ZOOLOGY AND BOTANY, MICROSCOPY, ETC. 951 



his observations the author observed a stage in which the formative proto- 

 plasm was differentiated into two layers, the outer of which was finely- 

 granular, while the inner was more coarsely so, and contained large highly 

 refractive granules ; the nuclei of both these layers appear to be the 

 products of the segmentation nucleus. The division of the nucleus of the 

 outer layer gives rise to a small blastodermic disc, formed of a single layer 

 of hexagonal cells. 



The mesoderm is originally paired, and is formed by the division of the 

 ectodermal cells on the thickened borders of the ventral streak ; it con- 

 tinues even to develop during the naupliiform stage, in which three pairs 

 of rudimentary appendages have the form of small sacs, made up by a layer 

 of hexagonal cells. Corresponding with the segments indicated by these 

 appendages, the mesoderm undergoes a rudimentary segmentation. The 

 body-cavity is formed in the anterior part of the body by the absorption of 

 the yolk which is surrounded by the mesoderm ; in the hinder part of the 

 body the yolk is surrounded by endoderm, and the space between ectoderm 

 and endoderm is filled by isolated mesodermic cells ; these, later on, 

 become connected with the ectoderm and endoderm, and between them the 

 body-cavity appears. 



The types of segmentation hitherto observed in the Crustacea are essen- 

 tially four, three of which are holoblastic. In Palsemon Bobretzky has 

 found it to be complete and regular ; after the division of the nucleus the 

 ovum divides into two segmentation spheres ; the internal portions of all the 

 cells fuse into a central vitelline mass, which is surrounded by a blasto- 

 dermic layer. Mayer found in Eupacjurus Prideauxii that the nucleus divided 

 into two, four, eight parts ; the independent cells thus devolojied migrate 

 towards the surface of the egg, and there is then a total and regular seg- 

 mentation of the egg. Here also the internal ends of the cells fuse into a 

 single central vitelline mass. In Calianassa mediterranea and in Asellus 

 aquaticus the nucleus and the surrounding protoplasm undergo segmenta- 

 tion in the interior of the egg, and after the formation of a certain number 

 of cells these migrate, as in Eupagurus ; the yolk then commences to 

 undergo superficial segmentation in such a way that a vitelline segment is 

 difierentiated around each blastodermic cell, while the centre of the mass 

 undergoes no segmentation ; this type reminds us of what happens in 

 insects. In the Schizopoda and in Oniscus a mesoblastic segmentation 

 has been observed. 



Brain of Mysis flexuosa.* — M. E. Koehler finds that the elements of 

 the nerve-centre of Mysis flexuosa offer no special characters, but that 

 the dotted substance is a good deal reduced. The greater jjart of the 

 non-cellular portions are formed of packets of parallel fibres, which form 

 very distinct bundles ; the masses of granular dotted substance interposed 

 among the fibrils are neither numerous nor extensive, and most are easily 

 resolved, with a high magnifying power, into a close plexus of anastomosing 

 fibrils. The topographical relations were studied by sections taken along 

 varying planes, but the descriptions refer so closely to the illustrations 

 that a general account is here impossible. The structure of the ventral 

 chain is extremely simple, and the ganglia only project slightly beyond the 

 connectives ; in the abdomen the ganglia are even more reduced than in 

 the other parts of the body ; the connectives are formed of longitudinal 

 fibres ; those of the first three ganglia are separated by a certain quantity 

 of connective tissue, but beyond it they approach one another, and are only 

 separated by a delicate partition. 



♦ Ann. Sci. Nat.— Zool., ii. (1887) pp. 159-88 (2 pla.). 



