149 



with the keel an oval , circular, or irregular shaped disc of cells, of 

 from one to two thirds the diameter of the egg. The edges of the disc 

 grow down into the yolk so that they are sharply defined all around. 

 Either wholly or partly within this area, or upon its edges the embryo 

 is formed. Sections through the disc show that it consists of, for the 

 most part, a single layer of large columnar or cubical cells, gorged 

 with yolk. The edges bend down and grow into the yolk, entirely in- 

 dependent of the general epithelium which covers the rest of the egg. 

 The edges of each side unite in front and behind, so as to form a cul- 

 de-sac at each end of the disc, and the whole structure represents a 

 flattened bug, partly buried in the yolk , and in communication with 

 the latter by an opening below. Sometimes the whole disc sinks into 

 the yolk , so that the nauplius embryo is nearly covered by yolk mate- 

 rial. This recalls the condition of the insect embryo after the amnion 

 and serosa are formed. Throughout the whole extent of the embryo 

 there is a meteoric cloud of spore-like fragments, the degenerating 

 nuclei, which pass into the yolk and are absorbed. 



Degenerating nuclei occur in the fully formed egg-nauplius in 

 various parts, but are most noticeable perhaps in the region of the 

 stomadaeum and optic discs. I believe it will be found tliat the frag- 

 mentation and dissolution of cells is a common phenomenon among 

 the Crustacea and other Arthropods. I have already observed it in 

 several Decapods, and it seems to me quite possible that »secondary 

 mesoderm celisi which have been described in the Crayfish, may be 

 explained, in part at least, as cases of cell-degeneration. In the pecu- 

 liar »dorsal organ « of the lobster which I have described in a former 

 paper, many of the cells proliferate and then break up into degenerat- 

 ing products precisely as in Alpheus Saulcyi. The history of the de- 

 generating cells is very similar in the two forms. 



3. Die Entwicklung der Daphnia aus dem Sommereie. 



Von J. Lebedinsky, Assistent an der Universität Odessa. 



eingeg. 28. März 1891. 



Der Arbeiten, welche über die Cladocerenentwicklung handeln, 

 sind sehr wenige. Einige von ihnen haben nur ein historisches Inter- 

 esse ì ; die anderen betrachten nichts mehr als die äußere Gestalt des 

 Embryo in verschiedenen Stadien, ohne etwas Weiteres über die inne- 

 ren Entwicklungsvorgänge zu geben 2 . Die beste Abhandlung in Be- 



1 Juri ne, Histoire des Monocles. 1820. 



2 Zaddach 1854; Leydigl860; M et schnikoff 1866; P.E.Müller 

 1868; Dohrn 1870 ; Claus 1876. 



