34 MALL. [Vor. XIX. 
formed perfectly and the vertebral canal remains open. How- 
ever, it is easy to conceive of the second kind as a variety of 
the first, for in it the development went on normally, but the 
vertebral arches did not meet in the dorsal mid-line to pro- 
duce the vertebral canal. 
In 1892 O. Hertwig! published his remarkable article on 
open blastopore in frogs’ eggs and its relation to spina bifida. 
Hertwig was experimenting upon eggs to produce poly- 
spermy, after they had been kept for some time after matura- 
tion and found that many of these eggs developed abnormally, 
due to polyspermy, he believed. A part of the eggs segmented 
irregularly and developed in a peculiarly pathological fashion, 
that is the blastopore remained open much longer than it 
should. The further study of these specimens showed that 
they grew into embryo monsters with all kinds of deformity 
of the spinal canal, often producing quite typical specimens 
of spina bifida. Hertwig saw clearly the bearing of these 
experiments upon the general explanation of spina bifida and 
its relation to the blastopore. This he discusses at great 
length and with much ability. He was able to show the rela- 
tion of his work with that obtained by Lereboullet on the 
pike, by Oellacher on the salmon, and by Rauber on the 
trout. These investigations had shown that an open spinal 
canal or even a total fissure of the body may result when the 
germ ring does not unite properly to form the body of the 
embryo. 
It now became possible for the first time to follow spina 
bifida from its very earliest stages in amphibian and fish 
embryos up to a time when it is clear that the process is 
identical with that found in birds and mammals. ‘To be sure, 
in the latter cases we must take the specimens as they occa- 
sionally come to us, for it is impossible at present to experi- 
ment upon mammals successfully, and in chicks the ex- 
periments are not very satisfactory. This comparison was 
made by Hertwig with great acumen, using the excellent 
*Hertwig, Arch. f. mik. Anat., XX XIX, 1892. 
