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Analyses of Books. 
sac ; and its hinder end, at the edge of the blastoderm, appears 
to correspond with the same extremity of the mature body. 
Growth in length depends on a process of intersusception, and, 
until the full number of mesoblastic somites are formed, it is 
effected, as in the Chaetopods, by the constant addition of somites 
between the one last formed and the posterior end of the body. 
On the other hand, His and Rauber assert that growth in 
length is effected by a coalescence of the two halves of the 
thickened edges of the blastoderm in the median dorsal line. 
The arguments in support of this view are, briefly stated, the 
general appearance, certain measurements which to the author 
seem mainly to prove that the growth takes place by the addition 
of fresh somites between the end of the body and the last-formed 
somite, and, lastly, certain phenomena observed in double mon- 
sters. An examination of the growth of the embryo proves, 
however, that points brought forward by His and Rauber are 
far from conclusive, and that the growth in length of the greater 
part of the body certainly takes place, as in the Chaetopods, by 
the growth of fresh somites. Hence it would be strange if a 
small part in the middle of the body should grow in a different 
way. If the vertebrate blastopore had the same extent as the 
dorsal surface, as His and Rauber assert, this should appear in 
Amphioxus. But in this species it is at first placed quite at the 
hind end of the body, and nearly closes up before the appearance 
of the medullary groove or the mesoblastic somites. The me- 
dullary folds have nothing to do with its lips. Again, in the 
Vertebrates the food-yolk is situate on the ventral side of the 
body, and is enfolded in the blastoderm. Hence in all “ large- 
yolked ” forms the ventral walls are completed by the lips of the 
blastopore closing on the ventral side. 
On the “ concrescence ” hypothesis of His and Rauber the 
dorsal walls are also completed by the closing of the blastopore, 
so that the concrescence of its lips would form both the dorsal 
and the ventral, as well as the dorsal walls of the embryo. This 
consideration Dr. Balfour thinks amounts to a reductio ad ab- 
surdum of the whole theory. 
In Chapter XII. the author discusses the ancestral form of the 
Chordata. He considers it clear that the ancestors of the 
Chordata were segmented, their mesoblast being divided into 
myotomes, which even extended into the region in front of the 
mouth ; so that the head, as well as the trunk, was segmented. 
The only internal skeleton was the non-segmented notochord. 
Hence Dr. Balfour draws the corollary that the skeleton is of 
relatively little value for the decision of many fundamental 
questions. The region which now forms the oesophagus and 
stomach was in the ancestral forms severed by gill-clefts. The 
mouth had a sucftorial character, and had its place on the ventral 
surface behind the pre-oral lobe In the higher types it has gra- 
dually become modified to a biting structure, and has been 
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