ZOOLOGY AND BOTANY, MICROSCOPY, ETC. 43 



Teleostei are concerned no nuclei or cells arise in the periblast or in the yolk, 

 and that the nuclei of the periblast, after their separation from the blasto- 

 derm, degenerate, and take no direct share in the formation of the embryo. 



2. The development of heart and blood-vessels. — After the ventral closure 

 of the gut, a band of mesoderm cells is observed close behind the optic 

 vesicles on the lower surface. They arise from the indifferent mesoderm 

 cells of the head which wander round the gut. The mass of cells splits to 

 form a kind of pouch or sac — the incipient heart. He lays special emphasis 

 on the role of these actively wandering mesoderm cells, not only in forming 

 the heart, but also the vessels and other parts of the embryo. The 

 similarly mesodermic origin of the blood-vessels is noted, and the history 

 of both heart and vessels is briefly discussed. It is important to note that 

 the heart grows as an open bag into the segmentation cavity, and its lumen 

 is nothing else than part of the blastocoel. So too are the three main 

 yolk-vessels parts of the blastocoel. The relation of this fact to Biitschli's 

 theory of the metazoan vascular system, as well as to such points as the 

 blastocoele origin of the proboscis-sheath cavity and vascular system in 

 Nemerteans is briefly noted. 



3. In regard to the much disputed origin of the Mood, Wenckebach main- 

 tains at least that it is of purely mesodermic origin, and that neither 

 endoderm nor free periblast nuclei share in its formation. 



Relation of Yolk to Blastoderm in Teleostean Fish-ova.* — Mr. G. 

 Brook briefly traces the development of a teleostean ovum from its origin 

 in the germinal!* epithelium. The excess of nutriment supplied by the 

 follicle is stored up as yolk in small masses. He points out the difference 

 in the relative distribution of protoplasm and yolk in the pelagic group of 

 ova on the one hand, and the herring and others on the other. In Tra- 

 chinus, belonging to the former group, even while segmentation is in 

 progress there is always a thin film of protoplasm around the large 

 single yolk-sphere, and he compares it to a fat-cell. He draws attention 

 to the difference between holoblastic and meroblastic ova, and shows that 

 the real difference is not in the proportion of yolk to protoplasm, but in its 

 distribution; and this necessitates a different mode of assimilation. It 

 is through the agency of the parablast that this takes place, i. e. the 

 portion of the germinal protoplasm which is not included in the germinal 

 disc. So long as there is naked protoplasm around the yolk intracellular 

 digestion can take place, and the protoplasm elaborated from the yolk 

 necessarily takes a share in the formation of the embryo. 



Origin of Pigment-cells whicli invest the Oil-drop of Pelagic Fish- 

 embryos.f — In his examination of the embryos of Scomber scomber Mr. J. 

 A. Eyder noticed that before the tail had become prominent pigment-cells 

 began to appear on the side of the oil-drop ; around the latter was a layer 

 of protoplasm, continuous with the periblast enveloping the yolk. The 

 periblast is hypoblastic ; and the only source of the nuclei of the pigment- 

 cells must be the periblast ; therefore these cells are hypoblastic in origin. 



Segmentation of Selachian Ovum.;}: — In studying the development of 

 Selachia Prof. J. Kollmann observed the persistence of segmentation, at 

 a late stage, on the floor of the segmentation cavity and in the adjacent layer 

 of the yolk. This has been observed by Kupffer in reptiles, and by Gasser 

 in birds. The stages observed were those with oval germinal disc before 

 differentiation of layers, and those with round disc and axial differentiation. 



* Proc. R. Phys. Soc. Edinb., ix. (1886) pp. 187-93. 



t Amer. Natural., xx. (1886) p. 987. 



i Verhandl. Naturf. Gesell. Basel, viii. (188G) pp. 103-.5. 



