134 EMBRYOLOGY. 



or be further surrounded by calcareous deposits, the shell 

 proper, (Fig. 101, s.) The number of these eggs is large, in 

 proportion as the animal stands lower in the class to which 

 it belongs. The ovary of a herring contains more than 

 25,000 eggs ; while that of birds contains a much smaller 

 number, perhaps one or two hundred only. 



279. Ovulation. Having attained a certain degree of 

 maturity, which varies in different classes, the eggs leave 

 the ovary. This is called ovulation, and must not be con- 

 founded with the laying of the eggs, which is the subsequent 

 expulsion of them from the abdominal cavity, either imme- 

 diately, or through a special canal, the oviduct. Ovulation 

 takes place at certain seasons of the year, and never be- 

 fore the animal has reached a particular age, which is 

 commonly that of its full growth. In a majority of species, 

 ovulation is repeated for a number of years consecutively, 

 generally in the spring in terrestrial animals, and frequently 

 several times a year ; most of the lower aquatic animals, how- 

 ever, lay their eggs in the fall, or during winter. In others, 

 on the contrary, it occurs but once during life, at the period of 

 maturity, and the animal soon afterwards dies. Thus the but- 

 terfly and most insects die, shortly after having laid their eggs. 



280. The period of ovulation is one of no less interest to 

 the zoologist than to the physiologist, since the peculiar 

 characteristics of each species are then most clearly marked. 

 Ovulation is to animals what flowering is to plants ; and, 

 indeed, few phenomena are more interesting to the student 

 of nature than those exhibited by animals at the pairing 

 season. Then their physiognomy is the most animated, 

 their song the most melodious, and their attire the most 

 brilliant. Some birds appear so different at this time, that 

 zoologists are always careful to indicate whether or not a 

 bird is represented at the breeding season. Fishes, and 

 many other animals, are ornamented with much brighter 

 colors at this period. 



