UROCHORDA. 



11 



fy. 



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



(From Huxley; after Kowalevsky.) 

 Tlie embryos are represented in longitudinal vertical section. 



I. Commencing gastrula stage, fh. segmentation cavity. 



II. Late gastrula stage witla 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; 

 in. muscles. 



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

 extremity of neural tube with otolith projecting into it ; Eg. anterior swelling of the 

 spinal division of the neural tube ; /. anterior pore of neural tube ; Km. posterior part of 

 neural tube; o. mouth; Chs. notochord; A-/, atrial invagination ; dd. branchial region 

 of alimentary tract; d. commencement of oesophagus and stomach; dd'. hypoblast in 

 the tail; in. muscles; lip. papilla for attachment. 



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

 en. endostyle; Its. branchial sack; Iks, 2ks. branchial slits; bb. branchial vessel between 

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

 letters as before. 



