MEM. M. C. Z., IX. No. 2. — ECHINODEEi\LA.TA. 13 



PLATE III. 



Develop77ieut of Opuwnoiviisk. Figures from li,hi\s Metschnikofp, Hubert Ludwig, Alexander 

 Agassiz, T. H. Stewart, Max Schultze, and August Krohn. 



(1-20, 26-33, Viviparous Ophiurans.) 



1-14. AmpMura squamnla. From E. Metschiiikoff, Studien iiber die Eiitwickehmg der Echinoilei-mi:-ii und 

 Nemertinen. Mem. de I'Acad. Imp. des Scien.de St. Pe'tersbourg, VIP Serie, XIV. No. 8, Hs. III. B, 

 IV., 1869. 



b I. Blastoderm. o. Mouth. 



c. Cutis. c. CEsojihagus. 



: c. Provisional limestone rods ; probablj' the ho- p a. Dorsal pore of the water-system (madreporite). 



mologue of the Pluteus rods. p I. Eemnant of rudimentary Pluteus. 



cp I. The bilateral embryonic skeleton. t. Tentacles. 



. c s. Segmental cavity. v. Kudimentary water-system. 



d'. Left lateral disk. v a. Water-system. 



d-. Plight lateral disk. v t. First trace of digestive cavity. 



e})- Epidermis. vv. Lobed wjtter-system. 



me. Thin outer membrane surrounding blastoderm. op,vr. Circular canal. 



VI V. Thick inner membrane of same. 



1. Egg, surrounded by its two membranes (m e, mv) with the blastoderm (b I). 



2. The same in profile : the outer egg membrane is not figured. 



3. Somewhat more advanced stage, showing, in addition to the blastoderm, the first indication of the digestive 



cavity {vt) and of the large cells of the cutis (c). 



4. Elongated embryo still protected by the inner egg membrane (m r). The deposition of the provisional lime- 



stone rods has commenced {cc). On each side of the future cesophagus are placed the rudiments of the 

 'water-S3'stem (v). 



5. Somewhat older embryo, with longer limestone rods {cp I), seen in profile. 



6. Still older embryo, seen from the dorsal side, showing the greatly enlarged water-system (crOand its two 



lateral disks (rfi, d-). 



7. Somewhat older embryo, also seen from the dorsal side, showing the five lobes of the water-system {v a). 



8. Older stage, remarkable for the great development of the lateral disks (rf', d-), the change in the position of 



the digestive cavity and cesophagus, the great increase in the size of the lobes of the water-system (v v), 

 and the formation of a dorsal pore {pa), the future madreporic body. 



9. Somewhat older embryo, showing the horseshoe-shaped water-system, each primary fold of wliich {vv) has 



subdivided into four secondary lobes. 



10. Embryo seen in profile. The cesophagus is already surrounded by the water-system ; each fold of the water- 



system has subdivided into five smaller ones. The stone canal is most distinct. 



11. An embryo about in the stage of fig. 10, seen from the dorsal side : the provisional limestone skeleton is 



already disappearing. 



12. The water-system of the same embryo : v v, the blind sacs (tentacles) of the water-system ; v r, the circular 



canal ; 2> c-, the madreporite. 



13. The embryo in a stage immediately after passing from the bilateral to the pentngonal form, seen from the 



actinal side ; the peculiar arrangement of the tentacles (0 and the formation of the mouth skeleton are 

 specially to be noticed in this stage. 



14. Somewhat more advanced than the preceding stage, seen from the dorsal side, showing the si.\ reticulated 



plates of the abactinal surface, as well as the stem (p I), first described by Kiohn and Schultze, forming a 

 temporary connection with the ovary. This becomes atrophied in still older stages. 



