578 SUMMARY OF CURRENT RESEARCHES RELATING TO 



as mcsoflerra and cndoclcrm in the sense in which these terms are used." 

 The ectoderm is, by its position and function, more clearly defined from 

 the first. 



The enveloping chorion functions as an egg-sac. When the fertilized 

 nucleus divides, its products pass towards the surface until a syncytium 

 of eight nuclei is formed ; the yolk segments over the whole surface 

 simultaneously into the same number of partial pyramids ; each of these 

 latter has a large nucleus at its base, while its apex fuses with the common 

 yolk-mass in the interior of the egg. After a time, by retardation in 

 one half, the egg loses its radial symmetry, and becomes two-sided. 

 When the primitive blastoderm is formed, a general migration of nuclei 

 takes place from the surface to the yolk within ; this is followed by a 

 partial secondary segmentation of the food-yolk into balls. 



The gastrula is modified, a slight invagination occurring where the 

 superficial cells are thickest ; the included cells multiply rapidly, and 

 form a mass of similar elements, some of which pass into the yolk. The 

 protoplasm surrounding the nuclei of these colls is prolonged into a 

 reticulum which incloses myriads of small yolk-fragments, and probably 

 digests them intcrcellularly. 



At the beginning of the egg-nauplius period, when numerous yolk- 

 cells have passed forward and joined the inner sxirface of the embryonic 

 ectoderm, certain new bodies begin to appear in great numbers. These 

 are the secondary mesoderm cells, and they arise by a process of endo- 

 genous growth from the embryonic cells or nuclei, and chiefly from the 

 wandering cells. Some of them appear to become ordinary mesoderm 

 cells, while others seem to be converted directly into blood-corpuscles. 



The plasticity of the embryonic cells and layers and the comparative 

 slowness with which they are clearly dift'crentiatcd are very striking ; 

 the cell-mass developed round the blastopore cannot be artificially 

 divided into layers. The cndoderm, which does not appear definitely 

 till comparatively late, is developed from yolk-cells which assume a 

 peripheral j)ositiou. 



Moina bathycolor and the greatest depths at which Cladocera 

 are found.* — Dr. 0. Nordqvist refers to Herr J. Richard's paper on 

 Moina hatliycolor Vernet, and jioints out that last year he suggested that 

 this form was probably the same as Ilijocriiptus acutifrons Sars. Hyo- 

 criqjfus, Alona, and Eurycercus are the Cladocera which are found at 

 greatest depths — as far as 200 metres down. 



Vermes. 



a. Annelida. 



Embryology of Vermilia caespitosa and Eupomatus elegans.f — 

 Mr. W. A. Haswell has some notes on the development of these two 

 Annelids, in both of which artificial impregnation was readily effected. 

 In Vermilia segmentation is equal and regular, as in Se}j>ida and 

 Pomatoceros. The blastopore, which is at first nearly terminal, becomes 

 shifted to that side of the larva which will be the ventral ; at the same 

 time it becomes elongated and slit-like, the anterior end of the slit 

 widening to form the mouth, while the anus is formed near the posterior 

 end at a somewhat later stage. When the process of invagination 



* Zool. Anzeig., xi. (1888) pp. 264-5. 



t Proc. Linn. Soc. N. S. Wales, i. (1888) pp. 1032-4. 



