Miscellaneous. 69 



On the Development o/Amphioxus lanceolatus. 



By A. KOVALEVSKY. 



The knowledge of the development of Amphioxus lias certainly 

 hitherto been one of the most important desiderata of science. 

 Hence the investigations of M. Kovalevsky, incomplete as they may 

 be, and doubtful as may be some of his interpretations, are certainly 

 worthy of the greatest attention. 



The egg of Amphioxus is formed of a vitellus, surrounded by its 

 vitelline membrane. This vitellus has the appearance of an emulsion 

 of fatty corpuscles, and the germinal vesicle seems to be wanting in 

 it at the period of its maturity. 



The segmentation is complete and proceeds with great regularity. 

 As soon as repeated division has brought the number of segments td 

 thirty-two, we see in the interior of the ovule a cavity homologous 

 with the so-called cavity of Von Baer. Six or eight hours after depo- 

 sition the blastoderm presents at one point of its surface a slight 

 depression, which gives the germ approximately the form of a 

 hemisphere. This depression, increasing by degrees, soon causes a 

 restriction of the cavity of segmentation, which finally presents only 

 the appearance of a thin clear layer interposed between the two 

 cellular layers of the blastoderm. We may note, at present, that 

 the ceecal cavity formed by the depression just described must be 

 regarded as the primitive nutritive cavity, and that its single aper- 

 ture will afterwards become the anus. We may also remark that 

 the interior cellular layer represents the wall of the intestine, and 

 that the cavity of segmentation becomes transformed into the 

 perivisceral cavity. 



After the phase just described, the outer blastodermic layer 

 becomes covered with vibratile cilia, and the embryo begins to turn 

 slowly upon itself. In this state the embryo quits the egg, and at 

 the same time its movement of rotation becomes more intense. 



A couple of hours after exclusion the wide aperture, which places 

 the primitive alimentary cavity in communication with the outer 

 world, begins to narrow, in consequence of a multiplication of the 

 surrounding cells. At the same time the embryo becomes lengthened, 

 and henceforward presents the appearance of an elongated cylin- 

 drical larva. 



Upon this free larva there then makes its appearance a median 

 dorsal furrow, the margins of which, rising by degrees, finally unite. 

 Immediately after these dorsal ridges, some laminae, destined to be 

 transformed into lateral muscles, make their appearance. On the 

 following day a dorsal cord may already be distinguished below the 

 medullary tube. At the same time the mouth is formed at the 

 anterior extremity, as an aperture which penetrates from the outer 

 surface to the digestive sac. About the same period of larval life 

 there appears in the anterior part of the body a sort of notch, 

 which is soon covered with vibratile cilia, and constitutes the 

 olfactory organ. 



In the succeeding phnse appear the branchiae and the proble- 

 matic gland (liver?). The former are produced in a manner very 

 analogous to that in which we have seen the mouth to be formed : 



