UROCHORDA. 



FIG. 8. VARIOUS STAGES IN THE DEVELOPMENT OF PHALLUSIA MAMMILLATA. 



(From Huxley; after Kowalevsky.) 

 The embryos are represented in longitudinal vertical section. 



I. Commencing gastrula stage, fh. segmentation cavity. 



II. Late gastrula stage with flattened dorsal surface, eo. blastopore; ch. noto- 

 chord ; dd. hypoblast. 



III. A more advanced embryo with a partially-formed neural tube, ch, and dd. 

 as before; n. neural tube; c. epiblast. 



IV. Older embryo in which the formation of the neural tube is completed, dd. 

 hypoblast enclosing persistent section of alimentary tract; dd' . hypoblast in the tail ; 

 m. muscles. 



V. Larva just hatched. The end of the tail is not represented, a. eye; gb. 

 dilated extremity of neural tube with otolith projecting into it; Rg. anterior swelling 

 of the spinal division of the neural tube ; f. anterior pore of neural tube ; Rm. posterior 

 part of neural tube; o. mouth; Chs. notochord; kl. atrial invagination ; dd. branchial 

 region of alimentary tract ; d. commencement of oesophagus and stomach ; dd' . hypo- 

 blast in the tail ; m. muscles ; hp. papilla for attachment. 



VI. Body and anterior part of the tail of a two days' larva, kirn, atrial aperture; 

 en. endostyle; ks. branchial sack; iks, iks. branchial slits; bb. branchial vessel 

 between them; ch. axial portion of notochord ; chs. peripheral layer of cells. Other 

 reference letters as before. 



