184 AN INTRODUCTION TO ZOOLOGY 



Before considering the embryology of the earthworm, the 

 development of the eggs and spermatozoa and the history of these 

 previous to fertilization must be described. 



SPERMATOGENESIS. During the development of the spermato- 

 zoa, the primordial germ cells separate from the testes, and lie 

 in the cavities of the seminal vesicles. The nucleus of each germ 

 cell divides into 2, 4, 8, or 16 daughter nuclei, which become 

 arranged in a single layer near the surface of the cell. Cell walls 

 now appear, extending inward from the periphery, the result being 

 a colony of cells attached by cytoplasmic pedestals to a central 

 non-nucleated mass. The cells of the colony then divide, increas- 

 ing the number to 32. 64, 128 or more. A dissociation of the 

 colony into several parts now takes place, each part containing 

 a number of spermatogonia. Such a spermatogonial group finally 

 becomes a spherical morula of 32 primary spermatocytes, which 

 are still fastened by cytoplasmic threads to a central body called 

 the blastophore. Each group of primary spermatocytes gives 

 rise to 64 secondary spermatocytes, and these divide to form 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, the reduc- 

 tion 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 (138, 

 140). 



During the later stages of copulation the spermatozoa are 

 drawn into the rosette-like funnels (Fig. 93, SF) of the vas adeje- 

 rentia, pass through these ducts, and are transferred to the 

 spermathecae of the other worm. Here they are stored until the 

 cocoon passes over the openings of the spermathecae during the 

 withdrawal of the worm, when some of them pass into the nutri- 

 ent fluid which has been secreted into the cavity of the cocoon. 



OOGENESIS. The eggs develop in the ovaries (Fig. 93, O). 

 These are pear-shaped bodies composed of egg cells in various 

 stages of growth. The basal portion of each ovary consists of 



