i\l. E. Claparede on the Devdupiiient of Xeritiua iluviatilis. 15/ 



minute, and gradually increased in size, whilst the yclk-niass 

 disappeared in the same proportion. But they had exactly the 

 appearance of segmented eggs : they were yellow, transparen*^ 

 globules, 012-0*07 mill, in diameter, and appeared to be con.- 

 posed of a great many smaller spheres. The latter were per- 

 fectly transparent, their surface alone being sparingly sprinkled 

 with small, strongly refractive, vitelline granules. The sj)heres 

 presented the greatest resemblance to the egg of Modiolaria 

 marmorata iu the last stage of segmentation, as figured by Loven. 

 No vitelline membrane could be detected with certainty upon 

 them : when crushed, a sort of empty envelope was obtained, 

 but this deliquesced immediately. The small internal globules 

 (nuclei) presented a distinct contour, but they were not true 

 cells, contained no nucleolus, and, when pressed, broke up like 

 lumps of a gelatinous or fatty substance. Loven mentions the 

 same thing with regard to the nuclei of Modiolaria. 



The diameter of the spheres coutaiued in the capsules exactly 

 agreed with that of the mature eggs from the vipper part of the 

 oviduct; and it was scarcely probable that in their progress to 

 the vaginal orifice they would acquire fifty or sixty times that 

 bulk. New matter would certainly have exhibited a different 

 nature from the rest of the yelk, but the 45-GO spheres were 

 exactly similar. The capsules were therefore to be regarded as 

 containing numerous eggs. 



The development of the single embryo iu this case could not 

 take place in the way described by Koren and Dauielssen, as 

 the smallest embryos were not much larger than the segmented 

 eggs, so that the embryo must be formed in accordance with the 

 ordinary law, from a single egg. The embryo in the period of 

 rotatory movement is a round creature, ciliated on its whole 

 surface, with a diameter of 0*112 mill., or about the same as 

 that of a moderate- sized egg. It turned round within the vitel- 

 line membrane, which was now distinct, although soft ; its con- 

 tents formed an opake coarsely granular mass, a sort of emulsion 

 of larger or smaller fatty drops, in which the peripheric layer 

 was not cleai'Iy perceptible, probably from its being very thin. 

 In most known larvse of Mollusca, the rotating embryo is clothed 

 with cilia, which are either produced beneath the vitelline mem- 

 brane, through which they afterwards penetrate, or formed upon 

 that membrane itself; in Cardium, however, according to Loven, 

 the same thing occurs as in Neritina. The only embryo seen 

 in this condition died before breaking through the vitelline 

 membrane. 



The embryo developed from one egg devours the other eggs, as 

 was stated by Carpenter, iu opposition to Koren and Dauielssen, 

 to be the case in Purpura. In Purpura, as in Neritina, all the 



