— 
+! 
— 
in males young spermatogonia may be recognised. — The 
anterior portion of the gonad is covered by a thin layer of 
celomic epithelium, but the posterior portion, from whieh 
oocytes or spermatogonia are being shed is not covered by 
an epithelium. The genital products are shed at an early 
stage from the gonad into the coelomic fluid where they 
complete their growth. The oocytes leave the ovary when 
they have reached a diameter of ‘016 to ‘02 mm. While 
floating in the coelomic fluid they increase in size and the 
nucleus becomes vesicular, its diameter being about halt 
that of the oocyte. Very small yolk granules are 
deposited in the protoplasm, they are rather more 
abundant round the nucleus, the peripheral portion of the 
protoplasm contains less yolk. The protoplasm is sur- 
rounded by a thin vitelline membrane about Lu im thick- 
ness. Oocytes must be produced in the gonads at a great 
rate, for the body cavity of large worms is filled with them 
almost to bursting by about the end of February. Ripe 
ova are not spherical but discoidal. The face of the egg 
is either circular (usually) and about “15 mm. in diameter, 
or it is oval with diameters of ‘16 mm. and about ‘14 mim., 
while the third axis of the eg 
(figs. 67 and 68). 
On the surface of the posterior part of the testes 
g measures ‘08 to ‘09 mm. 
groups of two, four or eight cells, young spermatogonia, 
may be seen. They are shed into the celom: the youngest 
stage usually found in the ccelomic fluid is formed of eight 
spermatogonia arranged around a vesicular mass of proto- 
plasm—the blastophore (figs. 60 and 61). The cells 
undergo numerous divisions, the products of which remain 
attached to the central blastophore. There is then a 
period during which the cells do not divide but increase in 
size becoming spermatocytes. Kach of these divides 
probably twice successively (as in the better known 
