castle: embryology of ciona intestinalis. 269 



tind des Mesoderms hildet. Die Einstidpung der Entodermzellen ist ah 

 Gastndation zu betrachten. Es ist ein palingenetischer Process, den die 

 Cbordateu von ihren Vorfabren ererbt zu haben scbeinen, wo dieser Pro- 

 cess gleiclimassig und radial symmetriscb vor sicb ging, wie es sicb bei 

 einigen wirbellosen Tbieren beobacbten lasst. Die Einstidpung der Ekto- 

 dermzellen ist dagegen als ein coenogenetischer Process zu betrachten, der 

 mit der Bildung des Darmes nichts zu than hat und durch den die Bil- 

 dung der ektoblastogenen Anlage der Chorda und des Mesoderms eingeleitet 

 tvird. AVie in den folgenden Abschnitten gezeigt werden soil, lassen 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-mesenchyme ring) 

 nuist be regarded as taking part m a. primary invagination along with 

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

 soderm, viz. tlie 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 begiiining of gastrulation they clearly lie in the 

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

 witliin 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-mnscular 

 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 



