OP ARTS AND SCIENCES. 67 



The history of the origin of the entoderm, excluding the periblastic 

 portion of it, is involved in that of the embryonic ring (" Randwulst," 

 '• Randzone," " Keimwulst," "Keirasaum"). We have given con- 

 siderable attention to the formation and growth of tliis ring, with a 

 view to obtaining an accurate idea of its composition and its relations 

 to the embryo. Its mode of origin, composition, and history are the 

 same in all the ova we have examined. Van Beneden, Hoffmann, and 

 others, have insisted very strongly that this ring is not formed by a 

 process of invagination. Balfour (No. 20, pp. 64—70) contends that 

 there is no " true ingrowth or invagination of cells " in the elasmo- 

 branch development ; and has devoted considerable space to an at- 

 tempt to refute Haeckel's statements regarding such an invagination 

 in teleostean development. On the other hand, Gotte,^^ Ilaeckel 

 (No. 3), Henneguy,^^ Kingsley, Conn, and Van Vleck, are very posi- 

 tive that the ring is formed by invagination, or ingrowth from the 

 margin of the blastodisc ; and we have satisfied ourselves that this 

 view is essentially correct. 



We do not affirm that any sharply delimited portion of the blastodisc 

 is actually infolded ; but we have positive proof that the ring arises 

 as a centripetal ingroiotli of cells from the margin of the disc. Within 

 one hour from the time the ring begins^ the central area bounded by its 

 inner edge is reduced to about one half its original extent^ as shown 

 by camera drawings of Ctenolcdtrxis. This fact furnishes indubitable 

 evidence of the ingrowth, which has been so often denied and treated 

 as incompatible with the facts of vertebrate embryology. A com- 

 parison of optical sections of the blastodisc, taken at short intervals 

 during the formation of the ring, tells the same story. Actual sec- 

 tions simply furnish a verificatiou of observations made on the living 

 ovum. Precisely as was first shown by Gotte, we find the ingrowing 

 layer bending at the margin into the ectodermic layer. " Nach been- 

 digter Furchung," gays Gotte, " bilden die Zellen des Keimes eine 

 linsenfcirmige Scheibe, welche in einer entspi'echenden Vertiefung des 

 Dotters ruht. Darauf verdtinnt sich die Mitte des Keimes und lost 

 sich vom Dotter, so dass zwischen beiden die Keimhohle entsteht, 

 Dann schlUgt sich dcr Rand des Keimes avf einer Seite nach tinten 

 um itnd breitet sich an der unteren Fldche des Keimes cms. Dasselbe 



^ Gotte. Berlin, medicin. Centralbl., pp. 404-406, No. 20, 1869; and Arcliiv 

 f. mikr. Anat., IX., p. 679, 1873. 



59 Heniicguy. Bull. Soc. Phil, de Paris, 1880 (extract in Ann. Mag. Nat. 

 Hist., IV., 1880). Compt. Rend., XCV., pp. 1297-1299, 1882 (abstract in Journ. 

 Hoy. Micr. Soc, III., p. 190, April, 1883). 



