174 WORMS. 



form 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 downwards, 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. 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 prseoral lobe, undergo 

 no change, but the mesoblast has been active. 



As gastrulation proceeds, the rriesoblast 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 stomatodaeum. 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 : outside is the thickened 

 epiblast, next, the rows of cells which sank in, and inmost 

 the mesoblast rows. The mesoblast rows have met in the 

 middle line by dividing and widening out into a pair of flat- 

 tened 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 epib'lastic 

 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 



