Paper - Prenatal growth of the pig

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Lowrey LG. Prenatal growth of the pig (1911) Amer. J Anat. 12(2): 107-138.

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Historic 1911 paper, for more information see Pig Development.



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Prenatal Growth Of The Pig

Lawson G. Lowrey

Professor Of Anatomy In The University Of Utah

From The Anatomical Laboratory Of The University Of Missouri

(1911)

Five Figures

Introduction

Numerous observations on various phases of growth are to be found in the biological literature. Most of those concerning prenatal growth are upon the human embryo, although scattered observations are also recorded on other mammals, and a few on the lower vertebrates. The work presented in this paper was done in the attempt to trace, in the pig, the course of the prenatal growth of the body and especially the relative growth of the various organs. The results are also compared with the course of growth in the human species and in the lower vertebrates, so far as data are available, The work was done in the Anatomical Laboratory of the University of Missouri, under the direction of Dr. C. M. Jackson, to whom I am deeply indebted for his interest, aid and valuable suggestions.

Material and Methods

The material used for this paper consists of 22 litters of pig embryos, comprising about 130 individuals, of which number 105 were used. In most cases, all the pigs of the litter were used, in the others, three or four specimens about the average of the litter were studied. These litters of embryos were secured from the packing houses in Chicago (August 31, 1909), Kansas City (December 27, 1909), and Columbia (at various times, spring 1910). Wherever possible, the litters were worked up in a fresh condition. In the other cases, they were preserved in a 5 per cent aqueous solution of formalin for varying lengths of time (table 1).


For the ovum, some fifty adult ovaries were examined fresh, by opening the large follicles under a dissecting microscope. The ovum (including zona pellucida) was measured with an eye-piece micrometer, whose divisions had a known value. The largest ovum found is considered nearly, if not quite, the size of the mature ovum. No data were found in the literature as to the size of the mature ovum of the pig.


Of the fetal material secured, the litter at 15 mm. was the smallest which could be conveniently dissected and weighed. The largest litter examined averages 262 mm. in crown-rump length, and is not quite full term. However, the changes between this stage and birth are probably slight, except in the matter of abso- lute weight. That is, the relative size of the various organs would probably not change much, since the changes are relatively slight during the latter part of the fetal period.


For the data on the adult, a trip was made to the local (Columbia) packing house, and four hogs, probably about ten to twelve months old, were examined and weighed. The individual measurements so secured were averaged, and the averages used in constructing the various curves.


The method used for the fetal and adult material was that of weighing. The crown-rump length was also taken in all cases, as it forms another basis of comparison for the individuals and the different stages.


Each litter is considered as a unit. That is, the individuals in each litter, or the three or four average pigs which were used therefrom, were weighed individually and individual calculations made for the percentage which each organ forms of the whole. Where the intestinal contents were determined (186 mm. and above), their weight was subtracted from the gross body weight, giving the net body weight, which was used in calculating the percentages. The average of the percentages for each organ is then taken for the litter, and this average is used in the table of observations, the minimum and maximum percentages observed in the litter being also indicated in parenthesis. In constructing the various curves of relative growth, it was found convenient to group together certain of the litters closely related in size, the average of the litter averages being taken.


The following measurements were made on each pig: weight and crown-rump length were observed for the whole body ; the head, brain, eyeballs, spinal cord, thyroid gland, thymus gland, right lung, left lung, heart, liver, stomach and intestines (with mesentery and contents, also without contents where possible), spleen, pancreas, suprarenal glands, gonads, kidneys and Wolffian bodies, were each weighed separately.


The weights were taken carefully, the organs being placed in a closed glass vessel of known weight. For the larger fetuses, the organs were weighed to 0.001 g. (1 mg.), the body and head being weighed to 0.1 g. For the smaller embryos (18 mm., 25 mm., 37 mm., 41 mm.), the body and head were weighed to 0.001 g., and the organs (except those weighing more than 10 mg. in the 37 mm. and 41 mm. embryos) were weighed to . 0002 g. (0 . 2 mg.) . For the 15 mm. embryos, the body and organs were weighed to 0.0001 g. (0.1 mg.).

The head was divided from the neck on a plane passing just behind the angle of the mandible and the cranium. Variations in this plane, which to a certain extent are unavoidable, lead to variations in the observed weight, and therefore in the relative size of the head.


The organs were weighed with contained blood, except the heart, which was opened and cleaned of the blood in the cavities. The brain and spinal cord were weighed with the pia mater but without the dura mater.


Since the age of the specimens is unknown, it is impossible to construct accurate curves of growth either for the body as a whole or for the various organs. However, by arranging the figures representing the relative size (per cent of the net body weight) according to the crown-rump length, curves can be drawn which give an approximate idea of the changes in the relative growth of the various organs during prenatal life. But no definite conclusions can be drawn from these curves as to the rapidity with which these changes in relative size take place. The only exception to this is in the case of the body as a whole, where some data by Keibel on the age of young pig embryos make it possible to compare the growth in the early part with that in the remainder of the prenatal and with the postnatal period.


A possible source of error lies in the fact that some of the litters were preserved in formalin, while others were studied fresh. It is well known that specimens preserved in formalin show an increase in total weight, amounting sometimes to 10 per cent or 15 per cent of the total. It is, however, improbable that this increase will materially affect the relative size of the organs.

Observations

The observations have been condensed into a single table, from which curves expressing the relative growth have been made. A brief discussion of, and explanation for, the table and curves, follows.


Table 1 gives a summary of all the observations on the different litters used. In the first column will be found the serial number of the litter used. In the second column, the manner of preservation. . 'Form' indicates a 5 per cent aqueous solution of formalin. The length of time preserved is also indicated. In the third column is given the number of each sex of the individuals used (M-male; F-female).


In the fourth column is given the average crown-rump length, in millimeters, for the litter, the minimum and maximum lengths being given in parenthesis. Similarly in the fifth column is given the average gross body weight for the litter in grams, together with the minimum and maximum. The net body weight is also given for the later stages, in which the intestinal contents could be measured and subtracted.


The sixth column shows the average percentage by weight which the head forms of the entire (net) body weight in each of the various litters. The minimum and maximum percentage found in the litter is given in parenthesis. Similarly in the succeeding columns is given the average percentage of the entire (net) body weight (also minimum and maximum percentage) in each litter for the brain, eyeballs, spinal cord, thyroid gland, thymus, lungs, heart, liver, stomach and intestines with contents, stomach and intestines empty, spleen, pancreas, suprarenal glands, gonads, kidneys and (for the earlier stages) the Wolffian


bodies. In the last column is given the average percentage for all the viscera added together, including the brain and spinal cord. Figures for all the viscera were secured by adding up the average percentages for the litter for each of the organs. The ovaries and testes were averaged together, and the resulting figures added. The weight of the stomach and intestinal contents is excluded.

Table 1

The complete individual data on which the averages in this table are based will be deposited in The Wistar Institute of Anatomy, Philadelphia, where they will be accessible to any who may desire to use them.


The following notes apply to the preceding table. The sex was undetermined in litters 13, 14, and 24. The intestinal and stomach contents were too small for measurement in the earlier stages. In a few of the individuals in litters 5, 16, 23, 4, 3, 1, 47, 11, 10, 19, 9 and 20, and in three of the adults, the weight of the contents is estimated.


In a few other instances, the number of observations on individual organs is less than the number used of the litter. Thus, in litter 14, only two observations were made on the lungs, two on spinal cord, and three on stomach and intestines. In litter 8, only three observations were made on the thymus, and in litter 5, there were seven observations on thymus, and six on eyeballs. Litter 23 contains eight observations on eyeballs; litter 4, only one spinal cord. In the adult, only one observation each was made on the brain, eyeballs and thyroid gland; two on pancreas and suprarenals, and three on kidneys. The crown-rump length was measured on only one (the largest) adult.


From the averages of the various litters, curves of relative growth for the different organs were constructed. The percentage which the various organs form of the entire (net) body weight is used for the ordinate, and the average crown-rump length for the abscissa. For convenience in drawing these curves, certam of the litters of nearly the same size were grouped together and their averages taken. Thus, litters nos. 17 and 21 were grouped together at an average of 39 mm. ; nos. 2 and 5 at 84 mm. ; nos. 16, 23 and 4 at 107 mm.; nos. 3, 18 and 1 at 130 mm.; and nos. 47, 11 and 48 at 157 mm. The resulting curves are not essen- tially different from those in which all the litters were used individually, and the simplification due to the elimination of occasional irregularities is an advantage, particularly when several curves are combined in the same figure.

The dotted portion of the line at the right indicates the trend of postnatal relative growth, from birth to maturity. It must, of course, be borne in mind that these curves give an idea only of the general changes in the relative size of the various organs, and no information concerning the age, or the rapidity with which the changes occur.


Fig. 1 contains four curves — one each for the head, liver, and brain, and one for all the viscera taken together.

Fig. 2 contains five curves — one each for the kidneys, Wolffian bodies, heart, stomach and intestines with contents, and the same empty.


1516 25 39 58 84 107 130 157

Body Length in Millimeters


191 215 242 262


Fig,. 1 Curves showing the relative growth of all the viscera, the head, liver and brain. These curves were secured by arranging the average percentage (for the litter) which the organs form of the net body weight, according to the average body length (in millimeters). In certain cases, several litters are combined, the average of the litter averages being taken (see above). The broken line at the right shows the trend of the post-natal relations, as indicated by the observations on the adults. The information thus gained is only general, as there are no indications concerning the age of the specimens, or the rnpidihj with which the changes take place.


Fig. 2 Curves showing the relative growth of the Wolffian bodies, heart, kidneys, stomach and intestines with contents and stomach and intestines empty. These curves were secured by arranging the average percentage (for the litter) which the organs form of the net body weight, according to the average body length (in millimeters). In certain cases, several litters are combined, the average of the litter averages being taken (see above). The broken line at the right shows the trend of the post-natal relations, as indicated by the observations on the adults. The information thus gained is only general, as there are no indications concerning the age of the specimens, or the rapidity with which the changes take place.


The figures for the human embryo are quoted from Jackson ('09), unless otherwise specified. Relatively few observations are available for other animals.


Fig. 3 contains three curves — one each for the lungs, spinal cord, and eyeballs.


Fig. 3 Curves showing relative growth of s])inal cord, lungs and eyeballs. These curves were secured by arranging the average percentage (for the litter) which the organs form of the net body weight, according to the average body length (in millimeters). In certain cases, several litters are combined, the average of the litter averages being taken (see above). The broken line at the right shows the trend of the post-natal relations, as indicated by the observations on the adults. The information thus gained is only general, as there are no indications concerning the age of the specimens, or the rapidity with which the changes take place.


Fig. 4 includes curves for the gonads (ovaries and testes), thymus and spleen.

Fig. 4 Curves showing the relative growth of the testes, ovaries, spleen and thymus. These curves were secured by arranging the average percentage (for the litter) which the organs form of the net body weight, according to the average body length (in millimeters). In certain cases, several litters are combined, the average of the litter averages being taken (see above). The broken line at the right shows the trend of the post-natal relations, as indicated by the observations on the adults. The information thus gained is only general, as there are no indications concerning the age of the specimens, or the rapidity with which the changes take place.


Fig. 5 includes curves for the suprarenal glands, thyroid and pancreas.

Fig. 5 Curves showing the relative growth of the pancreas, thyroid and suprarenal glands. These curves were secured by arranging the average percentage (for the litter) which the organs form of the net body weight, according to the average body length (in millimeters). In certain cases, several litters are combined, the average of the litter averages being taken (see above). The broken line at the right shows the trend of the post-natal relations, as indicated by the observations on the adults. The information thus gained is only general, as there are no indications concerning the age of the specimens, or the rapidity with which the changes take place.

Discussion of the Data and Comparison with Growth in Other Species

In the following pages, the body as a whole will first be considered, followed by a discussion of the viscera as a whole, and finally of each individual organ. Comparisons are made principally with the human species, for which the data are fairly complete.

1. The body as a whole

Table 1 gives the average body weights observed for the various litters examined, giving however, no indication of their ages. There are, however, some data available by which the age of one litter and the size at birth can be approximately estimated. From these may be roughly estimated the growth rate for the earlier and later stages.


The largest ovum found in those pig ovaries examined measured 0.177 mm. in diameter. This includes the zona pellucida which measures about 0.020 mm. in thickness (0.010 mm. counted twice in measuring the diameter). From this, the diameter of the mature ovum (including the zona pellucida) is estimated to be about 0. 18 mm. The corresponding volume would be about 0.000003 cc, and (assuming the specific gravity as 1) the weight about 0.000003 g.


Keibel ('97) describes three pig embryos, aged twenty-two days, which varied from 11.7 mm. to 14 mm. in length. In the smallest litter observed by me, the average crown-rump length was 13.6 mm., and the average weight 0.25 g. (litter 13, which contained eight specimens, only three of which were dissected). These embryos would probably not be more than 23 days old, according to Keibel's figures, although some allowance must be made for variation.


The duration of pregnancy in the pig is given by Coburn ('94) as 112 days, by Long ('06) as usually 112, varying from 110 to 116 days according to the age of the mother, and by Spencer ('98) as 16 weeks. This would place the usual time as 16 weeks or 112 days.


Long ('06) gives some figures for the weight of litters from one year old sows. The litters average 7 . 8 pigs to the litter, the total average weight being about 14 . 2 lbs, or about 6,442 gms. This is an average weight of about 826 g. per pig. For sows two to three years old, the pigs average about 1,190 g. each. For those about five years old, the pigs are still larger, weighing about 1300g. each on the average (individual data not given).


As the hogs which go to market are usually about ten to twelve months old, the average weight of the full term fetus from such animals would therefore be approximately 826 g. The oldest litter examined in my observations averaged 745 g. These pigs are therefore probably well along in the last month of fetal life.


During the 23 days immediately succeeding fertilization, the weight thus apparently increases to about 0.25 g., an actual increase of about 83,000 times the weight of the ovum. Even this figure is too small, as a part of the ovum goes to form the membranes, etc. But this gives an idea of the enormously rapid growth rate in the early stages. For the whole ensuing fetal period of 99 days, the fetus increases in actual weight from . 25 g. to 826 g., or about 3300 times the weight at 23 days. That is, in a period about four times as long, the increase is only about one twenty-fifth as great. If the early rate of increase were maintained throughout the fetal period, the newborn animal would weigh about 12 million billion grams!

The weight at birth, 826 g., represents an increase of over 275 million times the weight of the ovum.


The four adult hogs examined (age about twelve months) averaged about 104.5 kg. in weight. We may safely assume the average young adult weight to be about 100 kg. The increase from a weight at birth of 826 g. to an adult weight of 100 kg. represents an increase of only 121 times. This is certainly an enormous decrease in the relative growth rate, even when compared with the last 99 days of prenatal life. The total increase from the weight of the ovum to the weight of the one year adult (sixteen months prenatal and postnatal time) amounts to about thirty-three billion times the weight of the ovum.


Although these data are limited in extent, they show such enormous differences for the various periods that we may safely assume the conclusions of Muehlmann ('00), strongly emphasized by Minot ('07) and Jackson ('09), to be true also for the pig; that is, that growth takes place most rapidly in the earliest stages, the rate decreasing, at first rapidly, then more slowly, throughoat prenatal and postnatal life.


In table 2 some data are given for comparison of growth in the pig, human, rabbit, white rat and chick. The figures for the rabbit are derived from Waldeyer ('06), Fehling (77) and Minot ('07); those for the white rat from Donaldson ('06) and unpublished observations by Jackson; those for the chick from Welcker ('03) and Davenport ('08).

Table 2

1. Conclusions on comparative growth

From table 2 of the development of these various animals, it will be seen that the general course of relative growth of the pig agrees with that of all the others, in that the rate is very rapid in the early embryonic stages, decreasing at first rapidly, then more slowly, throughout prenatal and postnatal life, until the adult size is reached.


The rapidity of growth, however, is quite different in the various animals at corresponding periods. Thus, at the end of three weeks of prenatal life, the pig has increased about twenty times as rapidly as has the human embryo, but only about one-twentieth as rapidly as the white rat and one-fiftieth as rapidly as the rabbit. After birth of the rat and rabbit, however, their growth rate is greatly decreased; so that at 112 days the pig (newborn) has far outstripped the rat (275 million to 47 million), and is approaching the rabbit, which has reached a weight of about 333 million times the weight of the ovum. The human embryo still lags behind (30 million) but is approaching the rat. At one year postnatal (sixteen months total) the pig has reached 33 billion times the weight of the ovum. Next in order is the human (seven months old) with an increase of two billion times, while the rabbit and white rat have dropped far behind. Thus the animals with a short gestation period (rabbit, rat) have a more rapid growth rate during that period, but are thereafter overtaken and outstripped by those with a longer gestation period (pig, human).


At the time of birth, the growth rate always undergoes a rapid decrease, the prenatal rate always being far more rapid than the postnatal. There seems to be no definite relation, however, between the length of the gestation period and the ratio of new- born to adult weight. Of the five species considered, the human is relatively nearer the adult weight at birth, the ratio being about 1:19. The pig is farthest from the adult, the ratio being about 1:120. The rabbit, rat and chick take an intermediate position, their ratio being about 1:60 (table 3).


Table 3

2. Relative growth of the viscera

Under this heading will be considered the relative growth of all the viscera together (including the brain and spinal cord, and excluding the contents of the stomach and intestines) which were measured. In a negative way, this gives also the relative growth of the other structures of the body, chiefly skeleton, mus- culature and skin.


A reference to fig. 1 will show the general trend of the curve. The maximum relative size occurs in the 15 mm. litter, where all the visceral organs form 38.08 per cent of the total body. At 58 mm. they form 36.65 per cent, and in the late fetus, 16.23 per cent of the total weight. In the adult, they form only 7.85 per cent of the total weight.


From this it is evident that the skeleton and musculature are relatively small at first, increasing at first rapidly, then more gradually, until, in the adult, they form about one half more relatively than they did at the fetal maximum for the organs. This is based on the assumption that there is relatively little change in the relative weight of the skin, etc.


Corresponding figures for the human show that the viscera form about 36 per cent of the total body in the second fetal month, slowly decreasing to about 26 per cent at birth, reaching about 13 per cent in the adult.


The total relative weight of the viscera is apparently about the same for the pig and human in early fetal life; but at birth that for the pig is much below the human. The decrease for the viscera and the increase in relative size of the remaining structures are very marked between birth and maturity, both in the pig and the human.


According to Jackson, the brain and spinal cord in the human embryo, at the beginning of the second month, have nearly three times the volume of the organs lying ventral to the body axis; at birth they are about equal; while in the adult, the ventral organs are six times as large as the brain and spinal cord. In the pig, the nervous system is relatively much smaller. In the 18 mm. litter, the ventral organs are nearly three times as large as the brain and cord: at birth they are more than three times as large; while in the adult, they are about ninety times as large.

3. Relative growth of the various organs

The relative growth of the organs together having been shown, it is now necessary to take up the growth of each individual organ separately, as their growth is by no means uniform.


The Head

Table 1 and the head curve in fig. 1 show that the head was found relatively largest in the second stage examined (18 mm.), forming on the average, 29.69 per cent of the total. Throughout the earlier fetal stages it forms from 20 per cent to 28 per cent of the total, and in the later averages about 22 per cent to 23 per cent. Its minimum observed relative size is 19 . 65 per cent in the 108 mm. litter. In the nearly full term fetus it averages 22.3 per cent of the total. In the adult it forms 6.26 per cent of the total body weight. The decrease in the relative size of the brain is much larger than that of the entire head, so that the facial structures form an increasingly larger portion of the head in the later fetuses and the adult.


In the human, the head reaches a maximum relative size of 45 per cent during the second month. It then decreases gradually in relative size, forming about 26 per cent of the total at birth. The adult human head forms 6 to 9 per cent of the total body weight.


The head of the pig at no stage observed reaches as large a relative size as does the human at a corresponding stage, having, for the most part, a relative size about two-thirds that of the human. In the adult, the relative size is more nearly equal in the two species.


In all vertebrates, from the fishes upward, the embryonic head is relatively large, especially in the early stages. The extent to which this is true varies in the different forms, the head being in general best developed in the amniota. It is perhaps largest in bird embryos, where it may form more than half of the entire body (chick). There is also much variation in different species of mammals, as shown by the difference between pig and human.

Brain

(Table 1: fig. 1). The maximum relative size of the brain (table 1) was found in the earliest stages examined (15 mm. and 18 mm., accompanying the maximum relative sizes of the head). It then forms, on the average, nearly 9 per cent of the total body weight. The relative size then decreases, rapidly in the earlier stages, then more slowly. In the fetus nearly full term, it forms nearly 4 per cent of the total, and in the adult, 0.087 per cent.


A comparison of the relative size of the head and brain shows that, in the first stage examined, the brain forms about one-third of the head, at 86 mm. about one-fifth, and throughout the later part of fetal life and at birth, about one-sixth. In the adult it forms only about one-seventieth of the head.


This is a very small relative size when compared with the human, both in prenatal and postnatal stages. The maximum relative size for the human brain occurs during the second month (at the same time as for the head), when the brain forms about 20 per cent of the entire body. This is about one half of the en- tire head. At birth, the brain forms about 12 . 8 per cent or 14 . 6 per cent (still-born or live-born), which is about one-half of the entire head. Vierordt estimates the adult human brain at 2.16 per cent of the total body, forming about one-third to one-fourth of the head.


According to figures from Donaldson ('08), the brain in the new- born white rat forms about 5 per cent of the whole body, which is about one-fourth larger relatively than in the pig at birth.* In the adult rats the brain forms about . 7 per cent of the body, which is eight times as large relatively as in the adult hog.


In the dog-fish data by Kellicott ('08), show that at birth the brain forms 1.11 per cent of the total, while in the adult it forms only 0.085 per cent or about the same relatively as in the pig.


Some observations on the chick, recoi'ded by Welcker and Brandt ('03), indicate that, at the ninth day of incubation, the brain forms 28.2 per cent of the body; newly hatched, 3 per cent; adult, less than 0.5 per cent.

Spinal cord

(Table 1: fig. 3). The si:)inal cord in the pig embryo has its maximum relative weight (of the stages observed) at 18 mm., where it forms 1.87 per cent of the total. Decreas- ing, at first rapidly, then more slowly, at 156 mm. it reaches its minimum prenatal relative sizej about 0.21 per cent of the total. Throughout the remainder of the stages examined, it averages about 0.25 per cent. In the late fetus, however, it forms 0.33 per cent, while in the adult it forms 0.043 per cent of the total.

In the first stage observed, its weight is about one-fifth that of the brain, later in the fetal period, about one-twelfth to one-fif- teenth, in the late fetus reaching one-twelfth. In the adult the weight of the cord is about one-half that of the brain. Its rela- tive growth rate must be much greater in postnatal life than is that of the brain; or, otherwise expressed, the decrease in rela- tive size of the brain is much greater than that of the cord.


The cord is relatively larger in the human fetus than in the pig in the earlier stages, but smaller in the later stages. Its maximum in the human embryo is 4.85 per cent m the fifth week. At 17 mm., it forms 3.43 per cent, in comparison with 1.87 per cent at 18 mm. in the pig. At birth, the human cord forms about 0. 15 per cent of the total weight, which is only one-half the relative size of the pig's cord. In the human adult, the cord forms 0.06 per cent of the total (Vierordt), which is almost twice that of the pig. The conditions regarding comparative prenatal and postnatal growth of brain and cord are similar in both human and pig; that is, the brain has the more rapid relative growth rate in prenatal life. In postnatal life the relative growth rate of the cord is more rapid, as Donaldson has pointed cut for the human.


Figures from Donaldson ('08) show that at birth the spinal cord in the white rat forms, on the average, . 73 per cent of the total, while in the adult (10-12 months), it forms about 0.20 per cent. The decrease is therefore similar to that in the pig and human. Data by Welcker indicate a similar decrease for the chick, but not for the dog.


The Eyeballs

Table 1: fig. 3). The eyeballs, starting with an average relative size of 0.14 per cent of the total at 15 mm., increase rapidly to a maximum of 1 . 15 per cent at 86 mm. From this stage, they decrease slowly until, at birth, they form .41 per cent of the total. In the adult they form only 0.011 per cent of the total weight.


From a few scattering observations on the human eyeballs, it seems that they are relatively smaller than those of the pig in the early stages ; of about the same relative size in the later stages of prenatal life; relatively smaller at birth, and about twice as large relatively in the adult.


Welcker and Brandt record some observations on the chick embryo. At the eleventh day of incubation, the eyeballs form nearly 25 per cent of the total body. At birth they form about 3 per cent, and in the adult about 0.3 per cent.


Thyroid gland

Table 1: jig. 5). The thyroid makes its first appearance (being too small for acclirate dissecting earlier) in the 58 mm. litter, when it has its maximum observed relative size of . 045 per cent of the entire body weight. From then on until the end of the prenatal period, it averages from 0.02 per cent to 0.03 per cent, forming 0.026 per cent, at birth. In the adult it forms only 0.004 per cent of the total. The individual measurements are rather variable for this gland, but it averages very regularly throughout.


The human thyroid, forming . 035 per cent of the total embryo at two months, increases to . 11 per cent or . 125 per cent (still- born or live-born). In the adult, it forms 0.05 per cent of the total according to Vierordt. It is, therefore, at all stages, a relatively larger and probably more important organ in the human than in the pig.


Thymus. {Table 1: fig. 4)- The thymus is liable to a considerable amount of individual variation, and shows a gradual increase in relative size throughout prenatal life. Where first measured, at 58 mm., it forms practically 0.1 per cent of the total weight. Near full term, it forms 0.37 per cent of the total. It was not found in the adult, though it may possibly have been overlooked.


In the human, the thymus forms 0.008 per cent of the entire body at the end of the second fetal month ; and about . 3 per cent at birth. Vierordt gives the adult size as 0.04 per cent of the total weight.

In general, the relative size and course of growth of the thymus appears somewhat similar in the pig and in the human.


The Lungs

(Table 1: fig. 3). The lungs are considered together because, while there is a difference in size, their ratio to each other is fairly constant. The curve for both lungs together is given in fig. 3. Forming 0.6 per cent of the total weight at 18 mm., the lungs increase rapidly in relative size until, at 86 nam., they form 3.89 per cent of the total body weight. From here the decrease in relative size is somewhat irregular to the late fetus, where they form about 2 per cent of the total weight. In the adult they form about 0.69 per cent. In the smallest litters examined, there is apparently no appreciable difference in size between the two Itings. The difference between the two lungs in the later stages, while holding fairly constantly to the ratio (right to left) of 7 :5, is liable to considerable individual variation, as is the relative size of each lung.


Approximately the same course of growth occurs in the human. There is an initial rise, the maximum relative size occurring in the fourth fetal month, when the lungs average 3 . 29 per cent of the total weight. In the still-born they average 1.7 per cent; in the live-born, 2.18 per cent. In the adult they form 1 . 5 per cent of the total ( Vierordt) . In relative size therefore, the human lungs are approximately equal to those of the pig during prenatal life, but are about twice as large in the adult. In both, the maximum occurs early in prenatal life.


The right lung in the human is also larger than the left, averaging about 20 per cent larger. In the adult, the right lung is usually stated to be only 10 per cent larger than the left. In the pig, however, the right lung averages about 40 per cent greater than the left, both in the fetus and the adult.


The Heart

(Table 1: fig. 2). The heart has its greatest observed relative size in the earliest stage examined (15 mm.), forming 4.64 per cent of the total weight. An examination of earlier stages would possibly show a still larger maximum. It decreases in relative size rapidly at first, then more slowly, until at 125 mm. it forms only 0.66 per cent of the total body weight. In the later fetuses there is a gradual rise, the heart forming 1 .03 per cent of the total weight in the 26 cm. fetus (nearly full term). In the adult it forms 0.32 per cent, about one-third that of the late fetus.

In the human the heart also has its maximum early, being estimated at more than 5 per cent in a four-weeks embryo; 3 . 64 per cent in the fifth week (11 mm.) and . 85 per cent in the third month. At birth, it forms 0.7 per cent or 0.77 per cent (still-born or live-born). Vierordt estimates the adult human heart to form . 56 per cent of the total weight.

The heart is therefore similar in relative size in pig and human during fetal life. It appears, however, to be relatively smaller at birth, and larger in the adult, in the human.


Stomach and Intestines

(Table 1: fig. 2). The stomach and intestines (including mesentery) differ from all other organs observed, in that they increase greatly in relative size in the adult as compared with the fetus. This is the case whether they are considered with or without contents.


At 18 mm. they form 0.26 per cent of the total body weight, after which they increase, at first very rapidly, then more slowly (and with considerable individual variation) to a maximum (for prenatal life) of 6 per cent in the late fetus. In the adult they form 6.81 per cent of the total weight. These figures are for stomach and intestines plus contents, which, especially in the later stages, present a great deal of variation.


A better index of the growth is shown by the figures for the stomach and intestines without contents. In the stages up to and including the 42 mm. stage, there would seem to be no appreciable contents, and here the stomach and intestines amount to 1 . 62 per cent of the body weight. These organs gradually increase throughout fetal life, forming at 26 cm. about 3 . 59 per cent of the total body weight, being about 40 per cent less than the same with contents (5.99 per cent). In the adult they form (empty) about 4.79 per cent of the net body weight, about 30 per cent less than the tract with contents.


The range of variation is large, due to the extreme variations in contents, but the empty ti-act is, in later prenatal life, about 35 to 40 per cent less in weight than the tract with contents.


In the human, the intestinal tract is relatively small in the early stages, increasing rapidly to full term, when the empty tract forms 1.03 per cent of the total weight. The contents here are about twice as great as the empty intestinal tract. Vierordt estimates 2 . 06 per cent of the total body weight for the empty adult tract.


The empty intestinal tract of the pig is therefore relatively much larger than that of the human, both in the later fetal stages and in the adult, while the contents are apparently relatively smaller.


The Liver

Table 1: fig. 1). In the earliest stage examined (15 mm.), the liver forms 12 .25 per cent of the total body weight. In the next succeeding stages it forms the most prominent organ. It has an early maximum, reaching 15 . 88 per cent of the net body weight in the 25 mm. stage and remaining at about this relative size until the 58 mm. stage. From here it decreases sharply, forming 5.01 per cent at 109 mm. In the late fetus it forms 3. per cent of the total (net) body weight. In the adult it forms only about 1.38 per cent of the total body weight. The maximum relative weight may be somewhat more than 15 . 88 per cent, and come just preceding or following the 25 mm. stage.


In the human fetus, the liver never reaches more than 7 . 5 to 10 per cent of the total body, and this maximum size occurs during the second and third months. During the fourth month, the liver drops to an average of about 5 per cent, which it maintains throughout fetal life, averaging 5.23 per cent in live-born infant. Vierordt gives the average for the adult human at 2 . 75 per cent. Therefore, in the earlier fetal stages, the liver has a much greater relative size in the pig than in the human, but in the later fetal stages and in the adult, it is relatively much smaller.


fetus, it forms on the average 0. 17 per cent of the total weight. In the adult, it forms about 0.13 per cent.

Practically the same course of growth is found in the human, although here the spleen is relatively larger than in the pig. The maximum is over 0.4 per cent in the 8th and 9th fetal months. It forms . 43 per cent in live-born infants, and . 25 per cent for the adult (Vierordt). The individual variations would seem to be greater in the human than in the pig.

Pancreas

(Table 1: fig. 5). Starting with a relative size of 0.042 per cent of the total body weight at 42 mm., where it was first observed, the pancreas increases, at first rapidly, then more slowly to a maximum of 0. 19 per cent at the 215 mm. stage. At 26 cm., it forms 0.16 per cent of the total weight; in the adult, . 14 per cent. In the pig, the variations in individual size appear very great, due perhaps in part to difficulty in dissecting it out perfectly. During the greater part of fetal life, the pancreas averages about 0. 1 per cent to.0. 13 per cent of the total weight.

Judging from the data available for comparison, the pancreas is slightly heavier relatively in the pig than in the human through- out the greater portion of fetal life, and has about the same rela- tive size in the adult.

Suprarenal glands

(Table 1: fig. 5). At 25 mm., the suprarenal glands are readily visible with the dissecting lens, but, owing to their position and attachments, cannot be readily dis- sected out and weighed.


At 42 mm., the suprarenal glands form about 0. 11 per cent of the total weight. At 58 mm., they form 0.13 per cent of the total, which is the maximum observed relative size. Throughout the later part of fetal life, they average about 0.015 to 0.025 per cent of the total, forming 0.019 per cent in the latest stage exam- ined. In the adult they are very small organs, forming about 0.005 per cent of the total. There is no marked difference between the right and left in size.


In the human fetus, the left suprarenal is usually the larger of the two. During the second month, the suprarenals form about 0.3 per cent of the total body volume, increasing to a maximum of . 46 per cent in the third month. At full term they form about


As is the case in the pig and human, the liver is much smaller in the adult than in the embryo or new born in the chick, dog, stickleback and shrew (Welcker). But it is relatively larger in the adult for the salamander (Welcker) and dog-fish (Kellicott).


Spleen

(Table 1: fig. 4)- The spleen is liable to very great individual variations in relative size. First observed in the 42 mm. litter, it there has a relative size of 0.016 per cent of the total weight. It gradually increases in relative size, until, in the 26 cm. 0.24 per cent. In the adult, they form 0.01 per cent (Vierordt). The suprarenals, therefore, have a much larger relative size in the human than in the pig, both in prenatal and postnatal life.

Gonads

Table 1: fig. 4)- The gonads were first accessible by dissection at 25 mm., when they average 0. 13 per cent of the total weight (sex not determined). From this point on, there is considerable difference in the two sexes, the testes (including epididymis) averaging heavier relatively than the ovaries. Rising rapidly to a maximum, the testes form 0.24 per cent at 42 mm., decreasing to 0.038 per cent at 242 mm. At 58 mm. the ovaries have their maximum size of . 094 per cent of the total, decreasing to . 014 per cent at 242 mm. At 26 cm. the ovaries (average of two observations) form . 035 per cent of the total body weight. This is probably either erroneous, or an abnormal condition, hence the curve in fig. 4 was not extended to this point. In the adult (2 F.) the ovaries form 0.015 per cent of the total body weight. (No data for males).


A similar relation is also found in the human, the gonads being relatively larger in the earlier part of the prenatal growth and the testes being relatively heavier than the ovaries . In a human embryo of the fifth week (11 mm.), the anlages for the gonads (sex undetermined) form 0.085 per cent of the total body volume. In the later fetal stages, the human testes are about twice as large relatively as the ovaries, a relation similar to that in the pig.

Kidneys and Wolffian bodies

The kidneys (table 1 ; fig. 2) were first measured at the 25 mm. stage, where they average . 22 per cent of the total body weight. They increase rapidly to a maximum relative size of 2.59 per cent of the total weight at 58 mm. At 191 mm. they form . 92 per cent, which is the minimum for later fetal life. Near full term they form 1 . 01 per cent of the total, and in the adult, . 24 per cent. There is apparently no constant difference in size between the right and left.


In the human, the kidneys form about 1 per cent in the later fetal stages. Vierordt gives 0.46 per cent as the relative size in the adult. The human kidneys first appear in the second month, and from that time onward the left kidney is quite usually slightly larger than the right.

It would appear from this that the kidneys, throughout the later part of fetal life, have about the same relative size in the pig and human, but are larger relatively in the human adult.

Agreeing with the pig and human, the kidneys appear relatively smaller in the adult than in the embryo shrew, dog and chick; but they are relatively larger in the adult stickleback and sala- mander (Welcker and Brandt).

The Wolffian bodies (table 1 : fig. 2) of the pig have their maxi- mum very early in fetal life, probably at an earlier stage than can be readily dissected. At 15 mm. they have by far the largest relative size observed, — 12 . 1 per cent of the total weight. From this stage the decrease is rapid. In the 18 mm. pigs they form 7.45 per cent; 25 mm., 4.71 per cent; 37 mm., 2.43 per cent; disappearing about 125 mm.

The Wolffian bodies present a very interesting case, as they are the only structures observed (except postnatal thj'mus?) in which a decrease in absolute weight takes place.

The following table expresses the relations for this organ:

Table 4

Absolute and relative weight of the Wolffian bodies


Corresponding to the decrease in relative size of the Wolffian bodies, there is, as might be expected, an increase in the size of the kidneys. The first record of the kidnej'^s (0.22 per cent) corresponds to a size of 4.71 per cent for the Wolffian bodies. The maximum relative size of the kidneys (2.59 per cent) corresponds to a size of 1.41 per cent for the Wolffian bodies. At the time of disappearance of the Wolffian bodies^ the kidneys form 1 . 27 per cent of the total weight. Shortly after this, the kidneys increase to 1 . 59 per cent (table 1 ; fig. 2) .


In the human, the Wolffian bodies form 0.6 per cent of the total volume in an embrj'o of the fifth week (11 mm.), in which the renal anlages are just appearing. From this time onward, they become both absolutely and relatively smaller, just as in the pig, disappearing about the beginning of the third month (31 mm.).


The Wolffian bodies in the pig are therefore relatively much larger than in the human embryo, and persist for a longer time.

Differences according to sex

The data from the various litters were sorted out and grouped according to sex, the averages being calculated for each sex in each litter. Theresults were of little value, however, on account of the small number of specimens, only nine litters having two or more of each sex. Although it was therefore not thought necessary to reproduce this table, certain indications which it showed are worthy of mention.


In the first place, the total body weight almost constantly averages higher in males than in females of the same litter. It is known that the human males average heavier in weight than the females of the same age in prenatal life and at birth.


The liver in the pig fetus was found relatively heavier in the female in most cases where a comparison was possible. This is also almost constantly true in the human species.


The gonads, beginning with the time when the sexes are first distinguishable are found relatively heavier in the male than in the female pig, as already mentioned. This is also in agreement with the human species.


For other organs, no definite conclusions concerning sexual difierences can be drawn, and even those mentioned rest upon a very uncertain basis. Many more observations are necessary in order to distinguish variations due to sex from accidental variations.

Summary

The general conclusions concerning the growth of the pig may- be summarized as follows:

1 . The weight of the ovum is about . 000003 g. The increase in weight is at least 83,000 times during the first 23 days. At the end of 112 days (full term) the increase is 275 million. The weight of the (young) adult hog is about 333 billion times that of the ovum. Comparative figures given for the human, rabbit, rat and chick show that all these forms agree with the general law that the rate of growth is by far most rapid at the beginning of j:)renatal life, decreasing at first rapidly, then more slowly, throughout prenatal and postnatal life.

2. The viscera of the pig embryo (including the brain and spinal cord) taken all together have their maximum relative size, about 38 per cent of the whole bodj^ in the earliest stage examined (15 mm.), decreasing to about 16 per cent at birth. In the adult,, they form nearly 8 per cent of the whole body.

3. The head attains its maximum observed relative size at 18 mm. forming nearly 30 per cent of the total, decreasing to about 22 per cent at birth, and about 6 per cent in the adult. It is always relatively smaller than the human head at Corresponding stages.

4. The brain also attains its maximum observed relative size, at about the same time as the head, forming about 9 per cent of the total and about one-third of the head. In later stages it decreases until near full term it forms almost 4 per cent of the total, and about one-sixth of the head. In the adult it forms about 0.087 per cent of the body, and about one-seventieth of the head. The brain of the pig is at all stages relatively much smaller than that of the human.

5. The spinal cord has its maximum observed relative size 1.87 per cent of total body, at 18 mm. (probably larger in earlier stages), decreasing at first rapidly, then more slowly, and forming 0.33 per cent of the body at birth. In the adult, the cord forms about 0.04 per cent of the body.

6. The eyeballs attain their maximum observed relative size at 86 mm., forming 1 . 15 per cent of the body; decreasing to 0.4 per cent near birth. In the adult, they form only 0.011 per cent of the body.

7. The heart forms 4.64 per cent of the body weight at 15 mm., decreasing to about 1 per cent near birth, and averaging about 0.75 per cent during the greater number of the prenatal stages examined. In the adult hog, the heart averages 0.32 per cent of the body weight.

8. The lungs increase to a maximum of 3 . 9 per cent of the total body weight at 86 mm., decreasing irregularly thereafter to about 2 per cent near birth. The right lung is larger than the left in the approximate ratio of 7 to 5. The lungs in the adult form only about 0.7 per cent of the body.

9. The liver increases to a maximum relative size of 15.88 per cent of the body weight at 25 mm., decreasing to 3 . 1 per cent near birth. Its maximum relative size is over twice as large as that of the human. In the adult the liver of the pig averages 1 . 38 per cent, being only about half that of the human.

10. The kidneys increase rapidly to a maximum relative size of 2.59 per cent of the body weight at 58 mm., decreasing there- after to an average of 1.01 per cent near birth; forming about 0.25 per cent in the adult. The Wolffian bodies are at first relatively enormous, forming over 12 per cent of the body weight at 15 mm. They decrease rapidly, however, in relative (and, after 58 mm., in absolute) size.

11. The spleen, pancreas and thymus increase gradually from the beginning, averaging about 0.17 per cent, 0.16 per cent, and 0.37 per cent respectively, of the total body weight near the end of fetal life. In the adult the figures are 0. 13 per cent and 0.14 per cent for spleen and pancreas respectively.

12. The thyroid gland after 58 mm. decreases slightly in relative size throughout prenatal growth, averaging about . 026 per cent of the body at the close of the fetal period, and 0.004 per cent in the adult.

13. The suprarenals form about 0.13 per cent of the body at 58 mm., decreasing thereafter to about 0.019 per cent near birth, and about . 005 per cent in the adult.

14. The stomach and intestines increase gradually throughout the prenatal period, forming near full term about 6 per cent with contents, or 3.6 per cent empty. In the adult, they increase to 4.79 per cent empty or 6.8 per cent with contents.

15. During prenatal life the total weight of the body and the relative weight of the gonads are greater in the male, while the relative weight of the liver is usually greater in the female.

Literature Cited

Donaldson, H. H. 1906 A comparison of the white rat with man in respect to the growth of the entire body. Boas Memorial Volume, New York.

1908 Comparison of the white rat with man in respect to the growth of the brain and spinal cord. Jour. Comp. Neur. Psych., vol. 18, no. 4.

1909 On the relation of the body length to the body weight and to the weight of the brain and of the spinal cord in th« albino rat. Jour. Comp. Neur. Psych., vol. 19, no. 2.

CoBURN, F. D. 1894 Swine husbandry.

Davenport, C. B. 1908 Experimental morphology.

Fehling, H. 1877 Beitrage zur Physiologie des placentaren Stoffverkehres. Archiv. f. Gynaekologie. Bd. 2.

Jackson, C. M. 1909 On the prenatal growth of the human body and the relative growth of the various organs and parts. Amer. Jour. Anat., vol. 9, no. 1.

Keibel, F. 1897 Normentafeln zur Entwickelungsgeschichte des Schweines.

Kellicott, W. E. 1908 The growth of the brain and viscera in the smooth dog-fish. Amer. Jour. Anat., vol. 8, no. 4.

Long, J. 1906 The book of the pig.

MiNOT, C. S. 1907 The problem of age, growth and death. Pop. Science Monthly, vol. 71. (Also published in book form. New York, 1908).

MuEHLMANN, M. 1900 Uebcr die Ursache des Alters. Wiesbaden. (Quoted by Jackson).

Spenceh, S. 1S98 Pigs - Breeds and management.

ViKROKDT, H. 1906 Anatomische, Physiologisc^he und Physikalische Daten und Tabellcn. 3 Aufl.

Waldeyer, W. 1906 In Hertwig's Handbuch dcr Eiitwicklungslehre der VVirbeUiere. Bd. 1, Jena.

Welcker, H., AND Brandt, A. 1903 Gewichtswerte dcr Koorperorgane bei dem Monschen und d'.'ii Ticren. Archiv f. Anthrop. Bd. 28.


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