ZOOLOGY AND BOTANY, MICROSCOPY, ETC. 
459 
After describing the origin of mesodermic structures — somites, heart, 
and the like — Dr. Wilson concludes his important memoir with a dis- 
cussion of general morphological questions. The theory of His, that the 
vertebrate embryo is formed by the concrescence of two halves along 
the median dorsal line, receives no confirmation in the development of the 
Bass. Ziegler’s interpretation of Teleostean gastrulation is corroborated, 
and in regard to the significance of the germ-ring with respect to the 
amniotic gastrula, the author comes back to the Balfour-Rauber 
hypothesis. 
Polyspermy in Selachian Ova.* — Dr. J. Riickert has come to the 
conclusion that most, if not all, of the merocyte nuclei (vitelline or 
parablast nuclei) in the ova of Selachians are modified male pronuclei. 
They are present before the fusion of pronuclei ; they resemble male 
pronuclei in structure ; their appearance is preceded by the occurrence of 
several spermatozoon-heads ; transitions between them and spermatozoa 
were observed. From a study of the nuclear elements, Riickert infers 
that all merocyte nuclei of the early stages of segmentation which 
possess a reduced number of chromosomata result from spermatozoa. 
This is true of most, and perhaps all, of the merocyte-nuclei which 
originally occur in the germinal disc. The possibility is not excluded 
that some may result from migratory maternal cells, bnt this has not 
been proved. What Oppel has observed in Reptilian ova confirms 
Riickert’s conclusions. 
Ovarian Ova of Selachii.| — Prof. J. Riickert has studied these in 
Pristiurus , Scyllium , and Torpedo , especially in the first-named. The 
germinal vesicle of the smallest ova has, several nucleoli and numerous 
isolated chromosomata, forming what looks like a coil. It is very 
difficult to count these chromosomata, but there seem to be about 30-36, 
as there are in the mitosis of somatic cells in Pristiurus. 
As the first developmental period Riickert defines that which lasts 
until the ova have a diameter of about 1 • 5-2 mm. The germinal vesicle 
steadily increases in size, the nuclear membrane is distinct, the nucleoli 
increase in number. Each chromosoma is bordered by fine threads, as 
if outgrowths of the microsomata. Towards the end of the period the 
chromosomata become disposed in pairs, and their number is doubled, 
probably by longitudinal division. 
The second developmental period lasts from the time that the germinal 
vesicle has its maximum size until its dissolution (in ova 14-16 mm. in 
diameter). The chromosomata are shortened and thinned ; their micro- 
somata pass through a series of retrogressive changes; the pairs of 
chromosomata become more closely intertwined ; the whole mass is 
reduced. It can hardly be a tl Keimplasma” which undergoes such 
alterations of mass, it must be an involved somatic plasma. The nucleoli 
are also reduced in mass ; they seem to have some functional relation to 
the metabolism of the chromosomata. 
The third developmental period includes the disappearance of the 
germinal vesicle and the formation of the polar bodies. The chromatin 
elements, though not to be exactly counted, number only about a fourth 
* Anat. Anzeig., vii. (1892) pp. 320-33 (2 figs.), 
f Tom. cit., pp. 107-58 (6 figs.). 
