a2 



extending forwcird (iig. 4, cJi.). This baud separates off 

 from the liypoblast, which closes in beneath it, and thus 

 the notochord is formed (fig. 5, ch.). The same cells further 

 laterally and posteriorly become mesoblast, and separate 

 off as lateral plates which show no trace of metameric 

 segmentation (fig. 7a, m.h.). The remainder of the archen- 

 teron will become the branchial sac, and by further growth 

 bud off the rest of the alimentary canal. 



The medullary groove now becomes converted into the 

 closed neural canal by the growing up and arching inwards 

 (fig. 7a, n.c.) of the laminte dorsales, which unite with one 

 another from behind forwards in such a way that the 

 blastopore now opens from the enteron into the floor of 

 the neural canal, forming the neurenteric passage (fig. 5, 

 ii.e.c). For a time the anterior end of the neural canal 

 remains open as a neuropore. The posterior end of the 

 body is now elongating to form a tail, and the embryo is 

 rapidly acquiring the tadpole shape (fig. 6) characteristic 

 of the free larva. 



The tail grows rapidly, curves round the body, and also 

 undergoes torsion so that its dorsal surface comes to lie 

 on the left side. It contains ectoderm cells on its surface, 

 notochordal cells (in single file) up the centre (see fig. 7, 

 n.ch.), a neural canal dorsally, and a row of endo- 

 derni cells (/«?/.) representing the enteron ventrally to the 

 notochord. Later on the mesoblast also is prolonged into 

 the tail where it forms a band of striated muscle-cells at 

 each side of the notochord. When the ectoderm cells 

 begin to secrete the cuticular test it forms two delicate 

 transparent longitudinal (dorsal and ventral) fins in the 

 tail (fig. 7), and especially at its extremity where radial 

 thickenings form striae resembling fin rays. The ecto- 

 derm on the anterior end of the body grows out into three 

 adhering papillae (figs. 8 and 9). 



