TETHYODEA. 97 



Development. The segmentation is complete, and leads, according 

 to Kowalevski, to the formation of a blastosphere as in Amphi- 

 oxus (fig. 564). The wall of the blastosphere then begins to 

 invaginate. After the completion of the invagination the blastosphere 

 becomes a gastrula, with the remains of the segmentation cavity 

 between the ectoderm and entoderm (fig. 564, Fh}. The mouth of the 

 gastrula is at first wide, but soon becomes narrower and narrower, 

 until finally it becomes transformed into a small opening placed on 

 the dorsal surface at the hind end of the body. A flat median groove 

 on the ectoderm appears along the dorsal side of the already bilaterally 

 symmetrical embryo extending from the blastopore forwards. This 

 groove, into the hind end of which the blastopore opens, is the first 

 rudiment of the central nervous system. It is known as the medullary 

 groove. Its edges project and form the medullary folds which grow 

 round and close the narrow blastopore, and gradually fuse with one 

 another from this point forwards in such a manner as to convert the 

 groove into a canal, the walls of which separate from the external 

 ectoderm and give rise to the central nervous system. This canal is 

 known as the medullary canal : behind it is shut off from the exterior, 

 but communicates with the cavity of the gastrula (archenteron) by 

 way of the blastopore (fig. 564 c), which is now known as the 

 neurenteric canal ; while in front it remains open for some time. 

 Before these processes are completed two rows of the endoderm cells 

 of that part of the gastric wall which immediately underlies the 

 neural tube become different from the remaining endoderm cells and 

 give rise to the first rudiment of the notochord. The anterior pajrt 

 of the archenteron only gives rise to the pharynx and intestine 

 (fig. 564, e), while the posterior part furnishes the cell material not 

 only for the notochord, but also for the muscular system and the 

 blood corpuscles. It may accordingly be asserted that the meso- 

 dermal organs in the Ascidians arise from the entoderm, which is as 

 good as saying that the hinder half of the gastral sac has the value of 

 mesoderm. 



In the further course of development the somewhat elongated 

 spheroidal body grows out at the posterior and inferior end, opposite 

 to the blastopore and rather to the right,* into a tail-like prolonga- 

 tion, the axis of which is formed by the cells of the notochord (at this 

 period arranged in a simple row). The neural canal is prolonged into 



* In A. mammillata, according to Kowalevski, on the contrary, this growth 

 takes place at the other end towards the left, and therefore agrees with 

 that of Amphioxus. 



VOL. II. 7 



