16 



EMBRYOLOGICAL MONOGRAPHS. 



PLATE IV. 



Development of the TRACiiYMEDUSiE. Figures from Hermann Fol, John McCeady, Elias 

 Metschnikoff, Ernst H2eckel, and B. Uljanin. 



Figures 11, 12, 13, 19-27, 29, 30, were arranged by the author; the remainder were arranged by A. Agassiz. 



1-10. ruhjxcnia Icucoslijla. From Metschnikoff, Studien iiber die Entwickelung der Medusen und Siphonojihoren, 

 Zcit. f. Wiss. Zool., Vol. XXIV. PI. III. figs. 1, 2, 3, 4, 5, 7, 8, 9, 10. 



1. Free ovum just dropped in the water. 



2. Ovum with four segments. 



3. Segmented egg with eight segments. 



4. Segmented ovum more advanced. 



5. Morula. 



6. A ciliated larva with an external (epiblastie) layer, and a spongy mass of vitelline cells. 



7. An older larva, the body of which has beconu', veiy much elongated, and the extremities are about to become 



tenta<-les. hli. c. Hypoblast of the central region of the body. hb. ta. Hypoblast of the tentacles. Tlie 

 dillVrenee between the cells in the two regions is evident. 



8. Larva still ohlci', in which two tentacles are formed. 



y. Larva tlireo days old, still ciliated, showing two long tentacles and the 'beginning of a new pair (/n-). A 

 gastro-cavity is seen in the middle of the larva below the new tentacle. 

 10. Larva four days old, with four t«ntaclcs, a well-marked stomach, and mouth. In older larvie ol Pulyj-oiia 

 foin- otocysts, ench alternating with the tentacles, are next formed ; and later, au increase ol the uamber 

 of tentacles, unlil the adult form is reached. 



