368 CEPHALOCHORDA TA. 



DIAGRAM XX. 



Amphioxus. 



At the top is a figure of the entire animal (after Ray Lankester), 

 showing thenotochord {notoch.), the tentacles {tent.) around the mouth, 

 the reproductive organs {rep.), the myotomes, atriopore, anus, and fin- 

 rays. 



Below is a cross section (after Ray Lankester) showing the spinal 

 cord {sf. c), the notochord {notoch.), the pharynx {phar.) with dorsal 

 groove {d. g.), ventral groove {v. g.), and perforated walls, the " liver" 

 caecum {ciec), the atrium, the reproductive organs {rep.), the atrio- 

 coelomic canals, the lymph space (/. sp.) in the metapleural folds. 



Figs 1-8 (after Hatschek) illustrate the development. 



1. A ripe ovum with extruded polar globule. 



is. The stage with four cells, seen from above. 



3. The blastosphere, showing slightly larger cells at the inferior pole. 



4, 5. Sections of the blastosphere. 



6. Invagination, 



7. The gastrula ; arc. the archenteron. 



8. A cross section of the embryo, the ectoderm is the interrupted dark 

 line, the endoderm is solid black, the mesoderm is light. The medullary 

 or neural canal {m. e.), the archenteron {arc), the coelomic pouches 

 {c. p.), the incipient notochord {notoch.), are shown. 



9. A longitudinal section of the embryo, the head end {H.), the tail 

 end {T.), the medullary or neural canal {m. c), the archenteron {arc), 

 the neurenteric canal («. c. c), the mesodermic segments {m. s.), are 

 indicated. 



