OPHIURA BREVISPINA 421 



made from a specimen in which this epigastric invagination still 

 retains its communication with the exterior. This larva also 

 has the right anterior enterocoele in its proper position at the 

 side of the oesophagus. With this correction the developmental 

 history of the larva is the same as that formerly described. 



In Ophiura the coelomic structures are thus shown to be dif- 

 ferentiated morphologically not at one point from a single ag- 

 gregation of cells, as is the usual case in ophiurid development, 

 but at three points from three isolated groups of cells, as fol- 

 lows: one at the free end of the archenteron (the anterior pair 

 of enterocoeles), one in the midst of and on the ventral and 

 left side of the archenteron (the left member of the posterior 

 pair of enterocoeles and the hydrocoele), and one situated 

 entirely outside of the endodermal tract and within the ectoderm 

 (the right member of the posterior pair of enterocoeles). 



These observed differences between Ophiura and other ophiu- 

 rids in the places of origin of their evidently homologous coelomic 

 structures, and certain anomalous similar phenomena presented 

 by the developmental history of certain other echinoderms, 

 (text fig. 2, A-F), seem at first sight to indicate differences of 

 organization of a fundamental nature, which are opposed to 

 the idea of the genetic continuity of the group of echinoderms, 

 but, when viewed in the light of recent observations and experi- 

 mental results, they not only cease to present difficulties in the 

 way of unifying the group, but provide a basis for a further 

 advance in our knowledge of the fundamental organization of 

 the echinoderm egg. 



It was first shown by Morgan ('94) and later more definitely 

 by Boveri ('01) that the cytoplasm of the eggs of certain species 

 of sea urchin is differentiated into or contains different kinds 

 of material (formative stuffs), even before fertilization has taken 

 place, and that these materials become localized after the ma- 

 turation divisions into zones around the axis of the egg, one 

 zone occupying a position at and around the vegetative pole 

 and containing the material which is distributed during later 

 development to the mesenchyme cells; a middle zone of material 

 which is invaginated during gastrulation and distributed within 



