440 ZOOLOGY . SECT. 



Development. The egg of Nereis when first discharged is 

 enclosed in a transparent thick gelatinous envelope, within which 

 are two membranes an outer, very thin and delicate, and an inner 

 (zona radiata), thicker and very distinctly striated in a radial 

 direction. The protoplasm of the ovum contains a number of 

 oil-drops and yolk-spherules. When fertilisation takes place 

 the yolk-spherules move away from what is destined to become 

 the upper pole of the egg, leaving a polar area composed of granular 

 protoplasm. The zona radiata disappears, and the contents of 

 the ovum undergo for a time amoeboid changes of form. Then 

 the spherical form is reassumed, two small bodies the polar 

 bodies are thrown off at the upper pole, and the process of 



micro 



macro 



micro 



macro 



Fia. 361. Nereis. Early stages in the development. A, lateral view of eight-celled stage ; 

 .B, the same from above ; C, stage of the formation of the first somatoblast ; D, stage at which 

 both somatoblasts are present ; macro, megameres ; micro, micromeres ; som. 1, som. 2, first 

 and second somatoblasts. (After Westinghausen.) 



segmentation or cleavage begins (Fig. 361). This is of the spiral 

 type, and up to a fairly advanced stage corresponds very closely 

 with the segmentation of the Polyclad oosperm as described on 

 page 268. The oosperm divides first into two parts, then into four. 

 One of these (A , B) is larger than the rest. From these four cells the 

 megameres there are separated off in succession three sets or 

 quartettes of micromeres, making twelve in all. One of these, 

 belonging to the second set, somewhat larger than the others and 

 differing from them in its subsequent history, is termed the first 

 somatoblast (0, D, som. 1) ; a second somatoblast (som. 2) is soon 

 given off in the fourth quartette from the same megamere that 

 gave origin to the first. 



The germinal layers are now "all established. The micromeres 



