The Earthworm 279 



experiments as follow: Keep a number of worms, each in a separate 

 flower-pot, from infancy to maturity ; kill a few and examine the con- 

 tents of their spermathecae (it is conceivable that a worm may be able 

 to pass spermatozoa into its own spermathecae) ; allow the remainder 

 to mate once, and note if cocoons are deposited ; kill some and examine 

 the contents of spermathecae; allow the rest to mate a second time, 

 pairing some with their former mates and others with different mates. 

 Kill all and examine spermathecae." 



In plants and animals, where both sperm and eggs are found in the 

 same individual, there is usually a different period for the maturing of 

 each, or some apparatus like this of the earthworm is brought into play 

 so that it is very seldom that the same organism can fertilize itself. 



The sperm-mother cells are derived from the testes and deposited in 

 the seminal vesicles. They are not fully developed, or as we say, 

 "mature," however, when they leave the testes, and so must continue 

 their development in the seminal vesicles. 



The sperm-mother cells, or primordial germ-cells, from which the 

 sperm are developed in the testes, have their nuclei divide into 2, 4, 8, 

 or 16 daughter nuclei which become arranged in a single layer near the 

 periphery of the protoplasm which has not divided. Cell walls then 

 appear, extending inward into the undivided protoplasmic mass. These 

 newly-formed cells now divide again, forming as high as from 32 to 128 

 cells, when the whole mass breaks up into smaller colonies. These 

 nucleated cells, which are to become sperm, are called spermatogonia. 

 These spermatogonial colonies become spherical, each containing 32 pri- 

 mary spermatocytes, all of which are still fastened by cytoplasmic 

 threads to the central protoplasm. This whole 32 celled colony is now 

 called a blastophore. 



Each colony of primary spermatocytes causes the formation of 64 

 secondary spermatocytes, and these divide into 128 spermatids. The 

 latter then metamorphose ( ) into spermatozoa. 



The number of chromosomes in the spermatozoa is sixteen. This is one- 

 half the number contained in the somatic cells, a reduction having taken 

 place during maturation by the union of the chromosomes two by two in 

 the secondary spermatocytes, and a subsequent separation when the 

 spermatids were formed. 



The head of the spermatozoon is practically all nuclear material. 

 The mid-piece is what was formerly the centrosome, while the cytoplasm 

 formed the tail. But as it is only the head which actually enters and 

 fertilizes the Ggg, the tail being used only for locomotive purposes, it 

 will be seen why nuclear material is considered so very important. 



OOGENESIS 



The egg-mother cells are found in the ovary in various stages of 

 growth, beginning at the basal end of each ovary where the most primi- 



