Paper - Prenatal growth of swine (1928)

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Warwick BL. Prenatal growth of swine. (1928) J Morphol. 40: 59-84.

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This historic 1928 paper by Warwick described prenatal pig growth and development.




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Prenatal Growth of Swine

B. L. WARWICK

Ohio Agricultural Experiment Station, Wooster, Ohio


Seven Figures

Author’s Abstract

Fetuses from forty-three gravid uteruses from sows of known breeding dates, as well as from 448 uteruses with unknown breeding dates, were studied. Growth curves are given for weight and length of fetus and for weight of fetal membranes. The weight of the fetus first reaches that of the fetal membranes between the sixtieth and seventieth days of preg- nancy. Degenerate fetuses were found in 3.68 per cent of the cases. They were found at all stages of gestation. Size of litter was found to decrease from 11.4 at the twentieth day to 6.8 at the 110th day. Also, the calculated per cent of ova lost up to each ton-day stage tends to increase as gestation advances. Crowding was found to be an important factor, but probably not the only factor, in causing degeneration. Genetic factors were probably responsible for part of the resorbing fetuses.

In the study of the normal fetuses, significant correlations were found between fetus length and weight of fetal membranes, as well as between fetus weight and weight of fetal membranes. Lower correlations, but probably significant, were found between total distance (spacing) between fetuses in the uterus and weight of fetal membranes. Correla- tions between size of fetus and total distance between fetuses were very low. As in the case of the degenerates, crowding has an important relationship to size of fetus, but is probably not the only factor involved.


A study of birth records of swine has shown that there is much variation in size of the offspring at birth. It seems to be generally assumed that variations, other than qualitative characters, such as color, are due more largely to the en- vironment in utero than to the genetic make-up of the differ- ent fetuses. An attempt is made to separate some of the in utero environmental factors which influence the develop- ment of the pig embryo from the genetic- factors. This resolves itself into a study of the positions and sizes of embryos and fetuses in the same uterus, comparison with the size of the fetal membranes, and comparison with the amount of space available.


'Paper from the Departments of Genetics (no. 85) and Veterinary Science, Wisconsin Agricultural Experiment Station, in cooperation with the Animal Husbandry Division of the IT. S. Department of Agriculture. Published with the approval of the director of the Station and the chief of the Bureau of Animal Industry, U. S. Department of Agriculture.


Material Studied

plant utenises and their contents were studied at pack- ages at Chicago, Illinois, Madison, Wisconsin, and ille, Maryland. Most of these nteruses were from sows niseellaneoiis herds with unknown breeding dates. To material to use as a guide to the ages of untimed s, fifty sows and gilts of the Poland-China, Duroc- p and Chester-White breeds were mated by the writer, lied at definite dates after breeding. Of these animals, were found to be pregnant. Eecords of three gravid

es from sows of known breeding dates killed at Belts- rere added. The fetuses from these forty-three gravid ses form the basis for the estimation of age of the uder.

ACKNOWLEDGMENTS

s work has been carried on as a cooperative investiga- n the Departments of Genetics and Veterinary Science, irsity of Wisconsin, and the Animal Husbandry Divi- if the U. S. Department of Agriculture. Profs. F. B. Lson and J. M. Fargo, of the Animal Husbandry De- lent of the University of W^iscousin, aided the investiga- .11 many ways, especially by placing animals and feed e writer’s disposal, which made it possible to obtain yologic material of known ages. These animals were tallied at the Serum Plant of the Veterinary Science rtment. Innumerable courtesies were extended by lur & Co., Chicago, and Oscar Mayer & Co., Madison,

! the writer vias studying material at their packing plants, erous individuals have extended help in various ways, work was initiated at the suggestion of Dr. L. J. Cole 3 he was chief of the Division of Animal Husbandry, >. Department of Agriculture, and has been continued

r his direction in the Department of Genetics, Uiiiver- of Wisconsin. It has received the continued support Ir. E. W. Sheets, the present chief of the Animal Hus- Iry Division. Dr. F. B. Hadley, of the Department of irinai^ Science, has also oifei’ed many helpful suggestions.


Study of Fetuses of Known Ages

In order to estimate the ages of the fetuses obtained from imtimed packing-house material, it was necessary to study some of knorvii ages. No data of this nature have been pre- sented since the limited amount given in Keibel’s Normen- tafeln in 1897. The tables given by Stockli (’22) and in Strangeways’ Veterinary Anatomy (’92) go back principally to Gurlt, whose publications appeared in the early part of the last century, and, so far as the writer can determine, are not available in this country. Types, as well as methods of feeding swine, have changed so greatly since then that it seemed essential to obtain some data from present-day swine. Furthermore, the above-mentioned tables are not definite enough to be of the most use.

Through the courtesy of the Animal Husliandry Depart- ment, fifty-three head of sows and gilts were placed at the WTiter’s disposal. These were kept at the Serum Plant and were self-fed corn and tankage. Conditions did not permit hand feeding, yet it seems logical to believe that these data are comparable to those obtained from the untimed material obtained at packing-houses. The writer made daily observa- tions and mated the animals as often as estrum was observed. He also kept all breeding, slaughtering, and other records. Several of the gilts were never observed in heat and were finally sold open. Some of the others proved to be non- pregnant, one contracted pneumonia and died, and forty were pregnant when slaughtered. In addition to tlie aliove, four gilts were bred at the Beltsville farm of the U. S. Department of Agriculture and examined by Dr. II. C. McPhee at the time of slaughter. Three of these were iiregnant, and he kindly forwarded copies of tlie records. An attempt was made to divide the slaughtering dates so that the time after breeding of each female would be at ten-day intervals. The earliest was twenty days after breeding. It was impossible to have the sows at Beltsville killed at exactly twenty or thirty days after breeding. Of the remainder, it was neces- sary for various reasons to vary the time one to two days each in three cases. The other thirty-seven head are on exactly ten-day intervals.

Through the courtesy of Oscar Mayer & Co., it was possible to obtain the nteruses as soon as the carcasses were opened. All measurements, weights, and other records of the uteruses and their contents were taken as soon as possible on the killing floor.

The following procedure was followed in obtaining the data. The uterus was placed in a measuring box, and each ovary


Pig. 1 Gravid uterus of gilt. The left ovary had Iiecn removed before breeding and adhesions formed which held the left end of the uterus in a twisted shape. In all other respects this photograph is typical of gravid uteruses as obtained for study on the killing-floor.

removed, weighed, and the corpora Intea counted. Each broad ligament was then cut away from its attachment to the corresponding horn of the uterus, and the uterus and contents weighed (figs. 1, 2, and 3). Then the uterus was returned to the measuring box, and the length of each horn measured in centimeters. Each horn was split longitudinally, by means of an enterotome, on a line opposite the line of attachment of the broad ligament. The location of each embryo or fetus, its sex when determinable, . and whether it was normal or degenerated were noted. The distance between two fetuses and between each end of the horn and the nearest fetus was next obtained in millimeters. The points used were the placental attachments of the umbilical cords. The length of each fetus was taken from crown to rump by means of calipers. After measurement, the umbilical cord was cut from the body of the fetus, and the fetus was weighed in grams. After each fetus was removed, its fetal membranes were carefully stripped from the uterine wall, and, after drainage of the excess of moisture, w^ere weighed in grams. After all of the contents of the uterus were removed, the empty uterus was weighed. Figure 2 shows the measuring box and an unopened uterus, with ovaries and broad liga- ments removed.


2 Gravid uterus of gilt, after removal of the ovaries and the broad ligament. This shows the measurmg box used in this study.


The data on number and sex of fetuses® of different ages together with the weights and measurements and weights of fetal membranes are presented in table 1.

The relation of length of fetus to age is shown graphically in figure 4, and of weight to age, in figure 5. The weights of the fetal membranes at known ages show a decrease toward the end of the gestation period (fig. 5), but this may be due to flnctnation of the sampling, for when larger numbers are used, from fetuses of estimated ages, the apparent dip in the curve almost disappears, as is also shown in figure 5.

’ While it is usual to refer to the fetus iu its earlier stages as au embryo, in tliis paper, for convenience, the term .fetus is used throughout the develop-


The weight of fetus first equals that of the fetal membranes between the sixtieth and the seventieth day after breeding, as is also indicated in figure 5.

It is not the purpose of this paper to discuss the question of increment of growth or growth cycles during intra-uterine life of the fetus. The primary object of the inclusion of the data from timed material is to furnish a basis for estimating the age of the material of unknown age. Special attention is


Pig. 4 Curve showing relationship of length of fetus to age.


called to the wide variation in size of the apparently normal fetuses in the same uterus, and different nteruses of the same stage of pregnancy. The sizes in the different age groups of the latter half of the gestation period overlap to a large extent, but those of the younger ages do not. For this reason, it seems reasonable to believe that, in estimating the age of nntimed fetuses, the average of the normal individuals of the litter should be used rather than actual measurements of any one individual. Length is commonly employed to designate the size of pig fetuses, so is used in our estimates of age. The writer realizes that, even by using these averages from timed material to estimate the age, some of the known- age fetuses themselves would be estimated somewhat older or somewhat younger than they actually are, but it appears to be the best ‘yard-stick’ we have.


Fig. 5 Curves showing relationship of weight of fetus to age, and of weight ®f fetal membranes to age.


Occourance of degenerate embryos and fetuses

Among 3967 fetuses examined in apparently healthy nteruses, 146, or 3.68 per cent, were found to be in various stages of degeneration. The distribution according to age is seen in table 2. These figures show a somewhat lower percentage of degenerates than was found by Hammond (’21) and by Corner (’23) in swine.

Table 2 demonstrates that the largest per cent occurred during the earliest stage examined, but almost as large per



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B. L. WARWICK


nate the size of pig fetuses, so is used in our estimates of age. The writer realizes that, even by using these averages from timed material to estimate the age, some of the known- age fetuses themselves would he estimated somewhat older or somewhat younger than they actually are, but it appears to be the best ^yard-stick’ we have.


Age in days after breeding of the dam.

’ Average weights fetuses

Average weights fetal membranes (all flats, Inojuding estimated ages)

Average weights fetal raembronos ( known ages)

Pig. 5 Curves showing relationship of weight of fetus to age, and of weight @f fetal membranes to age.

OCCUREENCE OP DEGENERATE EMBRYOS AND FETUSES ,

Among 3967 fetuses examined in apparently healthy uteruses, 146, or 3.68 per cent, were found to be in various stages of degeneration. The distribution according to age is seen in table 2. These figures show a somewhat lower percentage of degenerates than was found hy Hammond ( ’21) and by Corner (’23) in swine.

Table 2 demonstrates that the largest per cent occurred during the earliest stage examined, hut almost as large percents occurred at other stages, particularly the last two. The appearance of the degenerate fetuses showed that many of them were in a process of resorption. Undoubtedly, most of those occurring in the earliest stages had completely dis- appeared by the time the later stages were reached. Oc- casionally, degenerate fetuses remain until the sow farrows and are expelled during normal parturition. A study of the number of fetuses present at each stage of pregnancy gives results as shown in table 3. From this it may be seen that

TABLE 2

Appearance of all fetuses in hoth right and left horns


ESTIMATED AGE

TOTAL FETUSES

NORMAL

DEGENERATE

DEGENERATE

Days

Number

Number

Number

Fer cent

20

148

137

11

7.43

30

270

259

11

4.25

40

283

269

14

4.95

50

528

513

15

2.84

60

690

679

11

1.59 ■

70

658

638

20

3.04

80

466

441

25

5.36

90

347

341

6

1.73

100

194

181

13

6.70

110

383

363

20

5.22

Totals and average

3967

3821

146

3.68


the number of fetuses decreased from an average of 11.4 at the twenty-day stage to 6.8 at the 110-day stage. However, the number of corpora lutea was also decreased as pregnancy advanced, although to a much smaller extent. Whether this was an actual decrease or due to some error in determining the number is not known. It is possible that some corpora lutea of estrum persisted and were included in the counts of the earlier stages. No histologic study was made to determine this point. It may be assumed that the difference between the average number of corpora lutea at each stage and the number of fetuses present represents ova that have been lost by not being fertilized or by death and resorption later. Table 3 also shows the per cent lost at each stage. This is also shown in figure 6. The fitted line denotes a distinct upward trend in the per cent of ova lost at each stage as gestation advances.

Four and twenty-three hundredths per cent of the fetuses located in the right horns and 3.13 per cent of those in the left horns were degenerated (fig. 4). There is no significant dif- ference in the total number of fetuses in the right and left horns. Of the 146 degenerates observed, 57.53 per cent were in right and 42.46 per cent in left horns. If it is assumed


TABLE 3

Estimated loss of ova at each stage of gestation, as shown l)y com-parison of corpora lutea with fcUises


estimated

AGE

NUMBER OR

NUMBER OP CORPORA LUTEA

NUMBER OP PETUSES

OVA UNACCOUNTED POR

XJTERTJSES

Total

Average

Total

Average

Average

Per cent

Days

20

13

183

14.3

148

11.4-

2,9

20.28

30

30

361

12.0

270

9.0

3.0

25.00

40

31

340

11.0-

269

8.7-

2 3

20.09

50

,55

628

11.4

500

9.1-

2,3

20.17

60

81

889

11.0-

680

8.0

3.0

27.27

70

78

902

11.5

629

8.0

3.5

30.43

80

60

605

10.1-

454

7.5

2,6

25.74

90

46

439

9.5

347

7.8-

1.7

17.89

100

27

256

9.5-

193

7.1

2.4

25.26

110

54

513

9.5

371

6.8

2,7

28.42


that they are as likely to occur in one horn as the other, the probable error of the deviation from expectation is ± 4.07. The deviation from the expected 50 per cent in each horn is 7.53. This is only 1.8 times the probable error. This indicates that the difference is probably not significant. The numbers of fetuses in the right and left horns tend to become equal even when there are unequal numbers of corpora lutea in the two ovaries. Experimental proof of this has been presented in a recent paper by the author (’26). In this experiment sows were semispayed, bred, and then slaughtered during pregnancy. Even though all the eggs came from one ovary, the numbers of fetuses became equalized in the two horns.

Degenerate fetuses have been found in all possible numeri- cal positions from the bifurcation. The tabulation in table 5 shows that in each of the first five positions from the bifurca- tion the percentage of occurrence is about the same, but rapidly increases in the positions farther removed.

Each horn was then taken as a unit and classified as to the number of fetuses present. The per cent of horns containing degenerate fetuses was determined in each group. It was found that 8.66 per cent of all -the horns contained one or more degenerate fetuses. Table 6 gives the per cents accord- ing to the number of fetuses present.



Pig. (3 Per cents of ova missing at each estimated age, with the fitted line.


This indicates that there is a decided tendency for the degenerates to occur more frequently when there are larger numbers in the same horn, yet they occur even when only one fetus is present and there can be no crowding.

The average space per fetus was determined for each horn of each uterus whose fetuses were estimated to be ninety-five days of age or older. Each horn rvas then classified and all tiie horns were grouped in a frequency table. The number of horns containing one or more degenerate fetuses was next detei-mined for each group of the frequency table. The results are aiven in table 7.



TABLE 4


Occurretice of degen&rates m right mid left horns of the uterus


ESl'lMATED


RIGHT HORN

1

LEB’T HORN


AGJi

Total

Normal

Degenerate

Total

Normal

Degenerate

Bays

20

74

69

5

74

68

6

30

136

131

5 :

134

128

6

40

149

140 '

9

134

129

5

50

260

251

9

268

262

6

60

351

342

9

339

337

2

70

325

315

10

333

323

10

80

241

227

14

225

214

11

90

166

162

•4

181

179

2

lOO

96

88

8

98

93

5

110

190

179

11

193

184

9

Totals

1988

1904

84

1979

1917

62

Per cent


1

i 4.23

!


3.13


TABLE 5

POSITIONAL OCCURRENCE OF DEGENERATE FETUSES

ESTIMATED AGES IN DAYS \ ] — I —


POSITION

10

30

40

50

60

70

30

90


110

‘'I

ID

lO

3

K

ID

li-

o

1-

UJ

5E

K

2

LU

e>

ID

O

D

S

g

ID

WiD o 2

DC ID UJ<J CLUJ Q

<

h-

O


_l

<

P

o

h

I

If

I

c

A

o

1-

UJ

&

a:

bJ

z

ID

u

Q

-i

<

H

o

1-

DEGENERATE

-J

<

H

o

1-


_J

<t

H

o

1-

ID

oi

U4

Z

ID

O

lU

Q

<

w-

o

H

ID

DC

lU

2

U-l

O

ID

Q

_j

<

H

O

H

UJ

H

<

DC

UJ

Z

lU

O

lU

o

<

H

O

w-

ID

DC

U4

z

ID

o

UJ

o

<

h-

O

1-

1

UJ

V-

s

ID

Z

Ul

ct

ID

Q


y

59

3

64

3

115

4

167

4

163

3

123

6

91

2

53

3

in

5

972

34

3.5


Q

57

3

64

4

112


163

Q

157

3

1 19

6

89

0

49

2

106

5

942

29

3. 1

3R,L

26

y

52

1

59

4

109


146

y

136

Z

105

5

79

0

43

3

83

5

838

26

3. 1

4R,L

23

y

42

1

47

3

67


104

y

95

1

65

S

54

Q

32

Z

57

4

606

20

3.3

5R,L

19

y

iQ

0

26

Q

64


59

y

60

1

35

2

2Z

2

14

2

19

1

348

12

3.5

6R,L

14

2

13

1

16

0

27


28

y

25

2

1 1

0

6

B

3

1

6

0

151

7

4.6

7R,L

10

2

8

1

7

0

6

0

13

0

10

J

3

1

2


m

0

1

n

60

4

6.7

BR,L

2

0

4

0

0

0

5

0

5

0

3

1

3

1

2

0

0

0

0

0

24

2

8.3

ea.L

2

0

3

0

0

0

3

0

3

0

3

3

1

0

0

0

0

0

0

0

15

3

20.0

lOR.L

0

0

1

0

0

0

0

0

1

0

3

1

t

1

0

0

0

0

0

0

6

2

33.3

im.L

0

0

0

0

0

0

0

0

1

1

1

0

0

0

0

0

■0

0

0

0

2

1

50.0

12R,L

0

0

0

0

0

0

0

0

0

0

1

1

0

0

0

0

0

0

0

0

1

I

100.0

13R.L

0

0

0

0

0

0

0

0

0

0

1

1

0


0

0

_j

0

0

0


1

1

lOO.O


j ■ •— J 1 1 1 1 1 — I 1 i

Number stands ^or position pom blprcotLon •, T, right horn ; Worn


TABLE 6

Distribution of degenerate fetuses in the horns according to the number of fetuses


NUMBER OF FETUSES IN HORN

TOTAL NUMBER

OF HORNS

NUMBER OF HORNS WITH DEGENERATE FETUSES

PER CENTS OF HORNS WITH DEGENERATE FETUSES

1

29

1

3.4

2

104

5

4.8

O

232

13

5.6

4

257

15

5.8

5

198

18

9.1

6

89

18

22.2

7

36

6

16.6

8 ,

9

2

22.2

9

9

0

0.0

10

3

3

100.0

11

2

2

100.0

12

0

0

0.0

13

1

1

100.0


TABLE 7


Per cents of horns which contain degenerate fetuses when classified according to average space. Estimated age of all fetuses in this table ninety-five days and older


average SPACE

I’ER FETUS

1

TOTAL HORNS

HORNS CONTAINING DEGENERATES

PER CENTS OF HORNS CONTAINING DEGENERATES

cm.

Number

Number


15- 19

1

1

100.00

20- 24

6

4

66.66

25- 29

11

3

27.2

30- 34

29

5

17.2

35- 39

26

1

3.8

40- 44

25

1

4.0

45- 49

16

0

0

50- 54

12

0

0

55- 59

7

0

0

60- 64

9

0

0

65- 69

5

1 0

0

70- 74

5

1 ®

0

75- 79

2

i 0

0

80- 84

1

1 0

0

85- 89

1

1 ®

0

90- 94

1

1 ^

0

135-139

1

0

0


This demonstrates that when the average amount of space is extremely small, a very large percentage of the horns contain degenerate fetuses. No horns in this age group had degenerate fetuses when the average space per fetus was over 45 cm. These results are probably most largely de- pendent on the number of fetuses present, which has been discussed above.

VAEIATION IN SIZE OF NOEMAL FETUSES

The variation in size is not limited to a comparison of normal and degenerate embryos and fetuses. As noted above, the sizes of the normal fetuses of the ages beyond the middle of the gestation period show such extreme variation that the frequency distributions of the different age groups overlap. A study of the different correlations should throw some light on the probable causes.

The fetus derives its nourishment from the mother by means of the fetal membranes. Does the size of the fetal membranes directly influence the size of the fetus f The only satisfactory measure of the fetal membranes of the pig is weight. Length of fetus was correlated with weight of fetal membranes in each estimated age group, using only the nor- mal individuals. The lowest coefficient of correlation was .3178 ± .0240 in the 65- to 75-day group, and the highest was .6478 ±; .0266 in the 25- to 35-day group. Similar correlations, using weight of fetus instead of length, give higher coefficients. The lowest is .3770 + .0317 at 75 to 85 days, and the highest is .5914 ± .0195 at 45 to 55 days. These show that there is a direct relationship of fetus size to size of the fetal mem- branes.

If the fetuses and their membranes are crowded closely together, it seems that the development would be retarded, as Ibsen ( ’23 a, ’23 b) has reported in guinea-pigs. The amount of space on each side of the fetus was added together and called total distance. This is, obviously, more than the aver- age space, but should be of value from a comparative stand- point. The weight of the fetal membranes was correlated


PEEJSTATAL GROWTH OP SWINE


79


with total distance in each of the estimated age groups except the youngest. The lowest coefficient was .2379 ±: .0351 in the 85- to 95-day group, and the highest was .4880 it .0382 in the 95- to 105-day group. This shows that the amount of crowd- ing influences the size of the placenta to some extent. How- ever, the coefficients of correlation are not much above the border-line of signiflcance. No high correlations have been found in this material such as those reported by Ibsen in guinea-pigs.

Correlation of length of fetus with total distance and of weight of fetus with total distance did not give any markedly significant correlations. The small amount of correlation must come indirectly from the effect on the fetal membranes. All of the coefficients of correlation are shown in table 8 and plotted against age in figure 7. Owing to the relatively small number of fetuses of any knovm age, it was not possible to determine the relation of w^eight to number of fetuses.

The records of the fetuses of the three oldest groups, ninety days and over, were classified according to numerical posi- tion from the bifurcation, and their weights were averaged. Obviously, it would be misleading to combine different age groups to get averages unless the total number of fetuses were the same in each age group. Any influence due to numerical position would manifest itself toward the end of pregnancy. For this reason the above-mentioned age groups were selected. The average weights for each position are given in table 9 for the three age groups. This shows that there is no constant ranking, and so numerical position of itself does not deter- mine the size.


Fig. 7 Curves showing the coefficients of correlation at different stages of gestation, with the fitted straight line for each curve. The coefficients of correla- tion marked 1 to 7, inclusive, should each be preceded by a decimal point.


PRENATAL GROWTH OP SWINE


81


TABLE 8


COEFFICIENTS OF CORRELATION ON PRENATAL GROWTH

E5T.

AGE

DAYS


LENGTH-

FETAL

membrane

LENGTH-

TOTAL

DISTANCE

WEIGHT-

FETAL

MEMBRANE

WEIGHT-

TOTAL

DISTANCE

FETAL MEM-

BRANE-TOTAL

DISTANCE

30

.6352 i 0127

.6478^.0266

.07461.0418

.5406 1.0321

.0709 1,0419

.2767 1.0423

40


.34461.0371

.03701.0416

.40931.0350

.1012 1.0416

.4879 1.0321

50


.5527 1. 020b

.0552 1. 0299

,59141.0195

.1029 t.029b

.2300 ±.0203

60


.4019 t.02l7

.19241.0249

,43951.0209

.1934 ±.0293

.24661,0243

70


.3178 ±.0240

.06461.0266

.4146 ±.3206

.09051.0265

.33181.0380

80

.7600 + .0141

.5326 ±.2231

.3010 1.2919

.37701.03/7

.1856 1.0310

.37971.0275

90

.7504t.0l23

.3390 ±.0330

.0659 ±.0370

.4300 ±,0301

.0425 ±.0300

.23791.0351

100

.5501 ±.0139

.3942 ±,0423

.1554 ±.0489

.42 721.0410

,1476 1.0490

.4680 1.0302

no

.83I9±.I095

.4365 ±.0287

.2330 ±,0337

,4600 i.0273

.1330 ±.0550

.3693 1,0308


TABLE 9


Average fetus weights acGorciing to mmerical position from bifurcation


NUMEIlIO.^;J

i’o.si'rioN

90-day fetuses

100-DAY FETUSES

110-DAY FETUSES

Average

weights

Rank

Average

weights

Rank

A verage weights

Rank

Grams


Grams


Grams


1

449

6

566

2

734

6

3

425

8

533

6

735

5

3 !

446

7

598

1

713

7

4

453

5

546

4

750

(3 + 4)

5

456

4

563

3

750

(3 + 4)

6

491

3

543

5

906

1

7

516

2


781

2

8

562

1


...


...




Pig. 7 Curves showing the eoefS.eieiits of correlation at different stages of gestation, with the fitted straight line for each curve. The coefficients of correla- tion marked 1 to 7, inclusive, should each be preceded by a decimal point.


TABLE 8


E5T.

AGE

DAYS


LENGTH-

FETAL

MEMBRANE

LENGTH-

TOTAL

DISTANCE

WEIGHT-

FETAL

MEMBRANE

WEIGHT-

TOTAL

DISTANCE

FETAL MEM- BRANE-TOTAL DISTANCE


.6352 ± 0127

.6476^.0266


.5486 1.0321

.0709 1.0419

.2787 ±.0423

40


.34461.0371

.0370 ±.0416


.1012 t.0416

.4879 ±.0321

50


.05321.0299


.1029 1.0296

.2380 ±.0283

60


.4019 1.0217

.19241.0249

.4395 ±.0209

.19341.0293

.2466 ±.0243


.3176 ±.0240

.06461.0266

.4146 1.32 B6

.0905 ±.0265

.33181.0360


.760fli.0l41

.5526 ±.2231

.3018 ±.2919

.37701.03/7

.1856 ±.0310

.3r97±.0275

90

.7504i.OI23

.3390 ±.0330

.08591.0370

.4360 ±.0301

.O425±.O30O

.2379±.0351


.650/ i.OI39

.3942 ±.0423

.1554 ±.0489

.4272 ±.0410

.1478 ±.0490

.4680 ±.0382

no

.83I9±.I093

.4365 ±.0267

.2330 1.0337

.4806 ±.0273

,1338 ±.0550

.3693 ±,0308


TABLE 9


Average fetus weights according to numerical position from bifurcation


NUMliUICAL

1’OSITION

90-t)AY PETU.SES

100-DAY FETUSES

110-DAY’ FETUSES

Average

weiglits

Rank

Average

weights

Rank

Average

weiglits

Rank


Grama


Grams


Grams


1

449

6

566

2

734

6

3

425

8

533

6

735

5

3

446

! 7

i 598

1

713

7

4

453

5

546

4

750

(3+4)

5

456

4

563

3

750

(3+4)

6

491

3

543

5

906

1

7

516

2


781

2

8

562

1



...





Summary and Discussion

Fetuses from forty-three gravid uteruses of known breed- ing dates were studied. The growth curves show that length increases at a fairly uniform rate throughout the gestation period, while weight increases much more rapidly during the last twenty days than earlier. The weight of the fetus first reaches that of the fetal membranes between the sixtieth and seventieth days of the gestation period. There is much varia- tion in the sizes of the fetuses of the same age, especially during the last half of pregnancy. For this reason it seems logical in any study in which mitimed pig fetuses are used, to estimate the age by use of the average of all of the normal individuals of the litter rather than by the measurement of any particular individual.

In addition to the timed material, the fetuses from 448 uteruses with unknown breeding dates were also studied. These uteruses contained 3967 fetuses, of which 146, or 3.68 per cent, were found to be in various stages of degeneration. These were found in each age group, with the largest per cent in the youngest group. However, there was no regularity from the youngest to the oldest. Due to the process of resorption, probably all the fetuses which became degenerate at early ages would become resorbed before the end of the gestation period. This is shown by the fact that litter size decreases from 11.4 at the twenty-day stage to 6.8 at the 110- day stage. Also the calculated per cent of ova lost up to each age tends to increase as gestation advances. The de- generate fetuses in the older material were probably of recent occurrence in most cases. However, there was so much varia- tion in the degree of resorption that it was impossible to estimate the length of time which had elapsed since the death of the fetus. It seems logical to believe that the death of the fetus is as likely to occur at one age as another.

Fetuses in the first five numerical positions from the bi- furcation included about the same per cent of degenerates in each. The other positions had much higher percentages of degenerates. This is probably due in part to the large number of fetuses present, wliicli in turn reduced tlie amount of space available for each fetus. It was found that both a large number of fetuses per horn and small average space was ac- companied by a marked increase in number of horns con- taining degenerates. This would imply that crowding was the cause of degeneration of the fetus. Crowding is without doubt an important factor involved, particularly during the latter half of the gestation period. It is probably, however, not the only factor involved. A degenerate fetus was found in one case where it was the only fetus present in one of the horns of the uterus. There was no crowding in this case. Other degenerates were found which showed no sign of crowding. Degenerates were most numerous during the earliest stages studied, even when the fetal membranes had not extended over the full area between two adjacent fetuses. Furthermore, when a fetus dies and resorption begins, the fetal membranes of the normal fetuses next to it tend to occupy its space. This would give the appearance of crowd- ing, even though crowding had nothing to do with the death of the fetus. Inherent genetic differences in the germ plasm of the fetuses themselves probably are responsible for part of these resorbing fetuses, as suggested by Corner (’23). It is even possible that all of the degenerates wmre the result of genetic factors. The fact that as many as nine normal fetuses have been observed in a single horn with no degener- ates would tend to support this hypothesis that degeneration is dependent primarily on genetic factors rather than over- crowding. It may be that fetuses which are inferior genetic- ally are particularly susceptible to the effect of crowding.

In the study of the normal fetuses, significant correlations were found between fetus length and weight of fetal mem- branes. Comparable correlations -were found between fetus weight and weight of fetal membranes. This shows that there is a direct relationship between the growth of the placenta and of the fetus. This is only natural, and it is sur- prising that the correlations are not even higher.


Lower correlations, but probably significant, were found between total distance between fetuses and weight of fetal membranes. This shows that crowding has some etfect in retarding the development of the fetal membranes. This being the case, since, as has been shown, size of fetus is cor- related with fetal membranes, the size of the fetus should also show positive correlations with total distance between fetuses. The correlations were positive, hut very low. As in the case of the degenerates, crowding has some effect on the size of the normals. Besides this the low coefficient of cor- relation shows that there must be other factors operating to increase the variability. These are probably genetic. It seems logical to infer that some of the fetuses carrying poor combinations of factors escaped death during the early part of pregnancy, but lacked the ability to develop into large vigorous pigs.


Literature Cited

Corner, Geo. W. 1923 The problem of embryonic pathology in mammals, with observations upon intra-uterine mortality in the pig. Am. Jour. Anat., vol. 31, pp. 523-545.

Hammond, J. 1921 Further observations on the factors controlling fertility and fetal atrophy. Jour. Agr. Sei., vol. 11, pp, 337-409.

Ibsen, H. L. 1923 a Some genetic experiments with guinea pigs and rats. Proc. Am. Soe. Animal Prod., Dec., 1922, pp. 99—102.

1923 b An environmental factor causing variation in weight at

birth of guinea-pigs. Anat. Eec., vol. 24, p. 413.

Keibel, P. 1897 Normentafeln zur Entwicklungsgescliichte der Wirbelthicre.

I. Normautafel zur Entwickluugsgeschiclite des Schweines (Sus scrofa domestieus ) . J ena.

Stockli, a. 1923 Beobachtungen iiber die Entwieklungsvorgange am Eump- feskelett des Schweines. Gegenbaurs Morpliologisches Jahrbuch Bd 52, S. 153-195. ’

Stkangeways, T. 1892 Veterinary anatomy. 4th ed. Revised and ed. by I. Vaughan. Edinburgh.

Warwick, B. L. 1926 Intra-uterine migration of ova in the sow. Anat Rec vol. 33, pp. 29-33. ’

ADDENDUM

Since this manuscript has gone to press, Ibsen has published his complete data on “Prenatal growth in guinea-pigs,” which demonstrate the effect of crowding on fetus size in that species

Ibsen, H. L. 1928 Prenatal growth in guinea-pigs, with special reference to environmental factors affecting weight at birth. Jour. Exp Zool vol. 51, pp. 51-91.



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