512 BULLETIN 82, UNITED STATES NATIONAL MUSEUM. 



first and divides the four cells in such a way as to produce four larger and four 

 smaller daughter cells. In appearance the embryo now agrees entirely with the 

 normal eight-cell stage; but it has arisen through the appearance of what is 

 normally the third cleavage furrow in advance of the first and second. 



Among perfectly formed blastulse of eight and a quarter hours Seeliger found 

 an embryo of very extraordinary structure. Its markedly elongated form called 

 attention to it at once. Further examination showed a very large number of cells 

 and two gastrulalike invaginations, the two blastopores being slitlike and almost 

 at right angles to each other, at the blind ends of which a number of mesenchyme 

 cells had been given off. Apart from the double invagination, development had 

 progressed twice as fast as normally, for the embryo had reached a stage similar 

 to that of embryos of 16 to 17 hours. Seeliger suggests that the condition of this 

 embryo may be explained through double fertilization. 



On very many embryos of between 13 and 17 hours Seeliger found, usually 

 directly opposite the blastopore, though sometimes nearer it, a second invagination 

 just like the first, which extended inward as far as the blind end of the primitive 

 gut, in some cases even pushing this out of position. This second invagination 

 did not give rise to mesenchyme cells. The embryos themselves were of the 

 usual spherical form. At the 24-hour stage this second invagination had invariably 

 disappeared, and from that point onward the embryos developed normally to 

 free-swimming larvae. 



Among 60-hour embryos of normal form Seeliger found a number which 

 were remarkable in that they possessed external tentacles, thus resembling that 

 described by Busch, except that the tentacles were shorter and stiffer. Some 

 retained the ciliated bands and swam about freely in the water, while others, 

 which had already lost the cilia, were picked up from the bottom of the vessel. 

 The intestinal canal was much more extensive than in the normal individuals and 

 was entirely filled with a mass of cells undergoing dissolution. The left ccelomic 

 sack was here the one which was asymmetrically developed. The branch uniting 

 the two horseshoe-shaped side pieces in the posterior section ventrally had pushed 

 itself in between the gut and hydroccele. The most striking point of departure 

 from the normal was the possession of ventral tentacles. The vestibular invagina- 

 tion here appeared as a shallow groove, on both borders of which the tentacles arose 

 as peglike structures. The hydrocojle had not yet closed into a complete ring; 

 on its ventral side there arose five groups of three evaginations each, of different 

 lengths; the longest of these filled the ectodermal tentacle evaginations, while the 

 shorter at most produced exteriorly insignificant nipplelike elevations between the 

 tentacles. The ectodermal covering of the evaginations of the hydroccele is com- 

 posed of a single layer of cells, prismatic in form, with the nuclei near the middle 

 or in the outer half. 



Others of equal age, in which the ciliated bands had disappeared, had the gut 

 extended and balloonlike, but without the included mass of disintegrating cells; 

 into the lumen at various places pseudopodialike processes extended. It was only 

 on the ventral side, in the immediate vicinity of the vestibular groove, that the 



