Book - The development of the chick (1919) 3

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Lillie FR. The development of the chick. (1919) Henry Holt And Company New York, New York.

Chapter III Outline of Development, Orientation, Chronology

The preceding chapters have traced the development up to the time of laying. The formation of the germ-layers has begun; and the stage of development is fairly definite, though not absolutely constant. When the egg cools, after laying, the development ceases, but is renewed when the temperature is raised to the required degree by incubation.

On the surface of the volk is a whitish disc about 4 mm. in diameter, known as the blastoderm. Edwards gives the average diameter of the unincubated blastoderm (59 eggs) as 4.41 mm., of the area pellucida (50 eggs) as 2.51 mm. The central part of the blastoderm is more transparent and is hence known as the area pellucida; beneath it is the subgerminal cavity. The less transparent periphery is known as the area opaca. In the course of development the embryo, and the embryonic membranes which serve for the protection, respiration, and nutrition of the embryo, arise from the blastoderm.

The embryo proper arises within the area pellucida, which becomes pear-shaped as the embryo forms; the remainder of the blastoderm beyond the embryo is extra-embryonic. From it arise the embryonic membranes known as the amnion, chorion, and yolk-sac. The allantois (Fig. 33 B) arises as an outgrowth from the hind-gut of the embryo, and spreads within the extraembryonic body-cavity; it thus becomes an extra-embryonic membrane secondarily. The growth of the embryo and of the extra-embryonic blastoderm are distinct, though interdependent, processes going on at the same time.

During the first four days of development the blastoderm spreads very rapidly (Figs. 32 and 33). 'Thus on the fourth day (Fig. 33 A) the greater portion of the yolk is already covered. Thereafter the overgrowth of the yolk proceeds much more slowly (cf. Fig. 33 B). In the opaque area there arise, as concentric zones, the area vasculosa distinguished by its blood-vessels and the area vitellina, which may be divided into inner and outer zones (Figs. 32 and 33). The development of the embryo during the same period is indicated in the same figures.

Fig. 32. — A. Hen's egg at about the twenty-sixth hour of incubation, to show the zones of the blastoderm and the orientation of the embryo with reference to the axis of the shell. (After Duval.) B. Yolk of hen's egg incubated about 50 hours to show the extent of overgrowth of the blastoderm. (After Duval.)

A. C, Air chamber, a. p., Area pellucida. a. v., Area vasculosa. a. v. e., Area vitellina externa, a. v. i., Area vitellina interna. Y., Uncovered portion of yolk.

The blastoderm early becomes divided in two layers as far as the margin of the vascular area. The outer layer, known as the somatopleure, is continuous with the body-wall, which is open ventrally in the young embryo. The inner one, known as the splanchnopleure, is continuous with the wall of the intestine which is likewise open ventrally. The space between these two membranes, the extra-embryonic body-cavity, is continuous with the body-cavity of the embryo. Ultimately, the splitting of the blastoderm is carried around the entire yolk, so that the latter is enclosed in a separate sac of the splanchnopleure, the yolk-sac, which is connected by a stalk, the yolk-stalk, to the intestine of the embryo. This stalk runs through an opening in the ventral body-wall, the umbilicus, where the amnion, which has developed from the extra-embryonic somatopleure, joins the body-wall (Fig. 33 B).

About the nineteenth day of incubation the yolk-sac is drawn into the body-cavity through the umbiUcus, which thereupon closes. The young chick usually hatches on the twenty-first day. Orientation. It is an interesting and important fact that the embryo appears in a definite relation to the line drawn through the axis of the entire egg, or to the line joining the bases of the two chalazse, which is usually the same thing. If the egg be placed as in Fig. 32 A, with the blunt end to the left, the head of the embryo will be found directed away from the observer when the blastoderm is above; the left side of the embryo is therefore towards the broad end, and the right side towards the narrow end of the egg. According to Duval this orientation is found in about 98.5% of eggs: of 166 eggs observed, in which the embryo was formed, Duval found 124 oriented exactly in this manner, 39 in which the axis of the embryo was slightly oblique, 2 in which the head was towards the broad end, and 1 in which the usual position was completely inverted. In the pigeon's egg the orientation of the embryo is equally definite, but slightly different. The axis of the embryo cuts the axis of the entire egg at an angle of about 45°, the head of the embryo being directed away from the observer to the right, when the broad end of the egg is to the observer's left as in Fig. 32 A.

Fig. 33. — A. Yolk of hen's egg incubated 84 hours. (After Duval.) B. Embryo and membranes of the hen's egg on the seventh day of incubation. (After Duval.)

AL, AUantois. Am., Amnion, a. v., (in B) Area vitellina. E., Embryo. S. t., Sinus terminalis. Other Abbreviations as in Fig. 32.

The definiteness of orientation of the embryo with reference to the axis of the egg enables one to distinguish anterior and posterior ends of the blastoderm before there is any trace of an embryo; and while there is no possibility of orientation by examination of the blastoderm itself, or when such orientation is otherwise extremely difficult. By the method of orienting the blastoderm w^ith reference to the axis of the shell, observers have been able to discover important features of the early development which would otherwise, no doubt, have escaped observation The relation is of interest in other respects discussed in their appropriate places. (See p. 15.)

Chronology (Classification of Stages)

The development of an animal is an absolutely continuous process, but for purposes of description it is necessary to fix certain stages for comparison with those that precede and those that follow. Each stage has a certain position in the continuous process, and the correct arrangement of stages is therefore a sine qua non for their correct interpretation. This may seem a very simple matter seeing that development is in general from the more simple to the more complex. And it would be so if it were not for the fact that embryonic stages, like the adult individuals of a species, vary more or less, so that no one embryo is ever exactly like another. These embryonic variations involve (1) the rate of development of the whole embryo, so that at a given time in the process no two embryos are in exactly the same stage; (2) the relative rates of development of different organs; (3) the size of the embryo, for embryos of the same stage of development may vary somewhat in size.

Although the total period of incubation is fairly constant in the hen's egg, about twenty-one days, yet there is great variation in the grade of development of embryos of the same age, especially during the first week. This is due to two main factors: first, variation in the latent period, that is the time necessar}^ to start the development of the cooled blastoderm after the egg is put into the incubator, and second, to variation in the temperature of incubation. Individual eggs may vary in rate of development when these two factors are constant, but this difference is relatively slight. Other things being equal, the latent period varies with the freshness of the egg; it is relatively short in eggs that are newly laid, and long in eggs that have remained quiescent some time after laying. It is obvious that the latent period will form a more considerable portion of the entire time of incubation in early than in late stages. Hence the difficulty of classifying embryos, particularly in the first four or five days of incubation, by period of incubation. Eggs procured from dealers usually show such great variations in degree of development, at the same time of incubation, that it is quite impossible to grade them with any high degree of accuracy by time of incubation. It is statf'd also that the rate of development varies considerably at different seasons, other factors being constant. But this has not been found to be a serious matter in my own experience.

Variations in temperature, either above or below the normal, also seriously affect the rate of development, and produce abnormalities when extreme. If the temperature be too low, the rate is slower than normal; if too high, the rate increases up to a certain point, beyond which the egg is killed.

The physiological zero, that is the temperature below which the blastoderm undergoes no development whatever, has been estimated differently by different authors. Some place it at about 28° C, others at about 25°; Edwards places it as low as 20-21° C. At the last temperature, apparently, a small percentage of eggs will develop in the course of several days to an early stage of the primitive streak, but most eggs show no perceptible development. In very warm weather, therefore, the atmospheric temperature m.ay be sufficient to start eggs. The following table is given by Davenport based on Fere's work:

Temperature .34° 35° 36° 37° 38° 39° 40° 41°

Index of Development 0.65 0.80 0.72 1.00 1.06 1.25 1.51

The index of development represents the proportion that the average development at a given temperature in a given time bears to the normal development {i.e., development at the normal temperature for the same time). There is an increase in the rate up to 41°; a maximum temperature, which cannot be much above 41°, causes the condition of heat-rigor and death.

There would seem to be no better way to determine the normal temperature for incubation than by measuring the temperature of eggs incubated by the hen throughout the entire period of incubation. This has been done very carefully by Eyclesh3mier, who finds the internal temperature of such eggs to be as follows:

Day of incubation

1

2

3

4

5

Temperature of hen

102.2

103.0

103.5

104.0

103.8

Temperature of egg

98.0

100.2

100.5

100.5

100.4

Day of incubation

6

7

8

9

10

Temperature of hen

105.0

104.6

104.5

105.0

105.0

Temperature of egg

101.0

101.8

102.5

101.6

102.0

Day of incubation

11

12

13

14

15

Temperature of hen

104.8

105.2

104.5

105.0

105.2

Temperature of egg

101.8

102.2

102.0

102.5

102.0

Day of incubation

16

17

18

19

20

Temperature of hen

105.0

104.6

104.8

104.5

104.5

Temperature of egg 103.0 102.4 103.0 103.0 103.0

The temperature of the hen is seen to be somewhat higher than that of the eggs. In an artificial incubator where 85 % of the fertile eggs hatched on the twentieth and twenty-first da3^s, the temperatures were as follows:

Day of incubation Temperature of incubator Temperature of egg

Day of incubation Temperature of incubator Temperature of egg

Day of incubation Temperature of incubator Temperature of egg

Day of incubation Temperature of incubator Temperature of egg

It would be possible then to establish a normal rate of development, by using perfectly fresh eggs incubated at a normal temperature. In practice I have found that the times given in Duval's atlas are approximately normal, and these are, therefore, adopted so far as given. But even under the best conditions the variations are sufficient to prevent close grading of stages by time of incubation in the first three days. This may be due to differences in the grade of development at the time of laying, owing to varia

1

2

3

4

5

102.0

102.0

103.0

102.0

102.5

99.5

100.0

101.0

100.5

100.5

6

7

8

9

10

103.0

102. 5

102.0

103.0

103.5

101.0

100.0

100.0

101.0

101.5

11

12

13

14

15

103.0

103.5

104.0

103.5

104.0

101.5

101.8

102.0

102.5

103.0

16

17

18

19

20

104.5

104.0

103.5

104.0

104.5

103.0

103.0

102.5

102.5

103.5

tioRS in the time of development in the oviduct and uterus, or to slow development before incubation in warm weather, or to individual variation. It becomes necessary, therefore, to find some other system. The method followed by a considerable number of investigators, namely to classify by the number of somites, has been found to be best between about the twentieth and ninetv-sixth hours of incubation. In the table which follows, therefore, this method of classification is used. For the sake of brevity throughout the book a stage reckoned by the number of somites will be w^ritten 1 s, 2 s, 3 s, etc. It is true that the relative rate of the development of organs varies slightly. Nevertheless, classification by number of somites is unquestionably the most exact method up to the end of the fourth day at least. Beyond this stage the method is difficult to apply, and after about the sixth day the number of somites becomes constant. After the fourth day the time of incubation is usually a sufficiently exact criterion for most purposes: the latent period has become a relatively inconsiderable fraction of the whole time of incubation, and the embryos that survive, assuming fresh eggs and normal temperature of incubation, are in about the same stage of development.

Classification of embryos by length is a favorite method particularly in Germany, and it offers many advantages in the case of some animals; under many conditions it is the only available method. But it offers considerable difficulties, the most serious of which come from the varying degrees of curvature of the embryo. In early stages of the chick, for instance, up to about 12 s, the total length of the embryonic axis may be measured, for the embryo is approximately straight. The cranial flexure then begins to appear, and slowly increases to a right angle; during this period there may be an actual reduction in length of the embryo (cf. table, 14-16 s). Conditions are also complicated by the fact that the head of the embryo is turning on its left side at the same time. The cervical flexure then appears and causes a second reduction of the total length (cf. table 2932 s). Later still the curvature of the trunk and particularly of the tail develops in somewhat varying degrees and makes bad matters worse. After these flexures are formed, let us say at about eighty hours in the chick, it is customary to take the so-called neck-tail measurement, that is, from the cervical flexure to the apex of the tail flexure. But even then it is questionable if this measurement is as accurate a means of classification as the age of normally incubated embryos; particularly as the cervical flexure is secondarily eliminated by raising of the head. It is probable that the measurement from the tip of the head to the apex of the cranial flexure (head-length) would be best for classification of chick-embryos by measurement. This dimension may be readily taken, after the cranial flexure begins, throughout the entire period of incubation. However, it has been relatively little used up to the present time.

The following tables give the chronology of development up to the end of the fourth day, the period usually covered in laboratory courses. For the later chronology the student is referred to Keibel and Abraham's Normaltafeln zur Entwickelungsgeschichte des Huhnes (Gallus domesticus), Jena, Gustav Fischer, 1900. In the various chapters of Part II, the later chronology of the various organs is given here and there throughout the text. It is believed that these references will be sufficient on the Avhole to enable the student to determine what embryos to select for the desired stage of most organs. The tables have been made practically continuous from 1 s up to 41s, because these cover the period of development in which the primordia of most organs are formed. They have been constructed mostly from entire mounts. The corresponding tables in Keibel and Abraham's work are noted by number in the right-hand column.

Chronological Tables of the Development of the Chick

I. Before Laying:

1. Maturation and fertilization; found in the oviduct above the isthmus.

2. Early cleavage up to about the thirty-two celled stage found in the isthmus of the oviduct during the formation of the shellmembrane (Patterson) .

3. Later cleavage, formation of periblast and entoderm, etc., found in the uterus up to time of laying. Data for the pigeon given in Chapter II; see legends to figures.

II. Incubation to Formation of the First Somite:

The period may be divided in three parts: (1) before the appearance of the primitive streak; (2) primitive streak formed but no head process; (3) after the appearance of the head-process. These stages may be subdivided by time or by length of the primitive streak.

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Lenmb ol loregul 1 1 mm. Two iiKenJ (utnin. .liU closed

-ihape of bean my Indlcuied

lead fold conn anlerior

put of hind brain

Mandibular archa begto loprojta

"

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deep pit

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Tail. fold barely indlcaled

iS-.i»

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iraoj

oral plate

upper angle

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n tail bud

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lead-fold coven to >otl> lomite

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Fouitb aortic arch termed

Cloaed

UUal eolargemeol nol l^iun

Tip of tall begin. 1" U dlrecicd fun>a/da

»s

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marked

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M mm. n«kmll, ,,,

Ahoal «6

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ESTtS

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01

mm, Ior»nM bnin

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fully tBimEil

equal lo

■llDUl i

breidih

68 T

to the apex ol if this measui the age of no] vical flexure ; It is probable the apex of tl classification ( sion may be throughout th been relativel} The follow to the end of ratory courses, to Keibel anc schichte des I 1900. In the of the various It is believed t to enable the the desired sU practically cor the period of d are formed. ' mounts. The work are no tec

Chronoloc

I. Before Layii

1. Maturatic

isthmu;

2. Early cle

the istb membn

3. Later cle^

in the Data for

II. Incubation 1 The period n of the primitive (3) after the app divided by time

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Cite this page: Hill, M.A. (2024, April 24) Embryology Book - The development of the chick (1919) 3. Retrieved from https://embryology.med.unsw.edu.au/embryology/index.php/Book_-_The_development_of_the_chick_(1919)_3

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© Dr Mark Hill 2024, UNSW Embryology ISBN: 978 0 7334 2609 4 - UNSW CRICOS Provider Code No. 00098G