XII BEACHIOPODA 409 



The cells of which the blastula consist are long and cylindrical 

 and carry long cilia. The cilia are inserted in basal granules of 

 unusually large size. The blastula becomes converted into a gastrula 

 by a wide-mouthed invagination. The invaginated cells as they pass 

 inwards change their form and become cubical, but of course still 

 retain their cilia. This change of form is the first histological 

 differentiation observed in development. The archenteron is very 

 spacious and fills up almost the entire interior of the gastrula, so 

 that the segmentation cavity or blastocoele is reduced to a narrow 

 slit. The embryo has now a somewhat conical shape, since, at the 

 place opposite to the blastopore, it is pointed. 



The next change which occurs is the division of the archenteron 

 into gut and coelom (Fig. 322). This takes place by the outgrowth 

 of a crescentic shelf of cells one layer thick, arising from what after- 



erd 



ent 



Fig. 323. — Later embryos of Terebratulina septentrionalis seen from the side — 

 in optical section. (After Conlilin. ). 



A, stage in which the enteron is nearly delimited from the coelom, but in which the blastopore is 

 still open. B, stage in which the blastopore has been closed, leaving a pit in its place, and in which 

 the hinder part of the archenteron has been obliterated. Wp, blastopore ; coe, coelom ; d.m.f, dorsal 

 mantle-fold ; d.m.g, dorsal mantle-groove ; ent, enteron ; l.coe, left lobe of coelom seen from behind the 

 enleron. 



wards is seen to be the anterior side of the archenteron. This shelf 

 arises from a position fairly high up on the wall of the archenteron, 

 and grows backwards and downwards into its cavity. It is bilater- 

 ally symmetrical, i.e. it is equally developed on the right and left 

 sides. By this outgrowth the archenteron is divided into an oval 

 gut above, and a broad flat coelom below, which underlies the enteron 

 and overlaps it at the sides. Tlie two chainbers, however, still open 

 into each other posteriorly, and the lower one communicates with 

 the exterior through the blastopore. 



The next change which occurs is that the embryo becomes 

 flattened in a dorso- ventral direction, and the coelom becomes 

 divided into right and left portions in the middle of its course, owing 

 to the gut being, as it were, pressed through it into contact with 

 the ventral ectoderm (Fig. 323). 



At the same time the blastopore becomes closed, and the manner 

 in which this takes place is important. It becomes changed in 



