440 



ZOOLOGY 



SKCT. 



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 



B 



micro 



macro 



macro 

 macro 



micro 



D 



micro 



macro 



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

 a, the same from above ; C, stage of the formation of the first sornatoblast ; D, stage at which 

 tn somatoblaste are present ; macro, megameres ; micro, micromeres ; som. 1, som. 2, first 

 and second somatoblaste. (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 2G8. 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 sett, somewhat larger than the others and 

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

 sornatoblast (C, 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. Tin- 



