ZOOLOGY AND BOTANY, MICROSCOPY, ETC. 
157 
heart, like that of other bony fishes, is formed by the union in the 
median plane of the lateral plates of the mesoderm. He describes his 
observations in detail and discusses the origin of the solid mass of 
enclosed cells which give rise to the endocardium. He points out that 
there are five ways in which these cells may be supposed to have arisen, 
and every one of these has been defended. For himself he thinks that 
this intermediate cell-mass arises from the lower inner margin of the 
protovertebrae. 
Yolk-syncytium and Blastoderm-rim in Salmonidse.* — Herr H. 
Virchow, with the help of several figures, describes the changes which 
take place during the development of the Salmonid egg in the yolk- 
syncytium (parablast, periblast, yolksac-endoblast) up to the time of its 
commencing degeneration. The various differentiations which the syn- 
cytium exhibits are shown to be connected with corresponding changes 
in the overlying segmented portion of the egg. The author is very 
sceptical as to any possible wandering of nuclei in the syncytium, as the 
distribution of the nuclei is generally uniform. The nuclei of the 
syncytium are admittedly derived from nuclei of the segmented part 
of the germ ; but the author opposes the idea that they undergo any 
changes of position other than the displacements caused by growth. 
Balfour’s view that the edge of the germinal disc represents the lip 
of an enlarged blastopore is supported. 
Formation of Mesoderm, Heart, Blood - vessels, and Blood in 
Salmonidse.j' — Herr Sobotta deals with the origin of the mesodermal 
tissues in the egg of Salmonidae. As stated by Virchow, the blastoderm 
at an early stage consists of a lenticular disc of cells separated from the 
yolk by a cavity. An internal proliferation of the circular edge of the 
lower (“ nervous ”) layer of the disc gives rise to a ringlike fold. This 
fold first appears at the hinder end of the blastoderm, but quickly 
extends along its whole periphery. The sides and anterior end of the 
fold consist only of mesoderm ; the hinder end of mesoderm and endo- 
derm. During the formation of the embryo as a swelling at the hinder 
end of the blastoderm, the hinder part of the meso-endodermal fold 
grows beneath the embryonic ectoderm, and separates in the middle 
region of the embryo into two layers — an upper layer of mesoderm, 
and a lower of endoderm. The notochord is differentiated from the 
endoderm at the time of the formation of the lateral mesoderm plates, 
and is at first connected with them ; it soon becomes marked off from 
the mesoderm, however, and in a later stage is separated from the 
endoderm. The body-cavity arises as a schizocoele. The endothelium 
of the heart is not derived from endoderm, but from certain cells which 
separate from the mesoderm masses and come to lie on the ventral side 
of the gut. The cardinal veins and aorta take their origin in a peculiar 
paired cell-mass lying beneath the notochord and derived from the 
inferior median angle of the mesoblastic somites. The pronephric 
tubules arise as true diverticula of the coelom; the pronephric duct, 
on the other hand, arises as a solid fold of the somatopleure in which 
a lumen subsequently appears. The ectoderm takes no part in the 
formation of the pronephros or its ducts. 
* Anat. Anzeig., Erganzungskeft, ix. (1894) pp. 66-77 (8 figs.), 
t Tom. cit., pp. 77-84. 
