CASTLE: EMBRYOLOGY OF CIONA INTESTINALIS. 269 
und des Mesoderms bildet. Die Einstülpung der Entodermzellen ist als 
Gastrulation zu betrachten. Ks ist ein palingenetischer Process, den die 
Chordaten von ihren Vorfahren ererbt zu haben scheinen, wo dieser Pro- 
cess gleichmässig und radial symmetrisch vor sich ging, wie es sich bei 
einigen wirbellosen Thieren beobachten lässt. Die Einstülpung der Ekto- 
dermzellen ist dagegen als ein coenogenetischer Process zu betrachten, der 
mit der Bildung des Darmes nichts zw thun hat und durch den die Bil- 
dung der ektoblastogenen Anlage der Chorda und des Mesoderms eingeleitet 
wird. Wie in den folgenden Abschnitten gezeigt werden soll, 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 
ho made his observations. However, in Ciona, at least, the cells which 
are destined to form chorda and mesenchyme (chorda-mesenchyme ring) 
must be regarded as taking part in a primary invagination along with 
the definitive endoderm. But plainly a very important part of the mo- 
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-muspular 
, 
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, tho 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 tho ectodermal mesoderm 
