1892.] Embryology. 783 
usually appears a single structure externally but may be quite deeply 
bifid or double from the base. Posterior to the tail a median groove 
may run in to the digestive tract as an anal pit. 
The third class of monstrosities presents only a slight departure ` 
from the normal, having a prominent yolk plug not closed in when 
the larva is even older than in the second class, This plug oceupies 
the position of the normal blastopore or anus of Rusconi, posterior or 
ventral to the tail, and is due to a failure of the ventral lip of the 
blastopore to grow up as soon as it should have done. 
In interpreting these peculiar abnormal embryos the author assumes 
that they are all cases of arrested development, that the yolk plug is 
in each case really the blastopore, which has failed to close at the 
proper time, thus causing the median dorsal parts of the embryo to 
appear as paired structures along the lateral lips of the huge, open 
blastopore, whereas, they normally would first appear as single strue- 
tures along the median dorsal line when the blastopore had closed 
there. The retardation in the closure of this dorsal blastopore has 
thus kept dorsal structures separated till they have so far developed as 
to form half structures widely apart; later, when the blastopore closes, 
these halves may grow together more or less perfectly and so produce 
a normal form. 
It is to be regretted that individual cases were not actually watched 
so that there might be no doubt concerning the real value of these 
great, dorsal, hernia-like yolk plugs. 
The author thus definitely adopts the position, hitherto held only by 
Roux and opposed by Schultze, that the frog larva develops along 
what was the light-colored side of the egg, the blastopore closing in 
successively from the head towards the tail along this aspect of the 
egg. He regards the blastopore in the frog as a median, dorsal open- 
ing extending the whole length of the trunk, normally closing in till 
the anus of Rusconi and the definitive anus are left as evidences of 
its posterior portion, while anteriorly a median “ riickenrinne” and 
the lateral origin of mesoblast and the relations of the notochord give 
evidence of its existence through the whole length of the animal. 
Increase in length would not take place anterior to the closing blas- 
topore so much as at the actual point of successive closing, the blasto- 
pore advancing posteriorly pari pasu with its gradual closing. 
Hertwig takes a definite stand as a supporter of the concrescence 
theory of His, modifying it somewhat when extending it to all verte- 
brates by regarding the neurenteric canal as also a part of this dorsal 
blastopore. 
