Prof. Allman on the Hydroid Zoophytes. 49 



the free Medusa, except the marginal tentacles, ocelliform bodies, 

 lithocysts, and velum (?). 



In the female sporosac, the generative product originates as a 

 voluminous plasma, between the endoderm and ectoderm of the 

 manubrium. It is evidently in more intimate relation with the 

 endoderm than with the ectoderm, and as it increases in bulk 

 it would seem to cause the absorption of the latter membrane, 

 which confined it in its young state. A portion of it now becomes 

 detached from the mass, and soon undergoes a special develop- 

 ment into an embryo within the cavity of the sporosac. We 

 must undoubtedly look upon this detached portion as an ovum, 

 though I have never succeeded in demonstrating the presence of 

 a germinal vesicle. The phsenomenon of yelk-cleavage is also 

 very obscure, but the entire ovum may be easily broken up into 

 cells filled with secondary cells. 



The ovum lies in contact with the remainder of the plasma, 

 and while in this position becomes developed into an actiniform 

 embryo, as has been pointed out by Van Beneden and others. 

 In the act of development it becomes first extended as a disk 

 over the residual plasma. Next, from the circumference of the 

 disk, short and thick processes radiate all round, which soon 

 elongate themselves into tentacula. The disk at the same time 

 gradually becomes more gibbous on the side turned away from 

 the axis of the sporosac ; its interior becomes hollowed out into 

 a stomach ; and a mouth makes its appearance in the centre of 

 the opposite side, or that in contact with the plasma. The embryo 

 now retreats from the plasma, the mouth is seen to be elevated 

 on a conical prominence, and a second circle of tentacula soon 

 makes its appearance immediately around it. In this state it 

 escapes from the sporosac, and after continuing free for a period 

 it ultimately developes, from the side opposite to the mouth, a 

 cylindrical stem, which soon clothes itself with a polypary, and 

 fixes the young Tubularia to some neighbouring object. 



While embracing the residual plasma in the sporosac, we are 

 strongly reminded of the relation which subsists between the 

 embryo and food-yelk, in those animals in which the embryo is 

 developed from only a portion of the yelk. The analogy, how- 

 ever, is not so close as it may at first appear ; and there is no 

 evidence that the residual plasma is absorbed by the embryo 

 during its development. 



After the escape of the embryo, or even during its develop- 

 ment within the sporosac, the remains of the plasma may still 

 throw off portions which become developed in a similar way into 

 free embryos. 



In the male sporosacs, the spermatogenous tissue is manifestly 

 composed of very delicate tubules, which are attached by one ex- 



Am,^ Mag, N. Hist* Ser. 3. VoUvf. 4 



