192 SEGMENTED WORMS OR ANNELIDA. 



is slightly unequal (Fig. 84 (1)), and exhibits considerable variation even 

 within the limits of a species. 



In about twenty-four hours a nearly spherical, one-layered blasto- 

 sphere or blastula is formed. It consists of only about thirteen cells. 

 During the next twenty-four hours the cells increase in number rapidly, 

 but the blastula remains one-layered. Two cells lying together do not 

 take part in this division ; they are rather larger than the rest, and their 

 inner ends project into the cavity, and are soon cut off as daughter cells. 

 Gradually the large cells still undergoing division begin to sink in, and 

 at last are quite included in the cavity (Fig. 84 (2)). Thus there arise 

 two parallel rows of cells within the blastula, and these define the 

 longitudinal axis of the embryo. This is the beginning of the mesoblast 

 which forms all the muscles of the trunk, and which thus takes origin 

 from two primary mesoblasts. 



After five to six pairs of secondary mesoblasts have been formed, the 

 blastula begins to flatten, and to elongate, becoming an oval disc. The 

 cells of the lower surface become clearer, and the hypoblast is thus 

 defined. The cells of the upper surface are smaller, and become very 

 much flattened ; they compose the epiblast. The mesoblasts lie side 

 by side near one end, forming two rows extending forwards and down- 

 wards, but divergent, because of the flattening of the blastula. The 

 hypoblast now becomes concave, and thus the blastopore arises, 

 occupying the whole of the lower surface (Fig. 84 (3)). The sides close 

 in and the blastopore becomes a slit, which further closes from behind 

 forwards, leaving only a small opening, — the future mouth. During 

 these processes the cells at the anterior tip of the blastopore, which 

 will give rise to the praeoral lobe, undergo no change, but the mesoblast 

 has been active. 



As gastrulation proceeds, the mesoblast rows grow forwards and 

 upwards, until they come near each other above the anterior tip of the 

 blastopore, while their middle portions are carried downwards until they 

 lie on the ventral surface. Over them the epiblast is thickened in two 

 bands. Two longitudinal rows of epiblast cells near the anterior end, 

 and ending behind in large cells, sink in just as the primary mesoblasts 

 did. The thickening now extends ventrally until the two bands meet, 

 and, passing into the blastopore, forms the stomodseum. Even before 

 this the embryo has begun to swallow the albumen in which it floats. 



There are now two lateral bands of cells called the germ bands, 

 composed of three layers (Fig. 84(4)): outside is the thickened epi- 

 blast, next the rows of cells which sank in, and innermost the meso- 

 blast rows. The mesoblast rows have met in the middle line by 

 dividing and widening out into a pair of flattened plates, but they still 

 end behind in the two primary mesoblasts. Ccelomic cavities develop 

 in the plates, and the anterior ends meet above the mouth. The 

 epiblastic rows which sank in (there were eight of them, four on each 

 side of the median line, and each ending in a large mother cell) go on 

 growing. The mother cells are apparently carried backwards as the 

 embryo lengthens, leaving a trail of daughter cells behind them. The 

 cells so formed also divide, the embryo rapidly lengthening and finally 

 becoming vermiform. Of the eight rows the innermost on each side 

 (neuroblasts) give rise to the nervous system, the next two rows on 



