The Intra-Uterme Growth-Cycles of the Guinea-Pig. 



v 



from the horizontal line to the hypotenuse along the ordinates, in- 

 creases as we pass from left to right, and approach the perpendicu- 

 lar line whose length represents the weight increase during gestation, 

 which was due to other factors than the weight of the litter. The 

 length of each ordinate (distance between the base and hypotenuse) 

 represents the increase in weight which is in excess of the litter's 

 weight on that day, just as the length of the perpendicular forming 

 one side of the triangle represents the difference at birth between 

 the total increase of weight during gestation and the weight of the 

 litter. The length of every second ordinate (i. e. every second day) 

 was obtained, and this figure subtracted from the weight for that 

 day in the second column of Table III. The remainder obtained 

 was set down in the third column of Table III. It represented the 

 weight of the litter on that day, the last figure in the column 

 being the total weight of the litter at birth. This correction was 

 made in all ten cases. The results are tabulated in Table IV. 



This method of eliminating the weight of the deciduae, growth 

 of the mother, and the accumulation of fat by the mother during the 

 progress of gestation is undoubtedly open to criticism. It seems, 

 however, to be the only feasible way of doing it and although there 

 are doubtless errors involved, still the essential point which I believe 

 that my data establish is the S-shaped form of the curve of growth 

 in utero. Now a brief consideration of the probable effect of the above 

 interpolation upon the form of -the empirical curve shows that the 

 method employed, far from exaggerating this result, would tend to 

 mask it for the following reasons. 



1) It is reasonable to assume that the growth of the placentae 

 and other foetal membranes will keep pace with the growth of the 

 embryos and follow the same curve which would represent their 

 growth. In assuming that the rate of growth is constant, as we 

 have in the method of eliminating their weight, we tend to make 

 the curve of the embryo's growth a straight line and thus straighten 

 out any curved lines which may rightfully be a part of it. 



2) What has been said regarding the deciduae may very well 

 be true also of the fat accumulated by the mother during pregnancy. 

 In the light of our present knowledge of hormones and the part they 

 play in life phenomena, especially those connected with reproduction, 

 we may well assume that this accumulation of fat is controlled by 

 internal secretions whose amount and activity in turn are controlled 

 by the growing embryos in utero. In eliminating this fat as if it 



