UROCHORDA. 



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Fig. 8. Vakious 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 partiaUy-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; gh. 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 ; llm. posterior part of 

 neural tube; o. mouth; C/is. notochord; kl. atrial invagination; dd. branchial region 

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

 the tail; m. muscles; hj>. papilla for attachment. 



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

 en. endostyle; ks. branchial sack; lAs, 2/i«. branchial slits; hh. branchial vessel between 

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

 letters as before. 



