60 HUBERT LYMAN CLARK ON 



closing of the anus. With the anus open, each muscular contraction of the animal 

 would tend to either force water out or draw it in through these openings, and in 

 that way spermatozoa could easily get into the body-cavity and thus fertilization 

 could take place within. This will appear more probable when it is remembered 

 that the animals are very social, and that the water around a mangrove root on which 

 there are hundreds of them, must contain countless spermatozoa. There is also a 

 possibility that spermatozoa enter the body-cavity, through the water-pore and stone- 

 canal, which, as we shall see, remain open in the adult, and also open into the body-cavity. 

 After fertilization a membrane forms around the egg and segmentation begins. It 

 seems probable that the extrusion of the polar bodies occurs before the formation of this 

 membrane, as Selenka ('S3) found no trace of them in the segmenting eggs of 8. digitata, 

 and I could not find them in any of the eggs of 8. vivipara which I examined, although 

 they were looked for with special care. Segmentation and the formation of the blastula 

 occur in practically the same manner as has been so well figured by Selenka ('83) for 

 S. digitata. The first plane of division forms two blastomeres of equal size and appear- 

 ance (Fig. 2). After a resting period of about twenty minutes, the second plane of 

 division occurs at right angles to the first, giving rise to four similar blastomeres (Fig. 

 3). The third plane is at right angles to the first two, and we now have an embryo of 

 eight equal cells with a segmentation-cavity between them (Fig. 4). The sixteen-cell 

 stage (Fig. 5) soon follows, the division plane being at right angles to the preceding. 

 The appearance of the embryo at this stage is very peculiar and characteristic, the cells 

 being arranged in a band or ring and the segmentation-cavity being open at each pole. 

 Another plane of division, again at right angles to the preceding, doubles the width of 

 the band and decreases the openings at the poles, but it does not divide the cells exactly 

 equally, so that the upper- and lowermost rows are of somewhat smaller cells than the two 

 middle rows and have a less diameter (Fig. 6 and 7). The subsequent divisions occur 

 with a fair degree of regularity in alternating plane*, each division decreasing the 

 openings at the poles until at last they are entirely closed. This occurs when the embryo 

 consists of approximately 256 cells, and so the blastula is formed (Fig. 8). The cells of 

 the four equatorial rows are somewhat larger than the rest, but the difference is not at 

 all noticeable and apparently has no significance. The divisions have followed on each 

 other with great rapidity so that the complete blastula is formed after about four hours, 

 while in S. digitata, according to Selenka ('83), the blastula is the result of twelve hours' 

 growth. It is for this reason, that I am inclined to think that the whole process of devel- 

 opment in S. vivipara is very rapid. 



