784 The American Naturalist. [September, 
In discussing the blastopore and concrescence in various vertebrates 
a sharp distinction is drawn between the true blastopore or depression 
leading into the digestive tract and the growing edge of the blasto- 
erm, “ Umwachsungsrand”’ as it may be called. Only part of the 
latter may, in some cases, become the blastopore. Thus in the bony 
fish the blastopore consists of a short transverse portion or sickle and 
a longitudinal constantly elongating and closing median groove run- 
ning forward from the sickle. The sickle is gradually formed more 
and more from the edges of the blastoderm, the “ Umwachsungsrand,” 
till the latter is eventually used up in this way, becoming converted into 
sickle-groove, which in turn is gradually closed in along the median, 
dorsal line of the embryo. In the shark, however, the “ Umwachungs- 
rand” soon leaves the sickle and the partly closed in portion of the 
blastopore and then closes by itself; is not then part of the blastopore. 
In the chick or in a reptile this separation is such an early one that 
the true blastopore is quite removed from the edge of the circular 
growing edge of the blastoderm, which then is not to be reckoned as 
part of the blastopore at all. 
The anus of vertebrates is regarded as the posterior part of this 
elongated blastopore, hence the vertebrate tail is morphologically, as 
in some of these monstrous frog embryos, a double structure growing 
out from the right and left lips of the blastopore. The tail, with its 
neural tube, notochord, mesoblastic somites and portion of the ento- 
blast is then not a prolongation of the trunk, but a dorsal outgrowth 
of different value. It elongates by a transfer of the “ Wachsthums- 
zone ” to its tips and in the same manner as the trunk elongated. 
How it is possible for the closing in process and growth to take place 
posterior to the tail and also at the tips of the tail the author does not 
explain. 
Having brought forward some arguments for his coelom theory and 
replied to certain criticisms of Gétte the author next discusses at length 
the relation of the blastopore to various abnormal forms in vertebrates. 
He takes the view that the formation of several embryos from a single 
egg is to be referred back to the formation of as many gastrula invag- 
inations in that egg. The difference between such multiple monsters 
in different groups of vertebrates is then due to the differences in the 
‘gastrulation, to the various possible ways in which multiple invagina- 
tions may arise in different sorts of eggs. The apparent absence or 
great rarity of double monsters in the Amphibia may be due either to 
the small size of the egg and difficulty of double invagination or it 
may be that such doubleness is early obliterated by following fusion 
