INTERRELATIONS OF THE MESONEPHROS, ETC. 193 



The total glomerular surface of each of the animals here 

 studied seems to have a definite relation to the rapidity of its 

 intra-uterine growth and the consequent length of the period of 

 its gestation. The size of the animal at birth must naturally 

 be taken into consideration, as no one would expect a large full 

 term pig fetus, for instance, to have developed in as short a time 

 as the new-born guinea pig. But in comparing the guinea pig 

 and the rabbit, one may be surprised that the former, smaller 

 animal should have a gestation period twice as long as the latter. 

 The number of embryos can have no influence in this case,- as 

 the rabbit normally has a much larger litter than the guinea 

 pig. There is obviously an actual difference in the growth 

 rate, and this is correlated with a difference in the excretory 

 surface in the embryo; the slower the growth rate, the less 

 active the cell changes, and also the less rapid the formation 

 of waste products to be eliminated. Other factors undoubtedly 

 underlie the causes of the differences, and it is not intended to 

 suggest that a small Wolffian body can cause slow growth, or 

 vice versa, but a comparison of the periods of gestation, as given 

 by Grosser, of the animals here studied with the table showing 

 the size and number of their mesonephric glomeruli shows that, 

 if the size of the animal be considered, the growth rate is evi- 

 dently correlated with the glomerular surface. Grosser gives 

 as the period of gestation for the rabbit, 28 days; for the rat, 35 

 days; for the guinea pig, 63 days; for the cat, 65 days; the pig, 

 four months; the sheep, five months; man, nine months. The 

 pig has a larger glomerular surface than the sheep, and a shorter 

 period of gestation; man has less glomerular surface than either 

 pig or sheep, and a longer intra-uterine life. If we consider 

 the relative sizes of the cat and the sheep, the cat's period of 

 gestation might be regarded as proportionately the longer of 

 the two, perhaps comparable to that of man; and the total 

 glomerular surface of the cat embryo is very similar to that of 

 the human embryo. A systematic study of the comparative 

 metabolism of different animals has not as yet been attempted, 

 to my knowledge, though scattered facts such as the analyses 

 of many types of urines are available. From the differences 



