[24 Elementary Zoology. 



In spite, however, of these differences, which are as marked 

 as those which occur between the cells of the most diverse 

 tissues, there are abundant reasons for regarding the ova and 

 spermatozoa as equivalent and homologous bodies. In the 

 first place, and apart altogether from the special resemblances 

 that will be pointed out in their development, they are both 

 •cells. In each is a nucleus, and the nucleus is accompanied by 

 protoplasm. In the case of the ovum the protoplasm has given 

 rise to more or less yolk, and it is consequently inert. The 

 spermatozoon consists of a " head," which is the nucleus ; and 

 ,the protoplasm of the cell has been mainly converted into the 

 actively vibratile " tail." A spermatozoon, in fact, may be 

 looked upon as a flagellate cell in which the flagellum has 

 acquired undue importance. The general correspondence 

 between ovum and spermatozoon is better shown by those rare 

 cases (the Nematoid worms) in which the spermatozoon is not 

 flagellate, but moves by thrusting out pseudopodia. 



As to the special resemblances between ova and sperma- 

 tozoa, they are both produced in identical "glands," the 

 gonads. The gonads, whether male or female, are always 

 local proliferations of the lining membrane of the coelom. In 

 the tadpole, for example, as an instance of an animal with 

 separated sexes, the gonads appear as a pair of ridges of the 

 peritoneal epithelium, the genital ridges, which are at first 

 absolutely indistinguishable. In hermaphrodite animals either 

 both ova and sperm are produced from the same gonad 

 {e.g. snail), in which no need arises for a comparison, or if two 

 kinds of gonads are present they are clearly homologous. Thus 

 in the earthworm the testes and the ovaries occupy identical 

 positions in the body. Moreover, they appear to correspond 

 exactly in number, for, although the adult worm has two pairs 

 of testes and one pair only of ovaries, there is in the embryo a 

 fourth pair of gonads in the twelfth segment. There is thus 

 a close correspondence, usually amounting to identity, in the 

 situations where the ova and spermatozoa are produced. There 

 is, furthermore, an exceedingly close likeness in the way in which 

 the ova and the spermatozoa respectively develop in those 

 gonads. 



