Miscellaneoua. )\'2.\) 



MISCELLANEOUS. 



Note on the Emhryogeny o/Salmacina Dysteri, Huxley. 

 By M. A. GiARD. 



The ovarian ovum of Sahnacina Dysten presents a transparent 

 vesicle containing, besides the niicleolus, a fine network of proto- 

 plasm analogous to that which has been described by 0. Herwig in 

 Toxopneustes lividus ; I have observed the same reticulum in the 

 ovular nucleus of Lamellaria perspicua. The egg when deposited 

 remains in incubation under the mantle of the adult and then 

 undergoes the first phases of its evolution. This egg possesses a 

 vitellus of a fine currant-red colour and a veiy distinct vitelline 

 membrane. After fecundation the germinal vesicle ceases to be 

 visible, and at one point of the surface of the egg we see appear a 

 finely granular circular spot, opposite to which we observe two polar 

 globules. The latter indicate the pole of the egg at which the 

 exodermic elements will subsequently be produced. The spot disap- 

 pears in its turn, and the egg undergoes a constriction, whicli is less 

 strongly marked on the side where the spot was than on the other 

 side. Towards the summit of each of the two halves of the egg, on 

 the side where the separation is best marked, stars are seen similar 

 to those described by Flemming in the segmentation of the egg of 

 Anodonta, and by other authors b). a great number of animals. 

 Soon there are formed, in place of the stars, nuclei situated at the 

 upper part of the globes which have become spherical. Each 

 nucleus is surrounded by a tolerably extensive zone of finely granu- 

 lar formative vitellus. The egg then divides into four equal spheres, 

 two of which touch each other, separating the two others, and thus 

 forming a cross. At the stage of 8 the plastic elements separate 

 from the nutritive elements and give origin to four small spheres 

 situated in a plane superior to the four mixed spheres and alter- 

 nating with the latter. The four little spheres are the first rudi- 

 ments of the exodemi ; the pole at which they are situated corre- 

 sponds to the ventral surface of the future embryo. 



The diff'erence between the segmentation of the ovum of Scd7na- 

 cina and that which has been described in other Annelides by 

 Clapard'de, Metschnikoff, and Hackel is the same as between the 

 segmentation of the ovum of numerous Eolidida) (KoUs aurantiaca, 

 A. & H., for example) and that of Purpura lupillus (Selenka) or 

 that of Drachionun (Salesky). The multiplication of the exodermic 

 elements is much more rapid than that of the nutritive spheres ; 

 nevertheless the latter increase in number, and the plastic part 

 contained in each of them becomes less and less considerable. Soon 

 an invagination is produced on the nutritive side, at the same time 

 that the epibolism of the exodermic elements completes the consti- 

 tution of the gastruhi. The prostoma {blastopore of Ray Lankester ) is 

 at first widely open ; but it soon becomes contract<Kl. Its contour i.s 

 not perfectly circular, but there exists at one point an emarginatioii 

 which is continued by a furrow of the exoderm. This furrow by 



Ann. tl; May. N. Ilist. Ser.4. Vvl. xvii. 22 



