48 Hjalmar Théel, 



of the sides of the stomach, and subsequently each of them presents 

 indistinctly three portions, an anterior, a median and a posterior, PL 

 VI, fig. 89 — 91, which are at first in open communication one with the 

 other, each vaso-peritoneal vesicle forming a single cavity. Even in 

 this stage, the left vesicle communicates directly with the exterior bv 

 the madreporic canal and the water-pore situated on the dorsal surface 

 and in its middle-line. I never was able to observe that the portions 

 of the vesicles separated completely, so as to form paired cavities as is 

 pronounced by Metschnikoff ') and Bury ^) to be the case in other 

 Echinoids. According to my experiences the right vesicle or rather 

 enterocœl remains undivided and its different portions never present 

 themselves very distinctly. The left vesicle on the contrary shows 

 clearly three portions, the middle one of which increases, separates gra- 

 dually and changes into the hydrocoel, while the remaining portions be- 

 come the left enterocœl. At first the madreporic canal appears to com- 

 municate with the anterior portion of the left vaso-peritoneal vesicle, 

 PL VI, fig. 89, but during the increase and separation of the hydrocoel 

 it passes over to this, PL VI, fig. 91. 



The first indication of the young sea-urchin presents itself in the 

 same manner as described by Agassiz ') and subsequently by Metschni- 

 koff *). In an early stage of the Pluteus, when the two posterior 

 dorsal arms of the larva begin to protrude, an ectodermic invagina- 

 tion takes place on the left side between the bases of the ventral and 

 dorsal posterior arms, PL F/, fig. 89 x. The invaginated portion soon 

 assumes the shape of a small bottle, the external aperture having been 

 considerably narrowed, PL F/, fig. 91 x., and its free expanded end grows 

 towards the hydrocoel in order to meet it. While this is going on, the 

 hydrocoel itself also increases and is finally placed side by side with 

 the invaginated sac, which has attained a noticeable size and gradually 

 become compressed so as to change into a rounded disc built up of 

 two layers, of which the under one is very thick while the upper is thin 



1) Embryologische Mittheihingen über Echinodermeu. Zool. Anzeiger. VII. 

 Leipzig. 1884. N:o 159. p. 62—65. 



2) Studies in the Embryology of the Echinoderms. Quart. Journ. Microsc. 

 Science. New Series, Nio 116. London. 1889. p. 412—414. 



3) On the Embryology of Echinoderms extr. from the Mem. of the Amer. 

 Acad. IX. 1864. p. 7. 



4) Studien über die Entwickeluug der Echinodermeu und Nemertimen. 1869. 

 p. 41—49. 



