castle: EMBKYOLOGY OF CIONA [NTESTINALIS. 269 



und des Mesoderms bildet. Die JEinstidpung der Mntodermzellen is/ als 

 Gastndatiou zu betrachten. Ks ist cin paiingenetischer Process, den die 

 Chordaten von ihren Vorfahreu ererbt ziuhaben scheinen, wo dieser Pro- 

 deichmassig und radial synimetrisch vor sich ging, wie es Bioh bei 

 einio'en wirbellosen Thieren beobachtcu lasst. Die Einstulpung der Ekto- 

 dermzellen ist dagegen ah ein coenogenetisclter Process zu betrachten, der 

 mit der Bildimg des Darmes nichts zu thun hat und durch den die Bil 

 dung der ektoblastogenen Anlage der Chorda und des Mesoderms eingeleitet 

 ivird. Wie in den folgenden Abschnitten gezeigt werden soil, lasseu sich 

 diese zwei Processe — die Bildung des Darmes und die Bildung der 

 ektoblastogenen Anlage der Chorda und des Mesoderms — auch in der 

 Entwickelung aller Wirbelthiere von einander unterscheiden." (Sepa- 

 rate, pp. 11, 12.) 



Whether Lwoff is right in including the chorda and the greater por- 

 tion of the mesoderm of Amphioxus and the Vertebrates in the second- 

 arily invaginated part of the inner germ layer, I do not attempt to say. 

 That question must be decided by an examination of the forms on which 

 he made his observations. However, in Ciona, at least, the cells which 

 are destined to form chorda and mesenchyme (chorda-raesenchyme ring) 

 must be regarded as taking part in a primary invagination along with 

 the definitive endoderm. But plainly a very important part of the me- 

 soderm, viz. the cells which form the longitudinal musculature, is carried 

 into the inner layer by a secondary invagination. The secondarily 

 invaginated cells are derived from the posterior segments of the neuro- 

 muscular ring. At the beginning of gastrulation they clearly lie in the 

 outer layer of the gastrula, but at the conclusion of gastrulation they lie 

 within the margin of the blastopore. 



Accordingly, I regard the definitive endoderm fundament and the en- 

 circling chorda-mesenchyme ring as constituting the primary endoderm 

 in Ciona. In the primary ectoderm, I would include the neuro-muscular 

 ring and the "ectodermal group" of cells, both of which lie entirely in 

 the outer layer of the gastrula when the closure of the blastopore begins. 

 If this view is correct, the mesoderm or middle germ layer of Ascidians 

 must be regarded as derived in part from the primary endoderm and in 

 part from the primary ectoderm. Lwoff reached a similar conclusion 

 concerning the origin of the mesoderm in Amphioxus and the Verte- 

 brates. In Ciona that part of the mesoderm which is derived from the 

 outer germ layer produces the longitudinal musculature of the larva. It 

 forms the whole of this tissue, and nothing else. Similarly in Amphioxus 

 and the Vertebrates, Lwoff concluded that the ectodermal mesoderm 



