190 Miscellaneous. 



The Bevslopment 0/ Daphnia /j-oki the Summer Ovum. 

 By J. Lebedinskt, Assistant at the University of Odessa. 



There are very few memoirs dealing with the embryology of the 

 Cladocera. Some of them have merely an historic interest*; the 

 others deal with nothing more than the external form of the embryo 

 at different stages, without giving any further details about the 

 internal processes of development t. The best treatise on the 

 developaient of the Cladocera is that by Dr. GrobbenJ, whose 

 observations were made upon Moina. Grobbon arrived at very 

 important results, which still serve as a basis for certain general 

 theoretical considerations on the laws of heredity. But even 

 Grobben's excellent investigations still loft gaps in the facts of the 

 Cladocerau development ; of the formation of the shell-gland our 

 knowledge is merely problematical, of that of the heart it is nil ; in 

 the same way the segmentation and " the origin of the separation of 

 the germinal layers in the blastosphere stage " still await the requi- 

 site elucidation. 



My investigations have been carried out upon Daphnia similis, 

 Cls, The animals were kept in aquaria, and deposited their ova at 

 the temperature of an ordinary room. The summer ovum of 

 DapTinia similis is perfectly spherical and •125 mm. in diameter; 

 it is enclosed in two membranes, the outer of which is a chorion, 

 the inner a vitelline membrane. The contents of the ovum consist 

 of (1) protoplasm, and (2) nutritive yolk. The latter is composed 

 of fat- and albumen-globules of different sizes, exhibits a concentric 

 arrangement, is of a green or blue colour, and renders the ovum 

 perfectly opaque. In every ovum there is found a large fat-globule, 

 always excentric in position, around which other smaller ones are 

 grouped. The protoplasm occupies a central position in the ovum, 

 and is represented by an amoeboid cell Avith lobulate nucleus, which 

 very greedily assimilates the surrounding yolk, increases in size, and 

 multiplies. 



Segmentation is superficial. The protoplasm alone divides ; no 

 furrows appear on the surface of the ovum, and the subjection of 

 the nutritive by the formative yolk during segmentation does not 

 take place. The amoeboid cell separates off a lump of plasma, 

 which lies at the periphery of the ovum, and in which a large 

 vesicle-shaped nucleus can be recognized; the lump of plasma is a 

 directive vesicle (according to "Weismann and Ishikawa). The 

 protoplasmic amoeboid cell, maintaining as before a central position, 

 divides into two equal daughter-cells, and each of these in a similar 

 way again divides into two, and so on. At stage 8 the directive 

 vesicle is still present ; but subsequently it is no longer recognizable. 

 I was not able to determine what becomes of it. 



* .Tmine, Ilistoire des Monocles, 1820. 



t Zaddach, 1854; Leydig, 18G0; Metschnilioff, 18CG; P. E. Muller, 

 1868; Dobm, 1870; Claus, 1870. 



J Grobben, Die Eutwicklimg der Moina rectirostn's, 1879. 



