FREE-SWIMMING SPOROSACS OF THE HYDROID GENUS DICORYNE. 275 



his theory. But Chun's statement in the passage quoted above will not bear this 

 interpretation. Indeed, the fact that he described the tentacles as occurring at the 

 aboral pole shows that he did not regard them as marginal. Unfortunately, Chun does 

 not show how his "complete medusoid" homology is to be applied in detail to the 

 sporosac, nor can we, after examination of his view, offer any interpretation which 

 will bear even the semblance of probability. If the sub-umbrellar cavity is closed and 

 has become filled with sexual products, then the ova and sperms should lie between 

 ectodermal layers, the inner one representing the ectoderm of the manubrium, and 

 the outer one either the ectoderm of the whole umbrella or that of its inner surface. 

 But there is no such complication in the sporosac of Dicoryne, nor in the develop- 

 ment is there any trace of a medusa rudiment (Glockenkern). Apart from the 

 impossibility of applying Chun's theory in detail to the cell-layers, there remain the 

 tentacles to be accounted for. With complacency Chun leaves them arising from 

 the roof of the supposed umbrella, offering no explanation of- so unlikely a condition. 



Carlgren (1909, p. 11) adopts Chun's view of the relationship of the sporosac 

 without adding to our knowledge of its interpretation. 



A third suggestion as to the possible medusoid homology of the sporosac of 

 Dicoryne has been made by Delage and Herouard : " On pourrait plutot admettre 

 que le corps n'est forme que par un manubrium nu, que l'ombrelle n'existe pas et que 

 les tentacules ont ete reportes a la base du manubrium par la regression de 

 l'ombrelle. . . . Des lors, le corps reproducteur serait non un simple sporosac, mais 

 un gonophore, puisqu'il aurait un rudiment d'endoderme ombrellaire " (1901, 

 pp. 64, 65). 



As we have seen above, however, the ectoderm of the sporosac of Dicoryne does 

 not correspond in mode of origin to the ectoderm of the manubrium of a medusa, and 

 there is no rudiment of umbrellar endoderm, nor, indeed, of any portion of an umbrella. 

 The view of Delage and Herouard is therefore entirely unsupported by the facts of 

 development and the structure of the fully formed sporosac of Dicoryne* 



The sporosac of Dicoryne (taking the female as an example) is not a preformed 

 structure into which migrate comparatively small oocytes, but is an elevation first of 

 ectoderm, later of both layers, following upon and seemingly called forth by the 

 presence at that point of a large and growing oocyte. 



Even in gonophores so reduced as those of Clava squamata (Harm, 1902) a 

 " Glockenkern " is present for some time, and subsequent to its disappearance a 

 primary endoderm lamella is found between the oocyte and the ectoderm enveloping 

 the gonophore. Such an endoderm layer is not visible in our preparations of the 

 early sporosacs of Dicoryne. Further, it may be noted that the ectoderm of the 



* Another argument of general bearing may be adduced against the homology of the sporosac of Dicoryne with a 

 manubrium. In the regression of medusie and medusoid gonophores the marginal tentacles are first lost, then the 

 manubrium suffers reduction or disappears, and only a hint of the medusoid rudiment is apparent. It is therefore 

 directly contrary to the general course of events to suppose that the manubrium should survive the umbrella, and 

 still more difficult to imagine that a marginal tentacle should remain after loss of the umbrella. 



