Chap. H. AURELIA FLAVIDULA. 15 



from the earliest stages; but this is a very difficult matter to decide upon, because 

 they have a great degree of contractility and expansibility, and moreover they can 

 change their shape, at least after the walls have become defined. 



In the earliest stages after segmentation, when the embryo has a perfectly 

 globular form {Fig. 25), it swims about with a rolling motion, ever changing its 

 axis of rotation, and proceeds in a zigzag direction hither and thither, now and 

 then shooting off, for a short distance, in a straight line. In order to reach 

 the pouches of the proboscis, they must of necessity swim in a more definite 

 direction than this, and so we find that the majority of those which have arrived 

 there are more or less elongated in form : these swim very swiftly, and in a 

 direct course, with one end forward, and roll upon the longer axis. Not only are 

 the young ciliated before they leave the ovary, but also the outer and inner walls 

 are apparent {Figs. 26, 27, and 28 a h), and the digestive cavity {d) has begun to 

 form ; and others have become oval {Figs. 30, 81, and 32), and the incipient for- 

 mation of the mouth {Fig. 30 c) may be recognized by a depression at one end. 

 A few ciliated globular embryos reach the pouches; but, when compared with the 

 elongated forms, they may be considered as exceptional cases. 



After segmentation has thoroughly done its office, the embryo is endued with 

 a covering of vibratile cilia (PI. X". Fig. 25). These cilia are very short, and so 

 exceedingly delicate that they might readily escape the eye of the observer; and 

 in numbers they are fully equal to the cells of the outer wall. Whether each 

 cell is furnished with a single cilium, or not, we cannot say. Notwithstanding that 

 the embryo at this age swims, revolving on a changeable axis, we may see, by 

 the decided and appropriate motions of the cilia, varying according to the direction 

 in which the body proceeds, that volition has to do with every turn the sphere 

 makes. At one moment these cilia are all bent in one direction, and at the 

 next they stop their vibrations and throw themselves, as if by preconcerted signal, 

 to an opposite side; and then, the body assuming a new axis of revolution, they 

 go on with their vibrations until a new course is adopted. It can hardly be said, 

 that the embryo, whilst in this, the globular state, pursues any particular course; 

 but rather that it progresses along a zigzag, or an irregular spiral path, and rarely 

 darts off in a straight line. Now and then one is seen to go for a considerable 

 distance in one direction; but this happens when it is in the midst of the older 

 oval forms, which sweep it along in the current. In this way sometimes, but very 

 rarely, the youngest globular embryos reach the pouches. 



at page 21, lie would seem to show that these were germinal vesicle nor the germinal spot." At the 

 not in the egg state proper, for he remarks that same time, however, he describes the segmentation 

 after they reach this place he "could not find the of the yolk as taking place in the pouches. 



