INTRODUCTORY EMBRYOLOGY. 



7 



uniting in the median line. This process of closure begins at 

 one end and proceeds towards the other, the end where the first 

 union takes place being the anterior. In some forms the 

 blastopore never closes completely, but persists in part as the 

 anus of the adult. In those forms where it closes completely 

 the anus later appears in the line of fusion. Another landmark 

 may be noted here, the blastopore closes along the median 

 line of the back ; and the region of this closure is known as the 

 primitive streak, the line of closure being the primitive groove. 

 From the region of the blastoporal lips (primitive streak) 

 there next grows into the segmentation cavity, on either side, 

 a third layer, the meso- 

 derm. In several forms 

 (Fig. 7) this mesoderm 

 clearly arises as an out- 

 growth from the entoderm 

 in the shape of a double 

 fold, its walls bounding a 

 cavity (coslom) which at 

 first is connected with the 

 archenteron. Later the 

 connection between these 

 coelomic pouches and the 

 archenteron is lost, the lips 

 of the outgrowth fusing, 

 and then the mesoderm 

 completely separates from 

 the entoderm. In other 

 cases the mesoderm ap- 

 pears as a solid outgrowth 

 from the same point, and 

 later it splits so as to form 

 a coelom comparable to that first described. 1 The result in 

 either case is that the segmentation cavity is still farther 

 reduced by the extension into it, on either side of the embryo, 

 of a flattened, mesodermic sac. In this sac two walls can be 



FIG. 7. Transverse section of Ambly- 

 stoma embryo showing formation of meso- ' 

 derm (mesothelium). a, archenteron ; 

 c, coelom ; et, ectoderm (outer layer) ; en, 

 ectoderm (nervous layer); m, medullary' 

 plate ; n, notochordal cells ; /, parietal 

 layer of mesothelium ; s, remains of seg- 

 mentation cavity; v, visceral (splanchnic) 

 layer of mesothelium. 



1 The type of coelom in the first case is called an enterocoele ; in the second a 

 schizocoele. 



