198 THE ASCIDIANS. 



Formation of Medullary Tube and Notochord. 



Here, as in Amphioxus, the dorsal wall of the embryo 

 flattens, while the ventral remains convex, and the ecto- 

 dermic cells on the dorsal side become marked off from 

 the rest by their larger size and columnar shape. The 

 medullary plate extends nearly to the front end of the 

 embryo, while posteriorly its cells form a ring round 

 the blastopore. 



In the formation of the medullary tube, however, there 

 is an important difference, and the Ascidian embryo con- 

 forms in this point more to the mode of development 



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Pig. 98. Transverse sections through embryo of Clavelina Rissoana, to show 

 mode of formation of medullary tube and mesoderm. (After DAVIDOFF.) 



A. Through anterior region of embryo, with medullary groove still open. 



B. Through posterior region, with closed medullary tube. 



ch. Rudiment of notochord. ec. Ectoderm, en. Endoderm. mes. Mesoderm. 

 m.g. Medullary groove, m.t. Medullary tube. 



which is typical of the higher Vertebrates than does 

 Amphioxus. In the latter the medullary plate sinks 

 bodily below the level of the surrounding ectoderm, which 

 then grows over it. Subsequently while underneath the 

 ectoderm the medullary plate assumes the form of a 

 half-canal open towards the ectoderm, and eventually its 

 margins come together and so form a complete tube. 



In the Ascidian embryo the overgrowth of the surround- 

 ing ectoderm and the folding up of the margins of the 



