ZOOLOGY AND BOTANY, MICROSCOPY, ETC. 
721 
geometer. As the shell itself is deposited by a process which involves 
the development of a statical equilibrium, it may be said that the shell 
has probably been developed statogenetically ; as the development of 
the figure of the egg is in all probability a purely dynamical problem, 
or one in which energy is applied in a definite manner to the plastic 
surface of a mass in statical equilibrium within the oviduct, we may 
say that the figure of the shell has been developed kinetogenetically. 
Mr. Ryder is of opinion that the origin of the egg-shell of the eggs 
of Birds and Reptiles may be traced^ 1 to |physiological causes acting 
automatically under the control of those instincts or intelligent efforts 
at self-preservation and protection extended to the young even while 
still in the form of the outwardly or apparently quiescent condition of 
the egg. He thinks that a retention of the ova in the oviduct would 
also distinctly tend to develope the amniote placental and viviparous 
forms of development, provided the retention of the eggs was from any 
cause prolonged. It would appear that the lines of demarcation between 
egg-laying and viviparous Vertebrates are in large measure arbitrary. 
There are now sufficient data to justify the doctrine that the various 
types of placentation were developed as the results of direct mechanical 
and physiological adaptation. 
Value of Mimetic Covering in the Struggle for Existence.* — Dr. 
Seitz thinks that too much stress has been laid on the relation of birds 
as destroyers of Insects. He has made some experiments with Monkeys, 
and relates how Trocliilium apiforme was shunned by Rhesus monkeys, 
while Cebus robustus took it. It appeared clear to him that the Rhesus 
monkeys knew wasps, respected their stings and were deceived by 
the mimicry. In the home of Cebus robustus the characteristic Vespa 
with its black and yellow rings is either absent or is very rare, and 
some of the mimicking Butterflies are like T. apiforme. 
“Urmund and Spina bifida.”t — Dr. M. von Davidoff points out how 
his own researches J on the development of Ascidians support the 
conclusions drawn by Hertwig § from observations on abnormal ova of 
the frog. Hertwig found that in such eggs the edges of the primitive 
streak formed a “ nerve-ring/’ Dr. M. von Davidoff finds that this 
condition, which persists throughout life in Actinia, and is abnormal 
in Amphibians, always appears temporarily in Ascidians. The central 
nervous system is formed by the apposition of the lips of the “ Urmund ” ; 
on this observation depends the conclusion of the paired origin of the 
central nervous system. The conditions in Ascidians also confirm the 
view supported by Hertwig that the dorsal organs, notochord, nervous 
system, &c., have all a paired origin, their points of origin having 
been once separated by the whole width of the blastopore, and being 
united by the fusion of its lips from before backwards. Further, with 
regard to the origin of the gastral mesoblast from the earlier peristomial, 
by the fusion of the lips of the blastopore, von Davidoff shows by 
means of quotations how closely his own earlier conclusions agree with 
those of Hertwig. He concludes that these interesting coincidences of 
* Zool. Anzeig., xvi. (1893) pp. 331-3. 
t Anat. Anzeig., viii. (1893) pp. 397-404. 
x MT. Zool. Stat. Neapel, ix. (1890) pp. 533-650 (1890). 
§ See this Journal (1892) pp. 585-6. 
