146 C. M. Jackson. 



by the observations of Falck (14) on the weight of the chick embryo 

 at various stages. The data by Welcker (45) also indicate that m 

 the chick the relative growth rate diminishes steadily from the 9th 

 day of incubation to the time of hatching. As Minot (34) has 

 pointed out, an inspection of Keibel's Normentafeln also demonstrates 

 the more rapid relative growth in the earlier embryonic stages. 

 Fischel (lY) states that this is likewise true for duck embryos. We 

 may therefore conclude that in the bird embryo the relative growth 

 rate is most rapid at the beginning, and decreases with age. Preyer 

 (39) gives a curve of growth showing the increase in the weight of 

 chick embryos, based on 42 observations by Potts, Growth is far 

 more rapid than in the human embryo of the same age, since the 

 chick reaches 30 grams in weight within 21 days. But on account 

 of the great individual variations a much larger series of observations 

 is necessary before it is posstible to construct an accurate curve of 

 growth for comparison with that of the human embryo. 



Por mammalian embryos also, the data available are not very ex- 

 tensive. Fehling (16) has shown that in rabbit embryos from the 

 15th day onward the relative growth rate decreases, at first rapidly, 

 then more slowly. Minot (34) has confirmed this result. For ear- 

 lier embryos apparently no data are available. As in the case of the 

 chick, however, it is easy to show that the rate of growth is in general 

 far more rapid in the rabbit embryo than in the human embryo of 

 the same age. Assuming the diameter of the mature rabbit ovum 

 to be .116 mm. (Marshall), its volume would be about .0000008 cc, 

 and its corresponding weight about .0000008 g., or about one-fifth 

 that of the human ovum. x\t the end of about 30 days, the rabbit 

 embryo has reached full-term, with an average weight of 38.35 g. 

 (Fehling), which is nearly 50 million times the weight of the ovum! 

 In the same length of time, as we have seen, the human embryo has 

 increased in size only about 10 thousand times. The human or- 

 ganism reaches finally a larger size than the rabbit or chick, in spite 

 of the lower growth rate, because growth continues for a much longer 

 period of time (Minot). ^ 



*Curves of growth by Donaldson (12) show apparently a slower pre- 

 natal growth in the white rat than in man. But for these curves the body 



