The Intra-Uterme Growth-Cycles of the Guinea-Pig. 717 



increased at a constant rate, we again tend to make the growth 

 curve of the young more nearly resemble a straight line. 



3) The hypotenuse of our triangle resembles more closely the 

 growth curve of the mother during the gestation period, for by this 

 time puberty is passed and the growth curve of the mother is ap- 

 proaching an asymptote and is nearly rectilinear. 



A few corrections have been made by interpolation in the weights 

 of the pregnant mothers in cases where all the animals were heavy 

 or light due to over- or under-feeding. But this was done only when 

 the difference exceeded twenty grams. In Table IV only 19 out of 

 314 figures have been thus corrected. 



Turning now to the consideration of the curve of intra- uterine 

 growth thus obtained (Fig. 1) certain features of it demand careful 

 attention. The first part of the curve representing the first four days 

 of gestation is not significant of the growth of the fertilized ova but 

 represents the rapid return of the mother to normal weight after the 

 loss occasioned by chasing and copulation. The dotted line, there- 

 fore, probably represents the correct course of growth for this period. 

 Although the most rapid cell division and relatively the fastest rate 

 of growth, occurs in the early cleavage stages, still such growth will 

 not be perceptible by the means employed in obtaining this curve. 

 So we should not expect a very rapid increase during the first eight 

 or ten days; for implantation of the ova does not take place till 

 seven days after fertilization 1 ). The slight increase in weight shown 

 may be due to the commencement of the accumulation of fat or to 

 other physiological processes in preparation for the coming pregnancy; 

 but this cannot be counted as growth of the litter in utero. 



That portion of the curve from ten to sixty days closely resem- 

 bles other growth curves which represent one cycle of growth. It 

 appears probable, therefore, that one cycle begins at the fertilization 

 of the egg and ends a week or ten days before birth, at which time 

 (or before) another cycle begins and continues on after birth. 



In Table V are to be found the weights and average weights 

 for twenty days after birth of the same twenty-eight young whose 

 growth in utero is represented by the first part of the curve. The 

 curve constructed from the averages in Table V is added to the curve 

 at the 68 day ordinate. There is a slight loss of weight at birth 



l ) GROSSER, Vergleichende Anatomic und Entwicklungsgeschichte der 

 Eihaute und der Placenta.* Wien u. Leipzig 1909. S. 162. 



Archiv f. Entwicklungsmechanik. XXXV. 47 



