1 98 



SEGMENTED WORMS OR ANNELIDA. 



M— 



anterior tip of the blastopore, while their middle portions 

 are carried downwards until they lie on the ventral sur- 

 face. Over them the epi blast 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 

 stomatodjeum. 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 epi- 

 blast, 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 flattened 

 plates, but they still end behind 

 in the two primary mesoblasts. 

 Coelomic 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 em- 

 bryo lengthens, leaving a trail of 

 daughter cells behind them. The cells so formed also 

 divide, the embryo rapidly lengthening and finally becoming 



Fig. 63. — .Stages in the 

 development of Earthworm. 

 (After WiLSOX.) 



/.c", polar 



stag. 



1. 2-celIed 

 bodies. 



2. Blastula, }!i^ a primarj- meso- 

 blast. 



3. Gastrula stage, Ec, ectoderm 

 or epiblast ; E.?i^ endoderm or 

 hypoblast ; ///, mesoblast cells. 



4. Longitudinal section in late 

 gastrula stage, cc, ectoderm ; en, 

 endoderm ; .1/, mouth ; st, stoma- 

 tod^um ; w, primarj' mesoblasts ; 

 «/', neuroblasts ; nc, nerve cord ; 

 n, nephridioblasts. 



