ZOOLOGY AND BOTANY, MICROSCOPY, ETC. 
21 
granulosa, and traces the subsequent changes that occur round the 
ripening ovum in the “ vitelline membrane” and “ follicular epithelium.” 
'The Pressure within the Egg of the Fowl.* — Sig. L. Tarulli finds 
that internal varnishing of the air-chamber of the egg prevents develop- 
ment, except in the first stages. The pressure is much affected, and the 
respiration but a little. External varnishing of the air-chamber is 
also followed by disturbing results, but the pressure after being in- 
creased returns to the normal, through the use of the unvarnished region. 
When the air-chamber is filled with oil, and thus completely varnished 
internally, there are no traces of development. Varnishing the egg not 
only hinders respiration, but affects pressure and temperature. The 
air-chamber regulates pressure, the surface of evaporation regulates tem- 
perature. It is only when these two conditions are naturally fulfilled 
that respiration can remain normal. 
Formation of a Double Embryo in the Hen’s-egg.t — Prof. W. 
Baldwin Spencer describes an egg in which two clearly formed embryos 
were developed within the limits of one blastoderm. The two embryos 
are precisely similar to one another ; each is in the stage at which the 
nervous system has the form of a tube, the anterior end of which is be- 
coming swollen out to form the vesicles of the brain ; at the posterior 
end the neural canal is still' widely open. Apparently every stage may 
be met with between this complete reduplication and that in which one 
portion only of the body is doubled. Prof. Spencer points out that 
there are three ways in which this doubling may have been brought 
about. As in the case of Lumbricus trapezoides there may be division 
of the at first single and normal embryo, but that could hardly have 
happened here, as the surrounding areas show no trace of division. 
Two distinct nuclei may have been inclosed abnormally within the proto- 
plasmic material of one ovum ; but then we should expect two blasto- 
derms. The third chance is the most probable, namely that the very 
first division of the nucleus was abnormal, and its products may have 
been qualitatively and quantitatively precisely similar, and not, as we 
may suppose to be the case in normal division, slightly different. This 
explanation will suffice also for the case, recorded by Mr. A. H. S. Lucas, | 
of a partially double chick embryo ; and in fact, all cases of incomplete 
division may be explained by it. In these abnormal segmentation, re- 
sulting in the production of two halves precisely similar, only takes 
place at a later stage, and so only affects certain cells (or their nuclei) 
which will give rise to certain organs of the body. The earlier in seg- 
mentation the abnormal division takes place, the larger is the part of 
the body affected. 
Maturation of Amphibian Ova.§— Dr. O. Bossi has been studying 
the maturation of the ovum in Triton and Bana . He finds that the ger- 
minal vesicle undergoes preliminary modifications within the ovarian 
ova, and that the ova from the base of the oviduct show no trace of 
germinal vesicle as such. He believes that a complete dissolution, and 
* Atti e Eend. Accad. Med. Chirurg. Perugia, ii. (1890) pp. 121-34. 
t Proc. Roy. Soc. Victoria, ii. (1890) pp. 113-5 (1 fig.). 
% T. c., pp. 111-2 (1 fig.). § Anat. Anzeig., v. (1890) pp. 142-3. 
