xm 



PHYLUM CHORDATA 



55 



development to be coelomic. Hence each gonad is surrounded by 

 a closed coelomic sac. 



When ripe the inner walls of the gonadic pouches burst, and the 

 ova or sperms make their way into the atrium and thence by the 

 atriopore to the external water. The laid eggs are covered by a 

 thin vitelline membrane, to which a second, inner peri-vitelline 

 membrane is added, the substance of which is derived from droplets 

 in the protoplasm. 



Development. After maturation (Fig. 758, A) and impregna- 

 tion, the membranes separate from the oosperm, leaving a wide 

 space around the latter. Segmentation is complete, there being 

 very little yolk ; it begins by a meridional cleft dividing the oosperm 

 into two (B), and followed by a second cleft, also meridional, at 

 right angles to the first (C, D). Next, an (approximately) equatorial 

 cleavage takes place, the embryo coming to be formed of eight cells 

 (E), of which the four belonging to the upper hemisphere, distin- 

 guished by the presence of the polar bodies, are smaller than the 



Fio. 759. Amphioxus lanceolatus. Three stages in the formation of the gastrula. 

 (From Korschelt and Heider, after Hatschek.) 



lower four. Apertures at the poles lead into a central cavity. 

 Further meridional and latitudinal divisions take place, and the 

 embryo becomes a blastula (I, K), enclosing a spacious blastoccele, 

 and having the cells on its lower pole (megameres) larger than the 

 rest (micromeres). The polar apertures disappear owing to the 

 closer approximation of the cells. 



Invagination then takes place (Fig. 759, A), occasioned by the 

 rapid multiplication of the micromeres ; then the lower pole of 

 the blastula becomes gradually pushed in until the whole lower 

 hemisphere is in complete contact with the upper hemisphere and 

 the blastocoale obliterated (B). The gastrula thus formed is at 

 first basin-shaped, having a very wide blastopore, but its cavity 

 (the archenteron) gradually deepens, and the blastopore is reduced 

 to a comparatively narrow aperture (C), afterwards destined to 

 form the anus. At the same time the aspects of the body are 

 marked out ; the dorsal surface becomes flattened, the ventral 



