644 



Comparative Morphology of Chordates 



produce scores, hundreds, or millions of relatively large eggs at a time, 

 the mammalian ovary produces very few mature eggs. The number of 

 young in a mammalian litter is under 25 and usually under 10. (There 

 is record of an African hedgehog, Centetes, bearing a litter of 22.) In 

 reptiles and birds the number of eggs produced is comparatively small 

 but the eggs are of enormous size. Consistent with these several facts, 

 the mammalian ovary is much smaller, relative to the size of the 

 animal, than the ovaries of other vertebrates. 



Comparing ovaries of mammals of different size, the organ is 

 larger in the larger animal. In a given species there is much variation 

 in size of the organ, depending on age, size, and physiologic condition 

 of the animal. In form, it varies from nearly spherical to a more or 

 less elongated and flattened ovoid. The roughly approximate average 

 longest diameter and the average weight of an ovary in several species 

 of mammal are as follows. (There is no constant ratio between the 

 diameters and the corresponding weights because of difference in form 

 of the organ in the several species.) 



Long 

 Diameter 



Weight 



Mouse 



Cat 



Man 



Horse 



Whale (humpback, Megapfera — 50 

 ft. long) 



4 mm. 

 10 mm. 

 37 mm. 

 75 mm. 



125 mm. 



0.02 Gm. 



0.20 Gm. 



6.00 Gm. 



75 . 00 Gm. 



500 . 00 Gm. 



It is, perhaps, to be expected that the larger mammal should have a 

 larger ovary but, taking into account certain other facts, a problem 

 presents itself. A mouse produces usually about six young in a litter. 

 (Exceptionally, as many as 14 have been reported.) The gestation 

 period is 21 days. Under ordinary circumstances, a mouse may produce 

 60 or more young in a year. The very large mammals bear usually only 

 one young at a time (twin whales have been reported) and have a 

 prolonged period of gestation — in the elephant, about 18 months; in 

 some whales, about 2 years. Further, among placental mammals there 

 is no important difference in the size of eggs. Rats and mice produce 

 eggs of nearly the same size (diameter 0.070-0.075 mm.); eggs of dog, 

 horse, gorilla, and man range in diameter from 0.130 to 0.145 mm. The 

 diameter of the mature egg of the humpback whale is given as from 

 0.100 to 0.130 mm., which makes it smaller than that of a cat, dog, 

 or rabbit. (Because of difficulties of measurement, there is much dis- 



