination of the trabeculae, project somewhat outwards on each side, giving the end of the process a rather bilobed appearance. These projecting portions of the fronto-nasal process form on each side the inner margins of the rapidly deepening nasal grooves, and are sometimes spoken of as the inner nasal processes. The outer margin of each nasal groove is raised up into a projection frequently spoken of as the outer nasal process, which runs downwards to join the superior maxillary process, from which however it is separated by a shallow depression. This depression, which runs nearly horizontally outwards towards the eyeball, is known as the lacrymal groove (see p. 155).

On the fifth day the inner nasal processes, or lower and outer corners of the fronto-nasal process, arching over, unite on each side with the superior maxillary processes. (Compare Fig. 81, which, however, is a view of the head of a chick of the sixth day.) In this way each nasal groove is converted into a canal, which leads

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FIG. 81. HEAD OF A CHICK AT THE SIXTH DAY FROM BELOW. (From Huxley.)

la. cerebral vesicles, a. eye, in which the remains of the choroid slit can still be seen. g. nasal pits. k. fronto-nasal process. 1. superior maxillary process. 1. inferior maxillary process or first visceral arch. 2. second visceral arch. x. first visceral cleft between the first and second visceral arches.

The cavity of the mouth is seen enclosed by the fronto-nasal process, the superior maxillary processes and the first pair of visceral arches. At the back of it is seen the opening leading into the throat. The nasal grooves leading from the nasal pits to the mouth are already closed over and converted into canals.

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from the nasal pit above, into the cavity of the mouth below, and places the two in direct communication. This canal, whose lining consists of epiblast, is the rudiment of the nasal labyrinth.

By the seventh day (Fig. 82), not only is the union of the superior maxillary and fronto-nasal processes completed, and the upper boundary of the mouth thus definitely constituted, but these parts begin to grow rapidly forward, thus deepening the mouth and giving rise to the appearance of a nose or beak (Fig. 82), which, though yet blunt, is still distinct. The whole of the lower boundary of the buccal cavity is formed by the inferior maxillary processes.

As we have before mentioned (p. 240), cartilage succeeded by bone is developed in the fronto-nasal process ; the pterygo-palatine osseous bar (membranous ossification) in the superior maxillary process; Meckel's cartilage the main part of which atrophies, the proximal end only ossifying as the articulare, and the quadrate succeeded by bone in the inferior maxillary process; the other bones which form the boundaries of the mouth in the adult are developed later after all external trace of these parts as separate processes has "disappeared.

At first the mouth is a simple cavity into which the nasal canals open directly. When however the various processes unite together to form the upper boundary of the mouth, each superior maxillary process sends inwards a lateral bud. These buds become flattened and form horizontal plates which stretch more and more inward towards the middle line. There they finally meet, and by their union, which is effected first in front and thence extends backwards, constitute a horizontal plate stretching right across the mouth and dividing it into two cavities an upper and a lower one.

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FIG. 82. HEAD OF A CHICK OF THE SEVENTH DAY FROM BELOW. (From Huxley.)

la. cerebral vesicles, a. eye. g. nasal pits. L fronto-nasal process. 1. superior maxillary process. 1. first visceral arch. 2. second visceral arch. x. first visceral cleft.

The external opening of the mouth has become much constricted, but it is still enclosed by the fronto-nasal process and superior maxillary processes above, and by the inferior maxillary processes (first pair of visceral arches) below.

The superior maxillary processes have united with the frontonasal process, along the whole length of the latter, with the exception of a small space in front, where a narrow angular opening is left between the two.

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In the front part of the mouth their union is quite complete, so that here there is no communication between the two cavities. Behind, however, the partition is not a complete one, so that the two divisions of the buccal cavity communicate at the back of the mouth. The external opening of * the mouth passes into the lower of these two cavities, which may therefore be called the mouth proper. Into the upper chamber the nasal ducts open ; it may be called the respiratory chamber, and forms the commencement of the chamber of the nose. In birds generally the upper nasal cavity becomes subsequently divided by a median partition into two chambers, which communicate with the back of the mouth by separate apertures, the posterior nares. The original openings of the nasal pits remain as the nostrils.

The spinal cord

On this day important changes take place in the spinal cord; and a brief history of the development of this organ may fitly be introduced here.

At the beginning of the third day the cavity of the neural canal is still of considerable width, and when examined in vertical section its sides may be seen to be nearly parallel, though perhaps approximating to each other more below than above.

The exact shape varies according to the region of the body from which the section is taken.

The epiblast walls are at this time composed of radiately arranged columnar cells. The cells are much elongated, but somewhat irregular; and it is very difficult in sections to make out their individual boundaries. They contain granular oval nuclei in which a nucleolus can almost always be seen. The walls of the canal are both anteriorly and posteriorly considerably thinner in the median plane than in the middle.

Towards the end of the third day changes take place in the shape of the cavity. In the lumbar region its vertical section becomes more elongated, and at the same time very narrow in the middle while expanded at each end into a somewhat bulbous enlargement, producing an hour-glass appearance (Fig. 65). Its walls however still preserve the same histological characters as before.

On the fourth day (Fig. 68) coincidently with the appearance of the spinal nerves, important changes may be observed in the hitherto undifFerentiated epiblastic walls, which result in its differentiation into (1) the epithelium of the central canal, (2) the grey matter of the cord, and (3) the external coating of white matter.

The white matter is apparently the result of a differentiation of the outermost parts of the superficial cells of the cord into longitudinal nerve-fibres, which remain for a long period without a medullary sheath. These fibres appear in transverse sections as small dots. The white matter forms a transparent investment of the grey matter; it arises as four patches, viz. an anterior and a posterior white column on each side, which lie on a level with the origin of the anterior and posterior nerve-roots. It is always, at first, a layer of extreme tenuity, but rapidly increases in thickness in the subsequent stages, and extends so as gradually to cover the whole cord (Fig. 83).

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FIG. 83. SECTION THROUGH THE SPINAL CORD OF A SEVEN DAYS' CHICK.

pew. dorsal white column, lew. lateral white column, acw. ventral white column, c. dorsal tissue filling up the part where the dorsal fissure will be formed, pc. dorsal grey cornu. ac. anterior grey cornu. ep. epithelial cells, age. anterior commissure, pf. dorsal part of spinal canal, spc. ventral part of spinal canal, af. anterior fissure.

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The grey matter and the central epithelium are formed by a differentiation of the main mass of the walls of the medullary canal. The outer cells lose their epithelial-like arrangement, and, becoming prolonged into fibres, give rise to the grey matter, while the innermost cells retain their primitive arrangement, and constitute the epithelium of the canal. The process of formation of the grey matter would appear to proceed from without inwards, so that some of the cells which have, on the formation of the grey matter, an epitheliallike arrangement, subsequently become converted into true nerve-cells.

The central epithelium of the nervous system probably corresponds with the so-called epidermic layer of the epiblast.

The grey matter soon becomes prolonged dorsally and ventrally into the posterior and anterior horns. Its fibres may especially be traced in two directions : (1) round the anterior end of the spinal canal, immediately outside its epithelium and so to the grey matter on the opposite side, forming in this way an anterior grey commissure, through which a decussation of the fibres from the opposite sides is effected : (2) dorsalwards along the outside of the lateral walls of the canal.

There is at this period (fourth day) no trace of the ventral or dorsal fissure, and the shape of the central canal is not very different from what it was at an earlier period. This condition of the spinal cord is especially instructive as it is very nearly that which is permanent in Amphioxus.

The next event of importance is the formation of the ventral or anterior fissure. This begins on the fifth day and owes its origin to a downgrowth of the anterior horns of the cord on each side of the middle line. The two downgrowths enclose between them a somewhat linear space the anterior fissure which increases in depth in the succeeding stages (Fig. 83, a/).

The dorsal or posterior fissure is formed at a later period (about the seventh day) than the anterior, and accompanies the atrophy of the dorsal section of the embryonically large canal of the spinal cord. The exact mode of its formation appears to be still involved in some obscurity.

It seems probable, though further investigations on the point are still required, that the dorsal fissure is a direct result of the atrophy of the dorsal part of the central canal of the spinal cord. The walls of this coalesce dorsally, and the coalescence gradually extends inwards, so as finally to reduce the central canal to a minute tube, formed of the ventral part of the original canal. The epithelial wall formed by the coalesced walls on the dorsal side of the canal is gradually absorbed.

The epithelium of the central canal, at the period when its atrophy commences, is not covered dorsally either by grey or white matter, so that, with the gradual reduction of the dorsal part of the canal and the absorption of the epithelial wall formed by the fusion of its two sides, a fissure between the two halves of the spinal cord becomes formed. This fissure is the posterior or dorsal fissure. In the process of its formation the white matter of the dorsal horns becomes prolonged so as to line its walls ; and shortly after its formation the dorsal grey commissure makes its appearance ; this is not improbably derived from part of the epithelium of the original central canal.

Meanwhile an alteration is taking place in the external outline of the cord. From being, as on the fourth and fifth days, oval in section, it becomes, chiefly through the increase of the white matter, much more nearly circular.

By the end of the seventh day the following important parts of the cord have been definitely established :

(1) The anterior and posterior fissures.

(2) The anterior and posterior horns of grey matter.

(3) The anterior, posterior and lateral columns of white matter.

(4) The spinal canal.

As yet, however, the grey masses of the two sides of the cord only communicate by the anterior grey commissure, and the white columns of opposite sides do not communicate at all. The grey matter, moreover, still far preponderates over the white matter in quantity.

By the ninth day the posterior fissure is fully formed, and the posterior grey commissure has also appeared.

In the centre of the sacral enlargement this commissure is absent, and the posterior columns at a later period separate widely and form the ' sinus rhomboidalis/ which is not, as has been sometimes stated, the remains of the primitive 'sinus rhomboidalis ' visible during the second day.

The anterior white columns have much increased on this day, and now form the sides of the already deep anterior fissure. The anterior white commissure does not however appear till somewhat later.

The heart

The fifth day may perhaps be taken as marking a most important epoch in the history of the heart. The changes which take place on that and on the sixth day, added to those previously undergone, transform the simple tube of the early days of incubation into an almost completely formed heart.

The venous end of the heart, though still lying somewhat to the left and dorsal, is now placed as far forwards as the arterial end, the whole organ appearing to be drawn together. The ventricular septum is complete.

The apex of the ventricles becomes more and more pointed. In the auricular portion a small longitudinal fold appears as the rudiment of the auricular septum, while in the canalis auricula ris, which is now at its greatest length, there is also to be seen a commencing transverse partition tending to separate the cavity of the auricles from those of the ventricles.

About the 106th hour, a septum begins to make its appearance in the bulbus arteriosus in the form of a longitudinal fold, which according to Tonge (Proc. of Royal Soc. 1868) starts, not (as Von Baer thought) at the end of the bulbus nearest to, but at that farthest removed from, the heart. It takes origin from the wall of the bulbus between the fifth and fourth pairs of arches and grows backwards in such a manner as to divide the bulbus into two channels, one of which leads from the heart to the fourth and third pair of arches and the other to the fifth pair. The free edge of the septum is somewhat Y-shaped, so that its two legs as it were project backwards towards the heart, further than its central portion ; and this shape of the free edge is maintained during the whole period of its growth. Its course backwards is not straight but spiral, and thus the two channels into which it divides the bulbus arteriosus wind spirally the one over the other. The existence of the septum can only be ascertained at this stage by dissection or by sections, there being as yet no external signs of the division.

At the time when the septum is first formed, the opening of the bulbus arteriosus into the ventricles is narrow or slit-like, apparently in order to prevent the flow of the blood back into the heart. Soon after the appearance of the septum, however, semilunar valves (Tonge, loc. cit.) are developed from the wall of that portion of the bulbus which lies between the free edge of the septum and the cavity of the ventricles.

These arise as six solid outgrowths of the wall arranged in pairs, a ventral, a dorsal, and an outer pair, one valve of each pair belonging to the one and the other to the other of the two main divisions of the bulbus which are now being established.

The ventral and the dorsal pairs of valves are the first to appear: the former as two small prominences separated from each other by a narrow groove, the latter as a single shallow ridge, in the centre of which is a prominence indicating the point where the ridge will subsequently become divided into two. The outer pair of valves appear opposite each other, at a considerably later period, between the ends of the other pair of valves on each side.

As the septum grows backwards towards the heart, it finally reaches the position of these valves. One of its legs then passes between the two ventral valves, and the other unites with the prominence on the dorsal valve-ridge. At the same time the growth of all the parts causes the valves to appear to approach the heart and thus to be placed quite at the top of the ventricular cavities. The free edge of the septum of the bulbus now fuses with the ventricular septum, and thus the division of the bulbus into two separate channels, each provided with three valves, and each communicating with a separate side of the heart, is complete, the position of the valves not being very different from what it is in the adult heart.

That division of the bulbus which opens into the fifth pair of arches is the one which communicates with the right ventricle, while that which opens into the third and fourth pairs communicates with the left ventricle (vide Fig. 93). The former becomes the pulmonary artery, the latter the commencement of the systemic aorta.

The external constriction actually dividing the bulbus into two vessels does not begin to appear till the septum has extended some way back towards the heart.

The semilunar valves become pocketed at a period considerably later than their first formation (from the 147th to the 165th hour) in the order of their appearance.

Towards the end of the fifth and in the course of the sixth day further important changes take place in the heart.

The venous end with its two very conspicuous auricular appendages, comes to be situated more dorsal to the arterial end, though it still turns rather towards the left. The venous portion of the heart undergoes on the sixth day, or even near to the end of the fifth, such a development of the muscular fibres of its walls that the canalis auricularis becomes almost entirely concealed. The point of the heart is now directed nearly backwards (i. e. towards the tail), but also a little ventralwards.

An alteration takes place during the sixth day in the relative position of the parts of the ventricular division of the heart. The right ventricle is now turned towards the abdominal surface, and also winds to a certain extent round the left ventricle. It will be remembered that on the fourth day the right ventricle was placed dorsal to the left.

The right ventricle is now also the smaller of the two, and the constriction which divides it from the left ventricle does not extend to the apex of the heart (Fig. 84). It has, however, a very marked bulge towards the right.

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

FIG. 84. TWO VIEWS OF THE HEART OF A CHICK UPON THE FIFTH DAY OF INCUBATION. A. from the ventral, B. from the dorsal side.

l.a. left auricular appendage, r.a. right auricular appendage. r.v. right ventricle, l.v. left ventricle, b. bulbus arteriosus. ++++++++++++++++++++++++++++++++++

At first the bulbus arteriosus appeared to come off chiefly from the left ventricle ; during the fifth day, and still more on the sixth, it appears to come from the right chamber. This is caused by the canal from the right ventricle into the bulbus arteriosus passing towards the left, and on the ventral side, so as entirely to conceal the origin of the canal from the left chamber of the heart. On the seventh day the bulbus arteriosus appears to come less markedly from the right side of the heart.

All these changes, however, of position of the bulbus arteriosus only affect it externally; during the whole time the two chambers of the heart open respectively into the two divisions of the bulbus arteriosus. The swelling of the bulbus is much less marked on the seventh day than it was before.

At the end of the sixth day, and even on the fifth day (Figs. 84, 85), the appearance of the heart itself, without reference to the vessels which come from it, is not very dissimilar from that which it presents when adult.

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FIG. 85. HEART OF A CHICK UPON THE SIXTH DAY OF INCUBATION, FROM THE VENTRAL SURFACE.

La. left auricular appendage, r.a. right auricular appendage. r.v. right ventricle, l.v. left ventricle, b. bulbus arteriosus.

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

The original curvature to the right now forms the apex of the ventricles, and the two auricular appendages are placed at the anterior extremity of the heart.

The most noticeable difference (in the ventral view) is the still externally undivided condition of the bulbus arteriosus.

The subsequent changes which the heart undergoes are concerned more with its internal structure than with its external shape. Indeed, during the next three days, viz. the eighth, ninth, and tenth, the external form of the heart remains nearly unaltered.

In the auricular portion, however, the septum which commenced on the fifth day becomes now more conspicuous. It is placed vertically, and arises from the ventral wall; commencing at the canalis auricularis and proceeding backwards, it does not as yet reach the opening into the sinus venosus.

The blood from the sinus, or, as we may call it, the inferior vena cava, enters the heart obliquely from the right, so that it has a tendency to flow towards the left auricle of the heart, which is at this time the larger of the two.

The valves between the ventricles and auricles are now well developed, and it is about this time that the division of the bulbus arteriosus into the aorta and pulmonary artery becomes visible on the exterior.

By the eleventh or thirteenth day the right auricle has become as large as the left, and the auricular septum much more complete, though there is still a small opening, the foramen ovale, by which the two cavities communicate with each other. Through this foramen the greater part of the blood of the vena cava inferior, which is now joined just at its entrance into the heart by the right vena cava superior, is directed into the left auricle. The left vena cava superior enters the right auricle independently; between it and the inferior vena cava is a small valve which directs its blood entirely into the right auricle.

On the sixteenth day the right vena cava superior, when viewed from the exterior, still appears to join the inferior vena cava before entering the heart ; from the interior however the two can now be seen to be separated by a valve. This valve, called the 'Eustachian valve/ extends to the opening of the left vena cava superior, and into it the valve which in the earlier stage separated the left superior and inferior vense cavse has apparently become merged. There is also on the left side of the opening of the inferior cava a membrane stretching over the foramen ovale, and serving as a valve for that orifice. The blood from the inferior cava still passes chiefly into the left auricle through the foramen ovale, while the blood from the other two vense cavse now falls into the right auricle, being prevented from entering the left chamber by the Eustachian valve.

Hence, since at this period also the blood from the left ventricle passes to a great extent to the anterior portion of the body, there is a species of double-circulation going on. The greater part of the blood from the allantois entering the left auricle from the inferior vena cava passes into the left ventricle and is thence sent chiefly to the head and anterior extremities through the third and fourth arches ; from these it is brought back through the right auricle to the right ventricle, from whence through the fifth arch it is returned along the aorta to the allantois.

From the seventeenth to the nineteenth day the right auricle becomes larger than the left. The large Eustachian valve still prevents the blood from the superior cavae from entering the left auricle, while it conducts the blood from the inferior vena cava into that chamber through the foramen ovale. The entrance of the inferior vena cava is however further removed than it was from the foramen ovale, and the increased flow of blood from the lungs prevents all the blood of the inferior cava from entering into the left auricle. At the same time the valve of the foramen ovale prevents the blood in the left auricle from entering the right auricle.

During the period from the seventh day onwards the apex of the heart becomes more marked, the arterial roots are more entirely separated and the various septa completed, so that when the foramen ovale is closed and the blood of the inferior vena cava thereby entirely confined to the right auricle, the heart has practically acquired its adult condition.

The pericardial and pleural cavities

The heart at first lies in the general body cavity attached to the ventral wall of the gut by a mesocardium (Fig. 86, A), but the part of the body cavity containing it afterwards becomes separated off as a distinct cavity known as the pericardial cavity. It is formed in the following way. When the two ductus Cuvieri leading transversely from the sinus venosus to the cardinal veins become developed (p. 170), a horizontal septum is formed to support them, stretching across from the splanchnic to the somatic side of the body cavity, dividing the body cavity for a short distance in this region into a dorsal section, {formed of a right and a left division) constituting the true body cavity (Fig. 86 B, p.p), and a ventral section (Fig. 86, B, p.c.), the pericardial cavity. The two parts of the body cavity thus formed are at first in free communication both in front of and behind this septum. The septum however is soon continued forwards so as completely to separate the ventral pericardial and the dorsal body cavity in front, the pericardial cavity extending considerably further forwards than the body cavity.

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

FIG. 86. TRANSVERSE SECTIONS THROUGH A CHICK EMBRYO WITH TWENTY-ONE MESOBLASTIC SOMITES TO SHEW THE FORMATION OP THE PERICARDIAL CAVITY, A. BEING THE ANTERIOR SECTION.

pp. body cavity, pc. pericardial cavity, al. alimentary cavity. au. auricle, v. ventricle, sv. sinus venosus. dc. ductus Cuvieri. ao. aorta, mp. muscle-plate, me. medullary cord

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

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

FIG. 87. SECTION THROUGH THE CARDIAC EEGION OF AN EMBRYO OF LACERTA MURALIS OF 9 M.M. TO SHEW THE MODE OF FORMATION OF THE PERICARDIAL CAVITY.

Atf. heart, pc. pericardial cavity, al. alimentary tract. Ig. lung. 1. liver, pp. body cavity, md. open end of Mullerian duct. wd. Wolffian duct. vc. vena cava inferior, ao. aorta, ch. notochord. me. medullary cord.

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

Since the horizontal septum, by its mode of origin, is necessarily attached to the ventral side of the gut, the dorsal part of the primitive body space is, as we have already mentioned, divided into two halves by a median vertical septum formed of the gut and its mesentery (Fig. 86, B). Posteriorly the horizontal septum grows in a slightly ventral direction along the under surface of the liver (Fig. 87), till it meets the abdominal wall of the body at the insertion of the falciform ligament, and thus completely shuts off the pericardial cavity from the body cavity. The horizontal septum forms, as is obvious from the above description the dorsal wall of the pericardial cavity.

After the completion of this separation the right and left sections of the body cavity, dorsal to the pericardial cavity, rapidly become larger and receive the lungs which soon sprout out from the throat.

The diverticula which form the lungs grow out into splanchnic mesoblast, in front of the body cavity, but as they grow they extend into the two anterior compartments of the body cavity, each attached by its mesentery to the mesentery of the gut (Fig. 87, lg.). They soon moreover extend beyond the posterior limit of the pericardium into the undivided body cavity behind.

To understand the further changes in the pericardial cavity it is necessary to bear in mind its relations to the adjoining parts. It lies at this period completely ventral to the two anterior prolongations of the body cavity containing the lungs. Its dorsal wall is attached to the gut, and is continuous with the mesentery of the gut passing to the dorsal abdominal wall, forming the posterior mediastinum of human anatomy.

The changes which next ensue consist essentially in the enlargement of the sections of the body cavity dorsal to the pericardial cavity. This enlargement takes place partly by the elongation of the posterior mediastinum, but still more by the two divisions of the body cavity which contain the lungs extending themselves ventrally round the outside of the pericardial cavity. This process is illustrated by Fig. 88, taken from an embryo rabbit. The two dorsal sections of the body cavity (pl.p.) finally extend so as completely to envelope the pericardial cavity (pc.), remaining however separated from each other below by a lamina extending from the ventral wall of the pericardial cavity to the body wall, which forms the anterior mediastinum of human anatomy.

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

Fig. 88. SECTION THROUGH AN ADVANCED EMBRYO OF A RABBIT TO SHEW HOW THE PERICARDIAL CAVITY BECOMES SURROUNDED BY THE PLEURAL CAVITIES.

ht. heart, pc. pericardial cavity, pl.p. pleural cavity. Ig. lung. al. alimentary tract, ao. dorsal aorta, ch. notochord. rp. rib. st. sternum, sp.c. spinal cord. ++++++++++++++++++++++

By these changes the pericardial cavity is converted into a closed bag, completely surrounded at its sides by the two lateral halves of the body cavity, which were primitively placed dorsally to it. These two sections of the body cavity, which in the chick remain in free communication with the undivided peritoneal cavity behind, may, from the fact of their containing the lungs, be called the pleural cavities.

Histological differentiation

The fifth day may also be taken as marking the epoch at which histological differentiation first becomes distinctly established and begins to make great progress.

It is of course true that loi)g before this date, even from the earliest hours, the cells in each of the three fundamental layers have ceased to be everywhere alike. Nevertheless the changes undergone by the several cells have been few and slight. The cells of epiblastic origin, both those going to form the epidermis and those included in the neural involution, are up to this time simple more or less columnar cells ; they may be seen here elongated, there oval, and in another spot spheroidal ; here closely packed, with scanty protoplasm, there scattered, with each nucleus well surrounded by cellsubstance ; but wherever they are found they may still be recognized as cells of a distinctly epithelial character. So also with the cells of hypoblastic origin, whether simply lining the alimentary canal or taking part in the formation of the compound glands. Even in the mesoblast, which undergoes far more changes than either of the other layers, not only increasing more rapidly in bulk but also serving as the mother tissue for a far greater number of organs, the alterations in the individual cells 1 are, till near upon the fifth day, insignificant. Up to this time the mesoblast may be spoken of as consisting for the most part of little more than indifferent tissue : of nuclei imbedded in a protoplasmic cell-substance. In one spot the nuclei are closely packed together, and the cell-substance scanty and compact; at another the nuclei are scattered about with spindle-shaped masses of protoplasm attached to each, and there is a large development either of intercellular spaces or of intracellular vacuoles filled with clear fluid. The protoplasm differs in various places, chiefly in being more or less granular, and less or more transparent, having as yet undergone but slight chemical transformation. Up to this epoch (with the exception of the early differentiated blood and muscles of the muscle plates) there are no distinct tissues, and the rudiments of the various organs are simply marked out by greater or less condensation of the simple mesoblastic substance.

From the fifth day onwards, however, histological differentiation takes place rapidly, and it soon becomes possible to speak of this or that part as being composed of muscular, or cartilaginous, or connective, &c. tissue. It is not within the scope of the present work to treat in detail of these histogenetic changes, for information concerning which we would refer the reader to histological treatises. We have already had occasion to refer

1 With the exception of the cells of the middle part of the inner layer of the muscle-plates, which we have seen become converted into longitudinal muscles on the third day (p. 187).

incidentally to many of the earliest histological events, and shall content ourselves by giving a brief summary of the derivation of the tissues of the adult animal from the three primary layers of the blastoderm.

The epiblast or upper layer of many embryologists forms primarily two very important parts of the body, viz. the central nervous system and the epidermis.

It is from the involuted epiblast of the neural tube that the whole of the grey and white matter of the brain and spinal cord appears to be developed, the simple columnar cells of the epiblast being apparently directly transformed into the characteristic multipolar nerve-cells. The whole of the sympathetic¹ nervous system and the peripheral nervous elements of the body, including both the spinal and cranial nerves and ganglia, are epiblastic in origin.

The epithelium (ciliated in the young animal) lining the canalis centralis of the spinal cord, together with that lining the ventricles of the brain, all which cavities and canals are, as we have seen, derivatives of the primary neural canal, is the undifferentiated remnant of the primitive epiblast.

The epiblast, as we have said, also forms the epidermis, not however the dermis, which is of mesoblastic origin. The line of junction between the epiblast and the mesoblast coincides with that between the epidermis and the dermis. From the epiblast are formed all such tegumentary organs or parts of organs as are epidermic in nature.

(¹The details of the development of the sympathetic system have only been imperfectly worked out in the chick. We propose deferring our account of what is known on this head to the second part of this work dealing with the Mammalia. We may here state, however, that the whole of the chain of the sympathetic ganglia is developed in continuity with the outgrowths from the wall of the neural tube which give rise to the spinal nerves.)

In addition to these, the epiblast plays an important part in the formation of the organs of special sense.

According to their mode of formation these organs may be arranged into two divisions. In the first come the cases where the sensory expansion of the organ of special sense is derived from the involuted epiblast of the medullary canal. To this class belongs the retina, including the epithelial pigment of the choroid, which is formed from the original optic vesicle budded out from the fore-brain.

To the second class belong the epithelial expansions of the membranous labyrinth of the ear and the cavity of the nose, which are formed by involution from the superficial epiblast covering the external surface of the embryo. These accordingly have no primary connection with the brain. We may also fairly suppose that the 'taste bulbs' and the nervous cells, which have lately been described as present in the epidermis, are also structures formed from the epiblast.

In addition to these we have the crystalline lens formed of involuted epiblast, and the cavity of the mouth and anus lined by it. The pituitary body is also epiblastic in origin. These are the most important parts which are derived from the epiblast.

From the hypoblast are derived the epithelium of the digestive canal, the epithelium of the trachea, bronchial tubes and air cells, the cylindrical epithelium of the ducts of the liver, pancreas and other glands of the alimentary canal, as well as the hepatic cells constituting the parenchyma of the liver, developed, as we have seen, from the hypoblast cylinders given off around the primary hepatic diverticula.

Homologous, probably with the hepatic cells, and equally of hypoblastic origin, are the more spheroidal c secreting cells ' of the pancreas and other glands. The epithelium of the salivary glands, though these so exactly resemble the pancreas, is of epiblastic origin, inasmuch as the cavity of the mouth (p. 119) is entirely lined by epiblast.

The hypoblast lines the allantois, and the notochord also is an hypoblastic product.

From the mesoblast are formed all the remaining parts of the body. The muscles, the bones, the connective tissue and the vessels, both arteries, veins, capillaries and lymphatics, with their appropriate epithelium, are entirely formed from the mesoblast.

The generative and urinary organs are also derived from the mesoblast. It is worthy of notice that their epithelium, though resembling the hypoblastic epithelium of the alimentary canal, is undoubtedly mesoblastic.

From the mesoblast lastly are derived all the muscular, connective and vascular elements, as well of the alimentary canal and its appendages as of the skin and the tegumentary organs. Just as it is only the epidermic moiety of the latter which is derived from the epiblast, so it is only the epithelium of the former which comes from the hypoblast.

Summary Day 5

The important events then which characterize the fifth day are:

1. The growth of the allantois.

2. The appearance of the knee and elbow, and of the cartilages which precede the bones of the digits and limbs.

3. The formation of the primitive cartilaginous cranium, more especially of the investing mass and the trabeculse, and the appearance of rods of cartilage in the visceral arches.

4. The developments of the parts of the face : the closing in of the nasal passages by the nasal processes.

5. A large development of grey matter in the spinal cord as the anterior and posterior cornua; considerable growth both of the anterior and posterior white columns, and the commencement of the anterior and posterior fissures.

6. The appearance of the auricular septum, of a septum in the bulbus arteriosus, and of the semilunar valves.

7. The establishment of the several tissues.

The Elements of Embryology - Volume 1 (1883)

The History of the Chick: Egg structure and incubation beginning | Summary whole incubation | First day | Second day - first half | Second day - second half | Third day | Fourth day | Fifth day | Sixth day to incubation end | Appendix

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