DEVELOPMENT OF FLUSTRELLA HISPIDA. 473 



In the figures of segmenting ova, the first four cells formed have been 

 lettered, merely for convenience of reference, "A," " B," " C," " D," and the 

 daughter cells arising from these latter are distinguished by index numbers, a 

 negative index indicating the generation to which a particular cell belongs, and 

 a positive index its place in that generation ; thus A| denotes the fourth cell 

 derived from A in the fifth generation (see also p. 452). 



All the figures, both of sections and of entire larvse, have been drawn by the 

 aid of the Zeiss camera lucida. 



PLATE 22. 



Fig. 1. — Transverse section of a young colony of Plustrella hispid a, 

 collected in March. The section is taken close to the apex of the colony, 

 and shovFS the position of the testes on the lateral walls in the front of one 

 zooecium, and the ovary lying on the funicle at the back of a neighbouring 

 zocecium. X 65. 



Tig. 2. — Section of a young ovary lying on the funicle, showing the follicle 

 cells commencing to grow in among the young ova. X 225. 



Tig. 3. — Section of an older ovary ; the follicle cells are now seen to have 

 increased in number, x 225. 



Fig. 4. — Section of a young ovary showing four young ova, the walls of 

 which are still unformed. The yolk nucleus is present in the form of small 

 dark granules, x 400. 



Fig. 5. — Section of a somewhat older ovary showing four ova around which 

 the follicle cells have not yet developed. The dark granules which represent 

 the yolk nucleus have increased in number, and in many cases are seen to lie 

 in close contact with the membrane of the germinal vesicle. X 400. 



Fig. 6. — Section of a young ovum more advanced than those shown in 

 Fig. 5. The granules of the yolk nucleus are seen to lie in four groups, each 

 containing two granules, and three of these groups are surrounded by clear 

 spaces (.r). x 400. 



Fig. 7. — Section of a young ovum of the same age as that shown in Fig. 6. 

 The granules of the yolk nucleus are grouped together and lie within a clear 

 space. X 400. 



Fig. 8. — Section of a young ovum somewhat more advanced than that 

 shown in the preceding figure. The majority of the dark granules have 

 become fused to form a single large yolk nucleus lying within a clear space (.r) 

 separated from the germinal vesicle. Two vacuoles have already appeared in 

 the yolk nucleus, x 400. 



Figs. 9 — 12. — A series of four sections through a slightly older egg, to 

 show the difference in the appearance of the egg according to the point at 



