546 



INVERTEBRATA 



CHAP. 



hours, it becomes constricted by a ring-like groove into an anterior 

 and a posterior vesicle (Fig. 403, B). The latter becomes very much 

 extended transversely, and narrowed in the middle, where it is 

 embraced by two horns of the anterior vesicle, a dorsal and a 

 ventral one, which grow backwards and surround the middle portion 

 of the posterior vesicle like a ring. 



acoe 



Fia. 403. Early stages in the embryonic development of .1 ntcdoii rtinacm. (After Seeliger. ) 



A, sagittal section through an embryo twenty-six hours old in the gastrula stage. B, longitudinal 

 frontal section through an embryo forty-eight hours old, to show the division of the archenteron into 

 two coelomic vesicles. C, longitudinal frontal section through an embryo fifty-seven hours old, to show 

 the belated formation of the alimentary canal. I), longitudinal sagittal section through an embryo 

 seventy-five hours old, to show the complete division of the coelom and the formation of the hydrocoele. 

 a.c, anterior coelom ; a.coe, anterior primary division of the coelom ; a.l, rudiment of alimentary canal ; 

 arch, archenteron ; in/, hydrocoele; /.y/.r, left posterior coelom; mes, mesenchvme ; y-.r<>c, posterior 

 primary division of the coelom ; r.yi.c, right posterior coelom. 



During the third day the embryo, which is now oval since it has 

 grown more quickly along one diameter than the other, acquires a series 

 of transverse ciliated rings which at first sight remind us of those of 

 the Holothurian pupa, for there are five of them formed, and in 

 addition an apical tuft of long cilia termed the apical field. The first 

 of the five rings surrounds this field but is incomplete ventrally ; the 

 rest, however, are complete. The ectoderm cells constituting these 



