DEVELOPMENT OF THE LEOPARD FROG 403 



in general terms in a preceding chapter (c/. p. 224). From the 

 testes the sperm are conveyed by the vasa deferentia to the kidneys, 

 whence they pass to the ureters, which function as ducts for both 

 excretory and reproductive products. Thus the spermatozoa, Hke 

 the ova, accumulate in terminal portions of the reproductive ducts 

 before their actual discharge during the spawning. In the act of 

 spawning the eggs pass slowly from the anal opening of the female 

 while sperm are emitted into the neighboring water by the male. 

 The union of ovum and spermatozoon, therefore, occurs in the open 

 water between the germ cells of individuals that are paired in sex- 

 ual union. Males that are unattached to females will sometimes 

 emit spermatozoa upon egg masses that have been laid some hours 

 previously, but the swelling that has taken place in the jelly usually 

 prevents the entrance of such spermatozoa into any eggs that have 

 not been fertilized at the time of laying. 



Maturation, Fertilization, and Organization of the Zygote. — 

 At the time of ovulation, the egg cell is naked save for the thin 

 vitelline membrane produced while it is still within the ovary by 

 the cells of its follicle. The jelly, as we have seen, is a secretion 

 from the oviduct. The first maturation division {cf. Fig. 114, p. 

 228) begins about the time the egg enters the coelome and is com- 

 pleted before it reaches the ovisac. As a result, the first polar body 

 appears during the passage through the oviduct as a small cell at 

 the animal pole of the egg (cf. Fig. 211). In the frog, this first 

 polar body does not divide into two cells, as shown in the schematic 

 representation of maturation (Fig. 114, p. 228). The second matura- 

 tion division, by which the second polar body is formed, does not 

 occur until after the spermatozoon has entered the egg at the time 

 of fertilization. By means of these two nuclear divisions, which 

 form the first and second polar bodies, the number of chromosomes 

 of the frog's egg is reduced one-half, in the manner described in 

 the gerieral account of maturation. The maturation of the sper- 

 matozoon occurs within the testis, as indicated in general by Fig. 

 113, p. 226. Thus, the nucleus of the ovum and that of the sper- 

 matozoon come to possess the haploid number of chromosomes, and 

 by the union of these two haploid groups at the time of fertiHza- 

 tion, the diploid number is restored. 



In fertilization as it occurs in the frog, a spermatozoon enters 

 the egg in the animal hemisphere somewhere in a zone from forty- 

 five to ninety degrees from the animal pole. The nucleus of the 



