^16 EMBRYONIC DEVELOPMENT 



and before they become epithelian, tlien, still in possession of their own cellular lite, 

 they, for a time, will constitute a little spheroid, which will acquire vibrillfe, m 

 accordance with ihe organic law of development of a large portion of the animal 

 kino-dom. These little bodies are the so-called CosmeUa arachnoides, (]escnhed by 

 by Alex. Nordmann as parasitic animals in the ovum of Tergipes edwardsii, a nu- 

 dibranchiate mollusc*. When this spheroid is of a certain size, it sometimes happens 

 tl'.at a small embryo issues from it, with a shape altogether similar to the embryos 

 formed by the entire mass of the yolk. 



It is my intention to dwell more at length upon these loose cells in another Paper, 

 now in preparation, and in which their physiological signification will be thorough- 

 ly examined. 



X. THE EMBRYO. 



§, 1. Between the period of the division of the yolk and that of the first manifesta- 

 tion of the embryo, there is a period of apparent repose, which lasts from four to five 

 days. The vitelline sphere seems to undergo no process of any kind ; the labor is 

 undoubtedly beyond the reach of our investigations, for, that something is going on 

 in, it cannot be questioned for a moment: life once started, keeps acting until it ceases 

 definitely its actions. During this period the vitelline mass, from opaque (figs. 46 

 and 47), acquires two transparent diameters (fig. 4S), wMiich are transformed into 

 four transparent internal cavities (fig. 49), sometimes a single, but larger central cavi- 

 t}^ is ol)served (fig. 50), which is less distinct in others (fig. 52 and 53), probablv in 

 the latter the embryonic substance has not yet acquired all its transparency: a phe- 

 nomenon at all events taking place during this period of rest and preceding that of 



motion. 



§, 2. For, now the embryo begins to move, the most wonderful sight which an ob- 

 server may behold. When numbers of them are witnessed at once, some may be seen 

 revolving around an ideal axis forty turns a minute, whilst in others the movement is 

 scarcely perceptible. And between these two extremes of motion, there are all in- 

 termediate degrees of velocity. According to the will of the animal, the velocity may 

 be increased or diminished, so that the same individual moves alternatively with dif- 

 ferent degrees of velocity. When passing from one degree into another, this docs 

 not take place according to the mechanical law of uniform acceleration, but the tvro 

 extremes may succeed each other in both directions. The axis of rotation is changing 

 as the form of the vitelline sphere itself temporarily changes, under the inces.sant 

 contractions of the embryonic mass. 



The direction, or movement of revolution is sometimes from right to left; in others, 

 from left to right, in an horizontal plane. In some others again, that movement is 



* Essai d'unc monographic du Tergipes edwardsii. — Ann. Sc. Nat. 3d Ser. V. 1S4C, 109, I'l. I. 



