36 COMPARATIVE ANATOMY 



rule, only one of the ova contained in a cocoon attains com- 

 plete f development. 



The ova are spherical and laden with small clear yolk 

 spherules and granules, which, however, are not so abundant 

 as to prevent its being fairly transparent. The early stages of 

 segmentation are irregular, and when seven or eight cells are 

 formed the embryo consists of two large lower cells called 

 macromeres, surmounted by a cap of five or six smaller cells 

 called micromeres. As segmentation proceeds a cavity is 

 formed between the micromeres above and the macromeres 

 below, and the embryo becomes a hollow sphere or blastula, 

 whose walls are composed of a single layer of cells, those at 

 one pole being larger than those at the other, though it is not 

 any longer possible to make a distinction between macromeres 

 and micromeres. After the establishment of the blastula the 

 cells composing its walls divide repeatedly by radial divisions, 

 excepting two large cells lying close together near the equator. 

 These do not divide, and therefore retain their primitive size, 

 whilst the other cells of the embryo become smaller and 

 smaller as the result of continued division. These two 

 large cells are known as the pole-cells of the mesoblast, or 

 mesomeres. Their inner ends soon project into the cavity 

 of the blastula (blastocoele), and presently these inner ends 

 are segmented off to form two smaller cells lying in the 

 blastocoele, the two large parent cells retaining their position 

 at the surface of the embryo. This process is repeated several 

 TTmesy s~6 "that two rows" of small cells formed by successive- 

 unequal divisions of the mesomeres lie side by side in the 

 ^.^blastocoele. During this time the mesomeres themselves 

 gradually sink below the surface and lie at the ends of the 

 two rows in the blastoccele. The mesomeres and the two 

 rows of smaller cells derived from them are the mesoblastic 

 bands, from which the musculature of the body-wall and gut, 

 the coelomic epithelium, the muscular septa, and the gonads 

 and their ducts are eventually derived. The mesoblastic 

 bands define the longitudinal or antero-posterior axis of the 

 future worm. The mesomeres are placed at what will be 

 the hinder end of the animal, the other ends of the bands, 

 which converge and unite in the middle line, consequently 

 represent the anterior end. 



The blastula has hitherto been nearly spherical in shape. 



