. Amphioxus and the ancestry of the vertebrates. Amphioxus; Sea squirts; Hemichordata. 112 DEVELOPMENT OF AMPHIOXUS. while the opposite side is flattened ; this is an indication that dorso-ventral differentiation has taken place, since the flattened side marks the dorsum or back of the embryo, while the convex side is ventral. It may be seen further that the blastopore is inclined towards the dorsal side of the embryo. The dorsal inclination of the blastopore is eminently characteristic of the vertebrate gastrula from the Ascidians up to the highest craniate forms. In the Inverte- brates (Anne
. Amphioxus and the ancestry of the vertebrates. Amphioxus; Sea squirts; Hemichordata. 112 DEVELOPMENT OF AMPHIOXUS. while the opposite side is flattened ; this is an indication that dorso-ventral differentiation has taken place, since the flattened side marks the dorsum or back of the embryo, while the convex side is ventral. It may be seen further that the blastopore is inclined towards the dorsal side of the embryo. The dorsal inclination of the blastopore is eminently characteristic of the vertebrate gastrula from the Ascidians up to the highest craniate forms. In the Inverte- brates (Annelids, Molluscs, etc.) the blastopore acquires a ventral inclination.* At the stage represented in Fig. 60 the embryo commences to ro- tate within the vitelline membrane, each ectodermic cell being now provided with a vibratile cilium. Fig. 61. — Elongated gas- . , trula. Optical longitudinal sec- The embryo next begms to elon- ^,^^_ xhe ciim are omitted gate, and the blastopore becomes ^^"^ *"= f °derm. (After ° ^ Hatschek.) still narrower (Fig. 61). A comparison of the accompanying figures will show that the narrowing of the blastopore is effected by the downward and backward growth of its dorsal border, while the ventral lip remains stationary. The dorsal ecto- derm, which is converted into the nicditUary plate, now shows indications of a shallow longitudinal groove. This is the beginning of the medullary groove which leads on to the formation of the central nervous system. * For a discussion of the phylogenetic relation of the blastopore or proto- stoma (Hatschek) to the mouth and anus, the following works should be consulted: Adam Sedgwick, Oh the Origin of Melameric Seginentcition, Quarterly Jour. Micro. Sc, XXIV., 1S84, and by the same author, Notes on Elasniobranch Development, lb. Vol. XXXIII., 1S91-92. Finally, Berthold Hatschek, Lehrbueh der Zoologie, Jena, Please note that these images are extracted from scanned page images that may have
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