236 



Beiträge zur Anatomie der Ophiuren« in the »Zeitschrift für wissen- 

 schaftliche Zoologie, 34. Bd. 1880« has distinguished however one of 

 them, the amptüla, with which the stone canal communicates, from the 

 axial sinus proper. This ampulla (probably of enterocoelic origin) is 

 already visible in my first stage. In the woodcut, Fig. 1, I have called 

 it sinus c and the two others sinus a and sinus h. Sinus a is formed 

 as an involution of the coelom underneath the rapidly growing rudi- 

 ment of the pseudo-heart; whilst h may be described as a chink bet- 

 ween the aboral part of the stone canal and the body wall. The later 

 history of these rudiments may be briefly related. Sinus c persists un- 

 changed, and never communicates with any other cavities than those 

 of the stone canal and the pore canal leading to the solitary madreporic 

 pore. Its size however is relatively less in the adult. Sinus h soon 

 closes, and grows round and embraces the basal part of the stone canal 

 and at a later period extends up on to the sides of the pseudo-heart. 

 Sinus a extends in a lateral direction as a tubular involution of the 

 coelom and forms the rudiment of the aboral »pseudohaemal sinus«. 

 It soon closes and the large cells in the aboral part of the pseudo-heart, 

 migrate along it forming the genital rachis. A similar occurrence has 

 been observed in Asterids by Cuénot in his «Contribution à 

 l'Étude Ana tomi que des Aste rides« in the Archives de Zoo- 

 logie Expérimentale. Tome V. supplémentaire. With regard 

 to the development of the genital organs, I have to confirm the guess 

 of Hamann (Anatomie der Ophiuriden, Jenaische Zeitschrift für Na- 

 turwissenschaft, 23. Bd.) that they are swellings of the genital rachis, 

 having traced all stages in the formation of the ovary. The testes are 

 formed a little later than the ovaries and develop rapidly , so that I 

 have obtained fewer stages in their development. 



The relations and distinctness of sinuses «, h and c I have veri- 

 fied on every series of sections I have examined : dealing only with 

 perfect series of well preserved embryos, the sections being in no case 

 thicker than 7V2 [J- These relations are shown in Fig. 2, where Ha- 

 mann' s »Urkeimzellen« are also clearly seen. Where proper precau- 

 tions are taken to ensure the penetration of the osmic acid, the struc- 

 ture of pseudo-heart appears to be that of a uniformly staining plasma 

 supported by a fibrous network with numerous cells. No lacunae are 

 visible. At its upper oral end it passes into the ordinary connective 

 tissue of the body wall and there is no trace of an oral haemal ring. 

 An aboral haemal ring in the aboral sinus certainly does not exist in 

 this species: the section of the sinus shewing only the »Urkeimzellen« 

 and surrounding endothelial cells. As to radial haemal vessels: from 

 an examination of most perfectly preserved and stained specimens it 



