SEA-FISHERIES LABORATORY. 251 



in males young spermatogonia may be recognised. The 

 anterior portion of the gonad is covered by a thin layer of 

 coelomic epithelium, but the posterior portion, from which 

 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 ccelomic fluid they increase in size and the 

 nucleus becomes vesicular, its diameter being about half 

 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 1/a in 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 mm., 

 while the third axis of the egg measures "08 to '09 mm. 

 (tigs. 67 and 68). 



On the surface of the posterior part of the testes 

 groups of two, four or eight cells, young spermatogonia, 

 may be seen. They are shed into the ccelom ; the youngest 

 stage usually found in the coelomic 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. Each of these divides 

 probably twice successively (as in the better known 



