ZOOLOGY AND BOTANY, MICROSCOPY, ETC. 35 



to the end of March, males and winter eggs were found. The season 

 corresponds to the late autumn of Europe, and the fact indicates that, 

 as in European forms, an autumnal breeding season occurs. 



Fresh-water Ostracods of Germany.* — Herr G. W. Midler com- 

 pletes his monograph, dealing with Cypridffi, Cytheridse, Darwinulidfe. 



Development of Leptodora hyalina.f — -Max Samter has undertaken 

 the study of this development, in the hope of settling some of the con- 

 troversies in regard to the phylogeny of the Cladocera. The present 

 instalment of the work includes an account of the development up to 

 the time when the embryo quits the vitelline membrane and begins life 

 on its own account. Like Weismann the author was unable to stain 

 the nucleus of the unsegmented egg deeply, and found that it appeared 

 as a light spot in all his preparations. Eroru the characters of the egg 

 and the nature of the gastrulation process, the author believes that the 

 yolk in the egg is a recent acquisition which has produced remarkable 

 modifications of the process of development. In the unsegmented egg 

 there is a nearly central mass of protoplasm containing the segmenta- 

 tion nucleus, and sending out protoplasmic prolongations through the 

 envelope of yolk in all directions to a peripheral layer of protoplasm. 

 The polar body was not clearly made out, though the author noted 

 appearances similar to those observed by Weismann in Daphnids. The 

 first and second segmentation furrows are internal, but at the eight- 

 cell stage the products of division approach the surface and fuse with 

 the superficial protoplasm. As division proceeds, the egg-surface shows 

 scattered cells connected by protoplasmic processes lying on a field of 

 yolk, and the internal yolk shows no sign of segmentation. A blasto- 

 sphere in the ordinary sense is therefore not formed before gastrulation 

 begins. This process takes place in the following way. The surface 

 of the egg is covered with cells which are only loosely connected, and 

 show numerous intracellular chinks and fissures through which the 

 yolk-spherules appear. At one spot these fissures are larger than else- 

 where, and this spot marks the origin of the endo-mesodermic primordium. 

 Round this area the smaller ectoderm cells proliferate, and at the same 

 time a movement of the endoderm cells begins, so that the endo-meso- 

 dermic disc gradually diminishes in size. The inwandering of endo- 

 derm cells is most pronounced in the posterior region of the disc, the 

 cells of which ultimately take up a position on the ventral surface and 

 form a closed cell-layer. Others of the endoderm cells are merely 

 overgrown by the proliferating ectoderm, so that generally it may be 

 said that the method of gastrulation is, as it were, intermediate between 

 invagination of the ordinary type and the immigration of isolated cells. 

 The relations of this method of gastrulation to other types found in 

 Crustacea, and the conclusions as to the position of Leptodora to be 

 drawn from its occurrence in this type, the author intends to discuss 

 in the general part of his work. He believes that the invagination of 

 the endoderm primordium is prevented by the resistance of the newly 

 acquired yolk, and therefore that the modified process of gastrulation is 

 accomplished in part by active movements of the separate cells, in part 

 by passive overgrowth by the ectoderm cells. 



* Zoologica, Heft 30, ii. (1900) pp. 49-112 (4 pis.). 



t Ztitschr. wiss. Zool., lxviii. (1900) pp. 169-2(30 (6 pla.). 



D 2 



